Ohm’s Law Calculator: Calculate Voltage, Current, Resistance and Power
Enter any two known values and this Ohm’s Law calculator finds the other two: voltage V, current I, resistance R and power P. The page also explains the Ohm’s Law formula, the VIR triangle, power formulas, units, series circuits, parallel circuits, LED resistor sizing, and school-level examples in one place.
Quick answer
Ohm’s Law states that the current through an ohmic conductor is proportional to the voltage across it and inversely proportional to its resistance, when physical conditions such as temperature remain constant.
Voltage equals current times resistance. If you know any two of these values, you can calculate the third. Combine it with P = V × I to calculate electrical power too.
Interactive Ohm’s Law Calculator
Enter exactly two known values from voltage, current, resistance, or power. Leave the unknown fields blank. The calculator converts common units automatically and shows the formulas used.
Ohm’s Law formulas, VIR triangle and electrical power equations
Use the formula table below to choose the correct equation for the value you need. Start with V = I × R for voltage, current and resistance, then use P = V × I for electrical power.
Formula wheel
P ÷ I
√(P × R)
P ÷ V
√(P ÷ R)
P ÷ I²
V² ÷ P
I² × R
V² ÷ R
How to read the wheel
The formula wheel is a compact way to show all twelve common formulas. Choose the value you want to find, then pick the formula that uses the two values you already know. For example, if you want current and you know voltage and resistance, use I = V ÷ R. If you want power and you know current and resistance, use P = I² × R.
The simple VIR triangle covers the three basic forms of Ohm’s Law. The larger wheel adds Watt’s Law for power, so the same page can help with circuit values, resistor checks, LED current limiting, and power dissipation problems.
| Value to find | Use when you know... | Formula | Plain meaning |
|---|---|---|---|
| Voltage V | Current and resistance | V = I × R | Volts equal amps times ohms. |
| Voltage V | Power and current | V = P ÷ I | Volts equal watts divided by amps. |
| Voltage V | Power and resistance | V = √(P × R) | Voltage is the square root of watts times ohms. |
| Current I | Voltage and resistance | I = V ÷ R | Amps equal volts divided by ohms. |
| Current I | Power and voltage | I = P ÷ V | Amps equal watts divided by volts. |
| Current I | Power and resistance | I = √(P ÷ R) | Current is the square root of watts divided by ohms. |
| Resistance R | Voltage and current | R = V ÷ I | Ohms equal volts divided by amps. |
| Resistance R | Power and current | R = P ÷ I² | Ohms equal watts divided by amps squared. |
| Resistance R | Voltage and power | R = V² ÷ P | Ohms equal volts squared divided by watts. |
| Power P | Voltage and current | P = V × I | Watts equal volts times amps. |
| Power P | Current and resistance | P = I² × R | Power lost as heat rises with current squared. |
| Power P | Voltage and resistance | P = V² ÷ R | Power equals voltage squared divided by resistance. |
Worked examples: calculating Ohm’s Law step by step
Work through these examples to see how the same formulas are used in homework-style circuit questions, resistor calculations, and power checks.
Example 1: Calculate current from voltage and resistance
Problem: A 12 V battery is connected to a 4 Ω resistor. Find the current and power.
Formula: I = V ÷ R
Calculation: I = 12 ÷ 4 = 3 A. Then P = V × I = 12 × 3 = 36 W.
Answer: The current is 3 A and the resistor dissipates 36 W.
Example 2: Calculate resistance from voltage and current
Problem: A small lamp has 9 V across it and draws 0.30 A. Find its resistance.
Formula: R = V ÷ I
Calculation: R = 9 ÷ 0.30 = 30 Ω.
Answer: The lamp resistance at that operating point is 30 Ω.
Example 3: Calculate voltage from current and resistance
Problem: A sensor circuit needs 25 mA through a 220 Ω resistor. Find the voltage drop across the resistor.
Convert first: 25 mA = 0.025 A.
Formula: V = I × R
Calculation: V = 0.025 × 220 = 5.5 V.
Answer: The voltage across the resistor is 5.5 V.
Example 4: Calculate power from current and resistance
Problem: A 10 Ω heating element carries 5 A. Find voltage and power.
Voltage: V = I × R = 5 × 10 = 50 V.
Power: P = I² × R = 5² × 10 = 25 × 10 = 250 W.
Answer: The heater has 50 V across it and dissipates 250 W.
Example 5: Calculate amps from volts and ohms
Problem: How many amps flow through a 2.2 kΩ resistor connected to 5 V?
Convert: 2.2 kΩ = 2200 Ω.
Formula: I = V ÷ R = 5 ÷ 2200 = 0.0022727 A.
Answer: The current is about 2.27 mA.
Example 6: Calculate resistor wattage
Problem: A 470 Ω resistor has 12 V across it. How much power does it dissipate?
Formula: P = V² ÷ R = 12² ÷ 470 = 144 ÷ 470 = 0.306 W.
Answer: A 0.25 W resistor would be too small. Choose a higher power rating, such as 0.5 W or above, depending on the safety margin and temperature conditions.
What is Ohm’s Law? Simple definition, statement and meaning
Ohm’s Law is one of the first rules students learn in electricity because it turns a circuit question into a clear calculation. It says that the voltage across a conductor, the current through it, and the resistance of that conductor are related by the equation V = I × R. A simple definition is: current increases when voltage increases and current decreases when resistance increases, as long as the conductor behaves in an ohmic way.
The common statement used in physics class is: At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across it. This means that if you double the voltage across a fixed resistor, the current doubles. If you triple the voltage, the current triples. The ratio V ÷ I stays the same, and that constant ratio is the resistance R.
The wording matters because it explains the relationship, while the formula lets you solve the problem. On a graph of voltage against current for an ohmic conductor, the graph is a straight line through the origin. The gradient of the graph is resistance.
Voltage V
Voltage is electrical potential difference. It is the push that causes charge to move through a circuit. Unit: volt, symbol V.
Current I
Current is the rate of flow of electric charge. It is measured in amperes, commonly called amps. Unit: ampere, symbol A.
Resistance R
Resistance is opposition to current flow. It is measured in ohms. Unit: ohm, symbol Ω.
Why the symbol for current is I
Students often ask why current is I instead of C. In electrical notation, I represents current intensity. That convention appears in many formulas, including V = I × R, P = V × I, and P = I² × R. The letter C is already used for electric charge unit names such as coulomb and for capacitance in many circuit formulas.
What is an ohmic conductor?
An ohmic conductor is a material or component that keeps an approximately constant resistance when conditions stay constant. A fixed metal resistor is the standard classroom example. Its voltage-current graph is a straight line. A diode, LED, thermistor or filament lamp is not perfectly ohmic across all conditions because its resistance changes as voltage, current or temperature changes.
Series and parallel circuit calculator helper
Use this helper after finding the total resistance of a series or parallel network. Once total resistance is known, apply Ohm’s Law to calculate total current, branch current, or voltage drop.
Parallel: 1/Rtotal = 1/R1 + 1/R2 + 1/R3. The same voltage appears across every branch.
How Ohm’s Law works in a series circuit
In a series circuit, the current is the same through every component because there is only one path for charge to flow. You add the resistances first. If a 12 V supply is connected to 100 Ω and 200 Ω in series, the total resistance is 300 Ω. The current is I = 12 ÷ 300 = 0.04 A, or 40 mA. The voltage drop across each resistor is then found with V = I × R. The 100 Ω resistor drops 4 V and the 200 Ω resistor drops 8 V.
How Ohm’s Law works in a parallel circuit
In a parallel circuit, every branch has the same voltage, but each branch can carry a different current. If a 12 V supply is connected across 100 Ω and 200 Ω in parallel, the branch currents are 12 ÷ 100 = 0.12 A and 12 ÷ 200 = 0.06 A. The total current is 0.18 A. The equivalent resistance is R = V ÷ I = 12 ÷ 0.18 ≈ 66.7 Ω.
LED resistor calculator using Ohm’s Law
This mini-tool helps with a common electronics problem: choosing a current-limiting resistor for an LED. It uses the same relationship R = V ÷ I, but the resistor voltage is the supply voltage minus the LED forward voltage.
LED resistor formula
The formula is R = (Vsupply − VLED) ÷ ILED. The current must be in amps, so 20 mA becomes 0.020 A. For a 5 V supply and a 2 V red LED at 20 mA, the resistor needs to drop 3 V. The calculation is R = 3 ÷ 0.020 = 150 Ω. The resistor power is P = I² × R = 0.020² × 150 = 0.06 W, so a common quarter-watt resistor is normally above that ideal dissipation.
Do not connect a bare LED directly to a voltage source unless the source or driver is designed to limit current. LEDs are non-ohmic devices, so their current can rise quickly after forward voltage is reached. The resistor is there to keep current within a safe range.
Ohm’s Law for Class 10, GCSE and school physics
For school physics, the most important point is not memorising a long list of formulas. The main idea is that a fixed resistor has a predictable relationship between potential difference and current. If the temperature of the resistor stays constant, the ratio of voltage to current is constant. That constant ratio is called resistance.
A clear school answer can be written like this: Ohm’s Law states that the current through a conductor is directly proportional to the potential difference across its ends, provided the temperature and other physical conditions remain constant. The formula is V = IR. The unit of potential difference is volt, the unit of current is ampere, and the unit of resistance is ohm.
If the question says “state and explain Ohm’s Law,” include the condition about constant temperature. Then explain direct proportionality: when voltage doubles, current doubles for the same resistor. On a graph with voltage on the y-axis and current on the x-axis, the line should be straight and pass through the origin. The gradient of a V-I graph equals resistance. On a graph with current on the y-axis and voltage on the x-axis, the gradient equals conductance, which is 1/R.
Exam wording
“At constant temperature, current is directly proportional to potential difference.” Then write V = IR and define each symbol.
Common mistake
Do not say resistance is always constant for every device. Resistance is constant only for an ohmic conductor under constant conditions.
Simple paragraph answer
Ohm’s Law describes the relationship between voltage, current and resistance in a conductor. It says that if the resistance remains constant, increasing the voltage increases the current in the same ratio. The equation is V = I × R. This means a 6 Ω resistor with 2 A of current has 12 V across it. The law is useful because it lets us calculate unknown electrical quantities in simple circuits.
Electric circuit for studying Ohm’s Law
A standard school experiment uses a cell or DC power supply, a fixed resistor or resistance wire, an ammeter in series, a voltmeter in parallel with the resistor, a switch, and a variable resistor. The aim is to measure pairs of voltage and current values and check whether voltage is directly proportional to current.
- Build the circuit.
Connect the resistor, ammeter, switch and variable resistor in series. Connect the voltmeter across the fixed resistor, not in series with it. - Start with low voltage.
Close the switch briefly and record the voltmeter and ammeter readings. Open the switch between readings to reduce heating. - Change the current.
Adjust the variable resistor or power supply to get a new current. Record another voltage-current pair. - Repeat several times.
Collect enough readings to draw a graph. Use sensible ranges and avoid overheating the resistor. - Plot the graph.
If the component is ohmic and temperature stays nearly constant, the graph of voltage against current should be a straight line through the origin.
Why temperature matters
As current flows through a resistor, electrical energy is converted to heat. If the resistor becomes hotter, its resistance may change. That is why school practical instructions often say to keep the current low, take readings quickly, and open the switch between readings. The condition “temperature remains constant” is part of the proper statement of Ohm’s Law.
What the graph shows
A straight-line graph shows direct proportionality. If the graph bends, the component is not behaving as a fixed ohmic resistor across that range. A filament lamp often gives a curved graph because the filament heats up and its resistance increases. A diode gives a strongly non-linear graph because it conducts easily in one direction only after a threshold voltage is reached.
Voltage, current, resistance and power: detailed guide
Voltage: the push that moves charge
Voltage is sometimes described as electrical pressure. A battery, power supply or generator provides potential difference between two points. When a conducting path is connected, charges move and current flows. In calculations, voltage is measured in volts. The symbol can be V, but some electronics texts use E for electromotive force. This calculator labels the voltage field as V or E because many older electronics problems use E in formulas like E = I × R.
Current: charge flow per second
Current is the flow rate of electric charge. The unit is ampere, shortened to amp or written A. Many electronics circuits use small currents, so milliamps are common. One milliamp is one thousandth of an amp. If a problem says 20 mA, you should calculate with 0.020 A unless the calculator converts the unit for you. This page supports microamps, milliamps, amps and kiloamps.
Resistance: opposition to current
Resistance is measured in ohms. A high resistance allows less current for the same voltage. A low resistance allows more current. Resistors are used to limit current, divide voltage, set bias conditions, sense current, pull signals up or down, and turn electrical energy into heat. Real resistors also have tolerance and power ratings, so a measured value can differ slightly from the printed value.
Power: how fast electrical energy is transferred
Power is measured in watts. The equation P = V × I says that power equals voltage multiplied by current. If a device has 12 V across it and 2 A through it, it is using 24 W. Power matters because components can overheat. Wires, resistors, switches, relays, motors and power supplies all have current or wattage limits.
| Quantity | Symbol | Unit | Unit symbol | Calculator question |
|---|---|---|---|---|
| Voltage / potential difference | V or E | volt | V | What is the voltage across the resistor? |
| Current | I | ampere | A | How many amps from volts and ohms? |
| Resistance | R | ohm | Ω | How many ohms are needed? |
| Power | P | watt | W | How many watts does the resistor dissipate? |
Unit conversion reference for Ohm’s Law
Most calculation mistakes happen because the numbers are entered in the wrong unit. The equation works cleanly when voltage is in volts, current is in amps, resistance is in ohms and power is in watts. The calculator handles unit conversion automatically, but it is still useful to understand the conversions.
| Prefix | Meaning | Example | Base value |
|---|---|---|---|
| micro, μ | one millionth | 250 μA | 0.000250 A |
| milli, m | one thousandth | 20 mA | 0.020 A |
| kilo, k | one thousand | 4.7 kΩ | 4700 Ω |
| mega, M | one million | 2.2 MΩ | 2200000 Ω |
| milliwatt, mW | one thousandth of a watt | 250 mW | 0.250 W |
| kilowatt, kW | one thousand watts | 2.4 kW | 2400 W |
Three significant digits
Some homework questions ask for an answer within three significant digits. This means you keep three meaningful digits, not necessarily three decimal places. For example, 0.0022727 A becomes 0.00227 A, or 2.27 mA. A value of 5.503 V becomes 5.50 V to three significant digits. This calculator displays friendly engineering prefixes so very small and very large values are easier to read.
Common mistakes when calculating Ohm’s Law
Typing zero for an unknown
Leave unknown values blank. Zero is a real value and can make division impossible or change the meaning of the problem.
Forgetting mA to A conversion
20 mA is 0.020 A, not 20 A. This is one of the biggest errors in LED and small electronics problems.
Mixing kΩ with Ω
10 kΩ is 10000 Ω. Enter the correct unit or convert before using the formula.
Using Ohm’s Law on an LED directly
An LED is not a fixed resistor. Use Ohm’s Law for the current-limiting resistor, not for the LED alone.
Ignoring resistor wattage
A resistor can have the right ohm value but still burn out if its power rating is too low.
Applying DC formulas to reactive AC loads
For capacitors and inductors, impedance and phase angle matter. Pure resistance is a simpler case.
Where Ohm’s Law is used in real circuits
Ohm’s Law appears in almost every practical electronics and electrical topic. It is used to size current-limiting resistors, calculate voltage drops, check whether a power supply can deliver enough current, estimate heating in a wire, choose resistor wattage, analyse voltage dividers, and understand why a short circuit produces a very large current.
Voltage dividers
A voltage divider uses two resistors in series to create a smaller voltage from a larger one. The same current flows through both resistors. Ohm’s Law explains why the voltage drop across each resistor depends on its resistance. Larger resistance gets a larger share of the total voltage.
Current sensing
A small resistor can be placed in series with a load to sense current. If the resistor is 0.1 Ω and the voltage across it is 0.25 V, the current is I = 0.25 ÷ 0.1 = 2.5 A. This idea is common in battery chargers, motor drivers and power supplies.
Wire voltage drop
Long wires have resistance. When current flows, a voltage drop appears across the wire. If a wire path has 0.2 Ω resistance and carries 5 A, the drop is V = 5 × 0.2 = 1 V. That lost voltage can affect motors, lights and low-voltage electronics.
Battery internal resistance
Real batteries have internal resistance. When a load draws current, some voltage is lost inside the battery. A battery with 0.05 Ω internal resistance supplying 20 A has an internal drop of V = 20 × 0.05 = 1 V. This is why battery voltage can sag under heavy load.
Fuses and circuit breakers
Protective devices are chosen by current. If you know the power of an appliance and the supply voltage, you can estimate current with I = P ÷ V. A 1200 W device on a 120 V supply draws about 10 A. A 1200 W device on a 240 V supply draws about 5 A.
Choose the right formula by question type
Find voltage
Use V = I × R when the problem gives current and resistance. Use V = P ÷ I when power and current are given. Use V = √(P × R) when power and resistance are given. The answer gives the potential difference across the component.
Find current
Use I = V ÷ R when voltage and resistance are known. Use I = P ÷ V when watts and volts are given. Use I = √(P ÷ R) when power and resistance are given. The answer is the current through the component in amperes.
Find resistance
Use R = V ÷ I when voltage and current are known. Use R = P ÷ I² when power and current are known. Use R = V² ÷ P when voltage and power are known. The answer gives the opposition to current flow in ohms.
Find power
Use P = V × I when voltage and current are known. Use P = I² × R for heating in a resistor when current is known. Use P = V² ÷ R when voltage and resistance are known. The answer gives electrical power in watts.
Ohm’s Law calculator FAQ
These questions cover the main ideas students need: voltage, current, resistance, power, the ohm triangle, units, formulas, and practical circuit use.
What is Ohm’s Law?
Ohm’s Law states that voltage equals current multiplied by resistance: V = I × R. It describes how electrical pressure, charge flow, and opposition to flow are connected in a simple resistive circuit when temperature and physical conditions remain constant.
What is the Ohm’s Law formula?
The main formula is V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. The same relationship can be rearranged as I = V ÷ R or R = V ÷ I.
How do I calculate voltage from current and resistance?
Use V = I × R. Multiply the current in amps by the resistance in ohms. For example, if I = 2 A and R = 6 Ω, then V = 2 × 6 = 12 V.
How do I calculate current from voltage and resistance?
Use I = V ÷ R. Divide the voltage by the resistance. For example, a 24 V source connected to a 12 Ω resistor gives I = 24 ÷ 12 = 2 A.
How do I calculate resistance from voltage and current?
Use R = V ÷ I. Divide the voltage by the current. For example, if a component has 9 V across it and carries 0.03 A, then R = 9 ÷ 0.03 = 300 Ω.
How do I calculate power with Ohm’s Law?
Use P = V × I when voltage and current are known. You can also use P = I² × R when current and resistance are known, or P = V² ÷ R when voltage and resistance are known.
What values do I need for this calculator?
Enter exactly two known values from voltage, current, resistance, and power. The calculator then finds the other two values and shows the formulas used.
What is the VIR triangle?
The VIR triangle is a memory aid for Ohm’s Law. Put V at the top and I and R at the bottom. Cover the value you want to find: V = I × R, I = V ÷ R, or R = V ÷ I.
What is the power triangle?
The power triangle places P at the top and V and I at the bottom. It helps you remember P = V × I, V = P ÷ I, and I = P ÷ V.
What is an ohm?
An ohm, written with the symbol Ω, is the SI unit of electrical resistance. A component has a resistance of 1 Ω when 1 V across it produces 1 A of current.
What does I mean in Ohm’s Law?
I means electric current. It is measured in amperes, or amps. The letter I comes from older electrical notation for current intensity.
What does V mean in Ohm’s Law?
V means voltage or potential difference. Voltage is the energy supplied per unit of charge and is measured in volts.
What does R mean in Ohm’s Law?
R means resistance. Resistance measures how strongly a component opposes electric current and is measured in ohms.
What does P mean in electrical formulas?
P means electrical power. It is measured in watts and tells you how quickly electrical energy is being transferred or converted.
Can this calculator handle milliamps and kilohms?
Yes. Choose mA for milliamps, μA for microamps, kΩ for kilohms, or MΩ for megohms. The calculator converts the units before solving.
How do I convert milliamps to amps?
Divide milliamps by 1000. For example, 20 mA = 0.020 A and 500 mA = 0.500 A.
How do I convert kilohms to ohms?
Multiply kilohms by 1000. For example, 4.7 kΩ = 4700 Ω and 10 kΩ = 10000 Ω.
Can Ohm’s Law be used for parallel circuits?
Yes, but first find the equivalent resistance of the parallel network. Then apply I = V ÷ Rtotal to find total current. Each parallel branch has the same voltage, so branch current is found with Ibranch = V ÷ Rbranch.
Can Ohm’s Law be used for series circuits?
Yes. Add all series resistances to get total resistance, then use I = V ÷ Rtotal. The current is the same through every series component, while voltage drops divide across the components.
Does Ohm’s Law work for AC circuits?
For a purely resistive AC circuit, use RMS voltage and RMS current with V = I × R. For circuits with capacitors and inductors, use impedance Z instead of resistance R.
When does Ohm’s Law not apply?
It does not directly describe non-ohmic devices such as diodes, LEDs, thermistors, transistors, and components whose resistance changes strongly with voltage, current, or temperature.
How do I choose a resistor for an LED?
Use R = (Vsupply − VLED) ÷ ILED. For example, with a 5 V supply, a 2 V LED, and 20 mA current, R = (5 − 2) ÷ 0.020 = 150 Ω.
What resistor wattage should I use?
Calculate resistor power using P = I² × R or P = V² ÷ R. Then choose a resistor power rating above the calculated value, often with a safety margin so the resistor does not run too hot.
Why does higher resistance reduce current?
For a fixed voltage, I = V ÷ R. As R gets larger, the denominator is larger, so the current becomes smaller.
Why does higher voltage increase current?
For a fixed resistance, I = V ÷ R. As V increases, current increases in direct proportion, assuming the component remains ohmic.
Is Ohm’s Law the same as Watt’s Law?
No. Ohm’s Law connects voltage, current, and resistance. Watt’s Law connects power, voltage, and current. Used together, they give the common 12 calculator formulas for V, I, R, and P.
What is an ohmic conductor?
An ohmic conductor is a material or component whose voltage-current graph is a straight line through the origin under constant physical conditions. Its resistance is approximately constant.
What is a simple school definition of Ohm’s Law?
A simple definition is: the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance, as long as temperature stays constant.
How many formulas are in the Ohm’s Law wheel?
The full wheel used by electronics students usually shows 12 formulas: three for voltage, three for current, three for resistance, and three for power.
Why does my real measured value differ from the calculator?
Real components have tolerance, temperature change, internal resistance, meter accuracy limits, and contact resistance. The calculator gives the ideal value for the numbers entered.