Ohm's Law Calculator: Voltage, Current, Resistance, Power

How to use it

Type the two values you know into their boxes, for example Voltage 12 and Resistance 470. Leave the other two boxes empty.
Read Current and Power, the two values that were worked out. The other two boxes show what you typed.
Open Working, step by step to see the formula and the numbers used for each result.
To try another case, clear one box and type a different value in its place. Fill in exactly two boxes.

The formulas

Ohm's law says that the voltage across a resistor is the current through it times its resistance. The power a component turns into heat is voltage times current. Combining the two gives three forms of the power formula, so any two of the four quantities are enough to find the others.

V = I × R        Ohm's law
P = V × I        power
P = I² × R       power with current and resistance
P = V² ÷ R       power with voltage and resistance

V  volts (V)     I  amperes (A)     R  ohms (Ω)     P  watts (W)

Known voltage and current: R = V ÷ I and P = V × I.
Known voltage and resistance: I = V ÷ R and P = V² ÷ R.
Known voltage and power: I = P ÷ V and R = V² ÷ P.
Known current and resistance: V = I × R and P = I² × R.
Known current and power: V = P ÷ I and R = P ÷ I².
Known resistance and power: I = √(P ÷ R) and V = √(P × R).

Prefixes and what the boxes accept

A prefix stands for a power of ten: m is milli (one thousandth), k is kilo (a thousand), M is mega (a million) and µ is micro (one millionth). A letter in the middle of a number takes the place of the decimal point, as on resistor markings: 4k7 is 4.7 kilohms and 2R2 is 2.2 ohms. Units typed after the number, such as 12V or 20mA, are ignored.

What Ohm's law does not cover

Components that do not follow Ohm's law, such as diodes and LEDs, whose voltage hardly changes with current. Use their datasheet curve or the LED resistor calculator.
Alternating current with capacitors or inductors, where impedance replaces resistance. The result is only correct for resistive loads or for the average of a steady supply.
The temperature of a resistor, its power rating and the safety margin you need: check the component's datasheet before you choose a part.

Worked example: 12 V across 470 Ω

The voltage is 12 V and the resistance is 470 Ω, so the current is I = V ÷ R = 12 ÷ 470 = 0.02553 A, which is 25.53 mA.

The power is P = V × I = 12 × 0.02553 = 0.3064 W, which is 306.4 mW. The same answer comes from P = V² ÷ R = 12² ÷ 470 = 0.3064 W.

The resistor therefore has to handle more than 0.3 W, and should be rated with a margin above that figure.

Frequently asked questions

What are the units in Ohm's law?

Voltage is in volts (V), current in amperes (A), resistance in ohms (Ω) and power in watts (W). A current of 25 mA is 0.025 A, which is why the calculator reads prefixes.

Can I type 4k7 or 10m in the boxes?

Yes. 4k7 is read as 4.7 kilohms (4700 Ω) and 10m as 10 milliamperes or milliwatts, depending on the box. The results are shown with the prefix that fits them best, such as 25.53 mA.

Why does the calculator refuse a current of zero?

Resistance is voltage divided by current, and dividing by zero has no answer. A zero current with a known resistance is still valid, because the voltage across the resistor is then zero too.

Which formula gives power when I know only current and resistance?

Use P = I² × R. For example, 0.02 A through 1 kΩ dissipates 0.02² × 1000 = 0.4 W, the same as the voltage 20 V times the current 0.02 A.

Does Ohm's law work for LEDs?

Not directly. An LED's forward voltage stays nearly constant as current changes, so the current is set by the resistor in series with it. The LED resistor calculator on this site uses the forward voltage for that reason.

Can I use this for AC circuits?

For a purely resistive load, yes: the same formulas hold for the RMS values. Circuits with capacitors or inductors need impedance, which this calculator does not include.

Sources

Figures and text last checked on Tuesday, November 10, 2026.

Result

Working, step by step