Ohm's Law Calculator

Enter any two of voltage (V), current (A), resistance (Ω) and power (W) to calculate the other two.

Fill in exactly two of the four boxes (leave the other two empty). Values must be 0 or more.

Voltage V

Current I

Resistance R

Power P

Result

Voltage V—

Current I—

Resistance R—

Power P—

Electrical safety: this is an ideal calculation for direct current (DC) and purely resistive loads. It does not include the power factor in AC circuits or changes in resistance with temperature. Follow the ratings in the device's datasheet. Working with high voltage or large currents can cause electric shock or fire, so check with a qualified person. Do not exceed the power rating of a resistor.

The text copied is the four values in base units (V, A, Ω, W), one per line, with 7 significant digits and no commas.

Ohm's law and the power formulas

For a DC circuit with a resistive load, voltage V (volts), current I (amperes), resistance R (ohms) and power P (watts) are related by

V = I × R (Ohm's law)
P = V × I (power)
which also gives P = I² × R = V² ÷ R

Which pair gives which result

You enter	You get	Formulas
V and I	R, P	R = V ÷ I, P = V × I
V and R	I, P	I = V ÷ R, P = V² ÷ R
V and P	I, R	I = P ÷ V, R = V² ÷ P
I and R	V, P	V = I × R, P = I² × R
I and P	V, R	V = P ÷ I, R = P ÷ I²
R and P	V, I	V = √(P × R), I = √(P ÷ R)

The calculation uses exact fractions. Only the R and P pair needs a square root (a square root that does not come out evenly is labeled "rounded value").

When you enter 0

A result that would need a division by 0 is shown as "Not available", and the other results are still calculated. If the two values you enter contradict each other, an error is shown.

V and I, with I = 0: P = 0. R is not available (you cannot divide by 0).
V and R, with R = 0 (a short circuit): I and P are not available, because the current is not determined.
V and P, with V = 0: if P is greater than 0 this is a contradiction (error). If P is also 0, I and R are indeterminate.
I and P, with I = 0: if P is greater than 0 this is a contradiction (error). If P is also 0, V and R are indeterminate.
R and P, with R = 0: if P is greater than 0 this is a contradiction (error). If P is also 0, V = 0 and I is indeterminate.
I and R: always calculable, even when one of them is 0.

Examples

V = 12 V, R = 4 Ω → I = 3 A, P = 36 W
V = 100 V, I = 0.5 A → R = 200 Ω, P = 50 W
I = 2 A, R = 5 Ω → V = 10 V, P = 20 W
R = 8 Ω, P = 50 W → V = 20 V, I = 2.5 A (a case where the square roots come out evenly)

Units and SI prefixes

The selector next to each box lets you choose a unit (mV or kV, µA or mA, mΩ, kΩ or MΩ, mW or kW and so on). Results are shown in base units (V, A, Ω, W), and when a more readable prefix applies it is shown in parentheses as well (for example, 0.02 A (20 mA)).

Notes

Enter exactly two values. With three or more, the values can contradict each other or be redundant, so an error is shown. Negative values are not accepted (only magnitudes are handled).
This calculator is for DC circuits with resistive loads. AC power factor, apparent power, capacitors and inductors are not covered.
Values can have up to 9 digits before the decimal point and 6 decimal places. Use a period as the decimal separator; commas are accepted only as thousands separators (1,234.5).

Frequently asked questions

How do I find the current when I know the power?

If you also know the voltage, I = P ÷ V. For example, a device that uses 1,500 W at 100 V draws 15 A. If you know the resistance instead of the voltage, I = √(P ÷ R).

What if I know three of the values?

Enter only two of them. You can then compare the third with the calculated result to check it.

How do I check a resistor's power?

Check that the power P that will actually be dissipated is smaller than the resistor's power rating (in watts). This calculator can find P from V and R (or from I and R).