Reference · updated 10 November 2026

Ohm's law calculator: volts, amps, ohms and watts

Q: How do I calculate resistance from voltage and current?

Divide the voltage by the current: R = V ÷ I. 12 V and 25.5 mA give 12 ÷ 0.0255 = 470 Ω. Enter the voltage and the current above, and the resistance and power are calculated for you.

Q: How do I calculate power from voltage and resistance?

Use P = V² ÷ R. At 12 V across 470 Ω that is 144 ÷ 470 = 0.306 W. If you know the current instead, P = V × I or P = I² × R.

Enter any two of voltage, current, resistance and power, and the calculator gives you the other two. Ohm's law is V = I × R; together with P = V × I it links all four quantities. The results are for DC, or for the RMS values of a resistive AC load.

iPhone and iPad (iOS 17 or later): free on the App Store. Android later.

Ohm's law calculator

This calculator needs JavaScript. Without it, use the formulas and the worked examples below.

Use the unit boxes for mV, mA, kΩ and so on. A decimal point or a decimal comma both work. If you fill in three or four fields, clear one: only two values are needed, and a third would be either redundant or contradictory.

The formulas

Voltage V in volts, current I in amperes, resistance R in ohms, power P in watts. Whichever two you know, the other two follow:

You know	Resistance R	Current I	Voltage V	Power P
V and I	`V ÷ I`	given	given	`V × I`
V and R	given	`V ÷ R`	given	`V² ÷ R`
V and P	`V² ÷ P`	`P ÷ V`	given	given
I and R	given	given	`I × R`	`I² × R`
I and P	`P ÷ I²`	given	`P ÷ I`	given
R and P	given	`√(P ÷ R)`	`√(P × R)`	given

What the formulas are not: Ohm's law describes a resistor, whose resistance does not change with voltage. A lamp, a diode, a motor or a battery does not follow it as a single fixed resistance, and AC reactive loads need impedance and power factor. For those, a calculated number is only a starting point; measure it.

Worked examples

Current and power in a resistor. A 12 V supply across 470 Ω. Current I = 12 ÷ 470 = 25.5 mA, power P = 12² ÷ 470 = 0.306 W. A ¼ W resistor would run at its limit, so use a ½ W part.

Known	Calculated
V = 12 V, R = 470 Ω	I = 25.5 mA, P = 0.306 W

Voltage across a resistor. 2 mA flows through 4.7 kΩ. Voltage V = 0.002 × 4700 = 9.4 V, power P = 0.002² × 4700 = 18.8 mW.

Known	Calculated
I = 2 mA, R = 4.7 kΩ	V = 9.4 V, P = 18.8 mW

Largest voltage for a power rating. A 1 kΩ resistor rated ¼ W (0.25 W). Voltage V = √(0.25 × 1000) = 15.8 V, current I = √(0.25 ÷ 1000) = 15.8 mA. Above 15.8 V it is over its rating.

Known	Calculated
R = 1 kΩ, P = 0.25 W	V = 15.8 V, I = 15.8 mA

Measure it live

To check the numbers on real hardware, connect a Bluetooth multimeter the app reads live, such as the OWON OW18B. Measure the voltage across the part and the current through it, then compare with the calculation.

The calculator in the app has preset formulas for Ohm's law and power, series and parallel, dividers, LED resistors, RC, dB and unit conversion, and takes engineering input such as 4k7. Cal Box fills it from your live reading and recent captures, and suggests Ohm's law. For several resistors on one node, see the parallel resistor calculator.

What the app does here, and what it doesn't

What it does
What it does	Plan
A calculator with SI prefix keys, engineering input such as 4k7 and a parallel operator.	Free
Presets for Ohm's law and power, series and parallel, dividers, LED resistors, RC, dB and unit conversion.	Free
Cal Box fills the calculator from your live reading and recent captures, and suggests Ohm's law.	Free

Questions

How do I calculate resistance from voltage and current?

Divide the voltage by the current: R = V ÷ I. 12 V and 25.5 mA give 12 ÷ 0.0255 = 470 Ω. Enter the voltage and the current above, and the resistance and power are calculated for you.

How do I calculate power from voltage and resistance?

Use P = V² ÷ R. At 12 V across 470 Ω that is 144 ÷ 470 = 0.306 W. If you know the current instead, P = V × I or P = I² × R.

How do I convert mA to A in Ohm's law?

Divide by 1000: 25.5 mA is 0.0255 A. In the calculator, pick mA in the unit box and type the number as it is; the conversion is done for you.

Does Ohm's law work for AC?

For a purely resistive load, yes, with RMS values. For coils, capacitors and motors the current and voltage are out of phase, and you need impedance and power factor instead of plain resistance.

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