Ohm's Law Calculator

Solve voltage, current, resistance and power from any two known values.

Find resistance and power from voltage and current.

Formula

R = V / I

Inputs

Voltage (V)
Current (A)

What it does

Ohm’s law links voltage, current and resistance in a resistive circuit: V = I × R. Give any two of voltage, current, resistance or power and the calculator finds the rest, plus a safe power rating for the resistor.

Where it's used

Sizing current-limiting resistors, working out how much current a load draws, or finding an unknown resistance from two meter readings.

How to use it

Pick what you already know in I know….
Type the two values — prefixes work: 20m = 20 mA, 4.7k = 4 700 Ω.
Read voltage, current, resistance, power and conductance. Your own inputs are shown muted.
Buy a resistor with at least the suggested rating (twice the dissipated power).

The formula explained

V: voltage across the part, in volts (V)
I: current through it, in amperes (A)
R: resistance, in ohms (Ω)
P: power turned into heat, in watts (W)
G: conductance = 1 / R, in siemens (S)

Worked example

A 24 V supply connected across a 1 kΩ resistor.

Inputs:

I know…: Voltage & Resistance
Voltage: 24 V
Resistance: 1k Ω

The calculator gives:

Voltage V: 24 V
Current I: 24 mA
Resistance R: 1 kΩ
Power P: 576 mW

Tips and common mistakes

Ohm’s law is exact only for “ohmic” parts such as resistors and wires. LEDs, diodes and lamps change resistance with current.
Mixing mA and A is the most common slip — type 20m rather than converting by hand.
A resistor run at its full rating gets very hot; keep at least a 2× margin.

Questions people ask

How do I pick a resistor to get a certain current?

Use R = V / I. To pass 20 mA from a 12 V supply you need 12 / 0.02 = 600 Ω. Standard values don’t include 600 Ω, so choose the next value up (620 Ω), which keeps the current just under 20 mA.

What happens if I double the voltage across a resistor?

The current doubles, but the power — the heat — goes up four times, because P = V² / R. A resistor that was comfortable at 6 V can overheat at 12 V.

Does Ohm’s law work for AC?

Yes for resistors, using RMS voltage and current. For capacitors and inductors the same idea holds with impedance Z in place of R: V = I × Z, where Z depends on frequency.

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