Ohm's law calculator

Fill in any two of volts, amps, ohms and watts to get the other two, with the working shown.

By the Calcist team · Checked against known answers · Updated October 27, 2026

Private: calculated on your device. Nothing you type is sent anywhere.

Next: Know the watts now? See what it costs to run with the electricity cost calculator.

The formulas

Ohm's law links voltage (V, in volts), current (I, in amps) and resistance (R, in ohms). The power law adds power (P, in watts). Together they give every combination:

V = I × R · I = V ÷ R · R = V ÷ I
P = V × I = I² × R = V² ÷ R
V = √(P × R) · I = √(P ÷ R) · R = V² ÷ P · I = P ÷ V

Worked example

A 12 V battery is connected to a 4 Ω resistor. The current is I = 12 ÷ 4 = 3 A, and the resistor turns P = 12 × 3 = 36 W into heat. Check it another way: I² × R = 9 × 4 = 36 W.

Unit prefixes

Prefix	Means	Example
µ (micro)	÷ 1,000,000	250 µA = 0.00025 A
m (milli)	÷ 1,000	20 mA = 0.02 A
k (kilo)	× 1,000	4.7 kΩ = 4,700 Ω
M (mega)	× 1,000,000	1 MΩ = 1,000,000 Ω

Limitations

Ohm's law doesn't apply everywhere:

AC circuits with motors, coils or capacitors: they have reactance as well as resistance, so P = V × I gives apparent power, not real power. You'd need the power factor.
Non-linear parts: LEDs, diodes and transistors don't have a fixed resistance.
Light bulbs and heaters: resistance changes with temperature; a cold filament has much lower resistance than a hot one.

This calculator takes positive values for a simple DC or purely resistive circuit.

Sources

How we checked this

Before every update, this calculator is run in a real browser with these known answers, and it must match all of them:

12 V across 4 Ω: 3 A and 36 W ✓
20 mA through 1 kΩ: 20 V and 400 mW ✓
60 W at 120 V: 500 mA and 240 Ω ✓
Three values filled in asks for exactly two ✓
1 mV across 1 MΩ: 1 nA, 1 pW ✓
1 nV across 1 TΩ: 1 × 10⁻²¹ A in scientific notation ✓

Last checked October 26, 2026. Spotted something wrong? The math is done in your browser with the formula shown on this page, so you can check any result by hand.

Add this calculator to your website (free)

Paste this code into any web page. The calculator works on your page, resizes itself, has no ads and no tracking, and nothing your visitors type is sent anywhere. More about embedding.

<iframe src="https://calcist.com/embed/ohms-law-calculator/" title="Ohm&#x27;s law calculator" width="100%" height="640" loading="lazy" style="border:0;max-width:760px"></iframe>
<p style="font:14px/1.4 sans-serif;margin:4px 0 0"><a href="https://calcist.com/ohms-law-calculator/">Ohm&#x27;s law calculator</a> by Calcist</p>
<script src="https://calcist.com/embed.js" async></script>

Questions people ask

What is Ohm's law?

Voltage equals current times resistance: V = I × R. Double the voltage across the same resistor and the current doubles.

How do I calculate watts from volts and amps?

Multiply them: P = V × I. 120 V × 2 A = 240 W.

What resistor do I need for an LED?

Subtract the LED's forward voltage from the supply and divide by the current you want: (5 V − 2 V) ÷ 0.02 A = 150 Ω. Enter 3 V and 20 mA here to check.

How many amps is 1,500 watts at 120 volts?

I = P ÷ V = 1,500 ÷ 120 = 12.5 A.

Related calculators

Electricity cost calculatorWhat an appliance costs to run per day, month and year.Scientific calculatorsin, cos, logs, powers, roots and more, with a history you can edit.Unit converterLength, weight, volume, temperature, speed, data and more.Exponent calculatorAny power exactly, negative and fractional exponents, or a missing exponent.