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Ohms Law Calculator

An Ohm's law calculator solves the relationship between voltage, current and resistance: enter any two and it returns the third, plus the power in watts. V = I × R, and P = V × I — the two equations behind almost every basic electrical calculation.

Your measurements

Result

It's the workhorse for sizing resistors, checking a load, or sanity-checking a circuit. Leave one field blank and the calculator fills it in, then gives you the power dissipated.

How it is calculated

Ohm's law: V = I × R, rearranged to I = V ÷ R or R = V ÷ I. Power: P = V × I (equivalently I²R or V²/R). These hold exactly for DC and for AC with a purely resistive, unity-power-factor load; reactive AC loads need the power factor applied.

Steps

Enter any two of voltage (V), current (A) and resistance (Ω).
Leave the third field at 0 — it will be solved.
Read the computed value and the power in watts.
Use it to size resistors, fuses or check a load.
For AC with motors or reactive loads, use the watts/amps calculator with a power factor.

Worked example

12 V across a 6 Ω resistor draws 2 A and dissipates 24 W (I = V ÷ R, P = V × I).

Common questions

What is Ohm's law?

V = I × R: voltage equals current times resistance. Rearranged, it gives current or resistance from the other two.

How do I calculate power?

P = V × I (watts). It also equals I² × R or V² ÷ R — handy when you don't have both V and I.

Can I use this for AC?

Yes for resistive loads (heaters, incandescent bulbs). For motors and other reactive loads, apply a power factor — use the watts-to-amps calculator.

What units do I use?

Volts, amps and ohms. Keep them consistent (e.g. milliamps → amps) and the power comes out in watts.