Ohm’s law calculator
given
given
derived; type to use instead
derived; type to use instead
- Voltage Vgiven
- 120 V
- Current Igiven
- 10 A
- Resistance RV ÷ I
- 12 Ω
- Power PV × I
- 1,200 W
DC and resistive AC only. For AC loads with a power factor below 1 (motors, ballasts), P = V × I × PF; use the watts-to-amps calculator.
Nothing typed leaves the browser; the inputs live only in the page address.
Type any two of the four quantities and the other two are struck beneath, each with the formula that produced it. The page keeps the last two fields you touched as the givens, so you can work around the wheel without clearing anything. For AC loads with a power factor below 1, use the watts-to-amps calculator instead.
Worked examples
- 120 V and 10 A
- R = 120 ÷ 10 = 12 Ω; P = 120 × 10 = 1,200 W. A 1,200 W space heater is a 12 Ω resistor. Open this run
- 1,500 W at 120 V
- I = 1500 ÷ 120 = 12.5 A; R = 120² ÷ 1500 = 9.6 Ω. On a 15 A circuit that is 83 % of the breaker; 210.23(A) limits a cord-and-plug load to 80 %. Open this run
- 0.1 Ω of cable at 20 A
- V = 20 × 0.1 = 2 V dropped; P = 20² × 0.1 = 40 W of heat in the cable. The voltage-drop calculator does this with the real Table 8 resistance. Open this run
- 60 W and 240 Ω
- V = √(60 × 240) = 120 V; I = √(60 ÷ 240) = 0.5 A. An old 60 W incandescent lamp at operating temperature. Open this run
How it is worked out
V = I × R, P = V × I; from which R = V ÷ I = V² ÷ P = P ÷ I², I = V ÷ R = P ÷ V = √(P ÷ R), V = P ÷ I = √(P × R), P = I² × R = V² ÷ R.
For AC with reactance, replace R with impedance Z and multiply power by the power factor; see the watts-to-amps and kVA pages.
The figures behind it
- Ohm’s law
- V = I × R (Georg Ohm, 1827) and Joule’s law P = V × I. Arithmetic, no table.
Questions people ask
- Can I use this for AC?
- For resistive loads (heaters, incandescent lamps) yes. For motors, transformers and electronics the current is larger than P ÷ V by the power factor; use the watts-to-amps calculator with PF.
- Which two should I enter?
- Whatever you know. The page treats the last two fields you typed in as the givens and derives the rest; clear a field to fall back to the others.
Embed this calculator
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<iframe src="https://necwire.com/embed/ohms-law-calculator" width="100%" height="760" loading="lazy" title="Ohm’s law calculator"></iframe> <p><a href="https://necwire.com/ohms-law-calculator">Ohm’s law calculator</a> by NECwire</p>