Ohm's law calculator
Type any two values. The other two appear as you type, along with the formula that produced them. Works for DC circuits and purely resistive AC loads such as heaters and incandescent lamps.
Fill in any two. Fields you haven't typed in are calculated.
Using I = V ÷ RP = V² ÷ R
See it: one battery, one resistor, one lamp
Drag the voltage up and watch the current (the moving charges) speed up. Raise the resistance and they slow down. The lamp shows power: it grows with the square of voltage, so doubling V quadruples the heat.
- Current I = V ÷ R
- 0.50 A
- Power P = V × I
- 6.0 W
The Ohm's law formulas
Georg Ohm published the relationship in 1827: for a given conductor, current rises in step with voltage. Add the power equation and you get twelve formulas, three for each quantity.
| To find | From V & I | From V & R | From I & R | From P & … |
|---|---|---|---|---|
| Voltage V | — | — | I × R | P ÷ I or √(P × R) |
| Current I | — | V ÷ R | — | P ÷ V or √(P ÷ R) |
| Resistance R | V ÷ I | — | — | V² ÷ P or P ÷ I² |
| Power P | V × I | V² ÷ R | I² × R | — |
Worked examples
- Heater element: a 1,500 W heater on 120 V draws I = 1500 ÷ 120 = 12.5 A and has a hot resistance of R = 120 ÷ 12.5 = 9.6 Ω.
- 12 V strip: an LED strip drawing 2.5 A from 12 V uses P = 12 × 2.5 = 30 W.
- Resistor wattage: 20 mA through a 470 Ω resistor dissipates P = 0.02² × 470 = 0.19 W, so a ¼ W part is fine.
Where Ohm's law stops working
It holds for resistive parts at a steady temperature. Diodes and LEDs are non-linear (current climbs steeply past the forward voltage), filaments change resistance as they heat, and AC loads with coils or capacitors need impedance and power factor. For those, use the power factor calculator or the LED resistor calculator.
Knowing the numbers doesn't make a circuit safe to touch. Mains voltage can kill at currents far below what trips a breaker. Switch off and lock out circuits and test before working on them.
Questions people ask
What is Ohm's law?
Ohm's law says the current through a conductor is proportional to the voltage across it: V = I × R. Combined with the power law P = V × I, any two of voltage, current, resistance and power determine the other two.
Does Ohm's law work for AC?
Yes for purely resistive loads (heating elements, incandescent bulbs). Motors, transformers, LED drivers and electronics have reactance, so use impedance (Z) instead of R and include power factor when working out real power.
How do I find resistance from watts and volts?
R = V² ÷ P. A 1,500 W heater on 120 V has a hot resistance of 120² ÷ 1500 = 9.6 Ω.
Why does my meter read a lower resistance than the calculator?
Many elements, especially tungsten filaments, have much lower resistance cold than at operating temperature. The calculator gives the operating (hot) value implied by the rated watts and volts.