Ohm’s law links the four numbers you measure most with a multimeter: voltage, current, resistance and power. Enter any two of them in the free calculator below and it works out the other two, shows the formula it used, and tells you which multimeter input to use for the current.
Calculate volts, amps, ohms and watts
Enter any two values and leave the other two boxes empty.
How to Use the Ohm’s Law Calculator
- Type the two values you know, for example the voltage and the wattage printed on a device label.
- Pick the unit next to each box: mV, V or kV for voltage, µA, mA or A for current, Ω, kΩ or MΩ for resistance, and mW, W or kW for power.
- Leave the other two boxes empty and press Calculate.
- Read the two new values (highlighted in blue) and the formula used for each. Press Reset to start again.
Ohm’s Law and Power Formulas
Every result in the calculator comes from these twelve formulas. V is voltage in volts, I is current in amps, R is resistance in ohms and P is power in watts.
| To find | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| Voltage (V) | V = I × R | V = P ÷ I | V = √(P × R) |
| Current (I) | I = V ÷ R | I = P ÷ V | I = √(P ÷ R) |
| Resistance (R) | R = V ÷ I | R = V² ÷ P | R = P ÷ I² |
| Power (P) | P = V × I | P = I² × R | P = V² ÷ R |
Worked Examples
1200 W heater on a 120 V outlet
Current: I = P ÷ V = 1200 ÷ 120 = 10 A. Resistance while it runs: R = V² ÷ P = 120² ÷ 1200 = 12 Ω. That is two-thirds of a 15 A circuit on its own, so avoid running it on the same circuit as other high-power appliances.
55 W car headlight bulb on 12 V
Current: I = 55 ÷ 12 ≈ 4.6 A. Resistance while lit: R = 12² ÷ 55 ≈ 2.6 Ω. Measure the same bulb cold with a multimeter and you will see a much lower reading, often well under 1 Ω, because a filament’s resistance rises sharply as it heats up.
220 Ω resistor on 5 V
Current: I = 5 ÷ 220 ≈ 22.7 mA. Power: P = 5² ÷ 220 ≈ 0.11 W. A standard 1/4 W resistor handles that with room to spare.
How to Measure Each Value with a Multimeter
- Voltage: set the dial to V (AC or DC) and touch the probes across the part, in parallel. See how to use a multimeter.
- Current: break the circuit and put the meter in series, with the red lead in the mA or 10A input. See how to measure amps with a multimeter.
- Resistance: turn the power off and measure the part on its own. See how to measure resistance with a multimeter.
- Power: a multimeter cannot read watts directly. Measure volts and amps, then multiply, or follow how to test watts with a multimeter.
Safety: never measure resistance on a live circuit, and never connect the meter in current mode across a voltage source. Both can blow the meter’s internal fuse or damage the meter.
Frequently Asked Questions
What is Ohm’s law?
Ohm’s law says that voltage equals current multiplied by resistance: V = I × R. If the resistance stays the same, doubling the voltage doubles the current.
How do I calculate watts from volts and amps?
Multiply them: P = V × I. For example, 120 V × 2 A = 240 W.
How do I calculate amps from watts and volts?
Divide the watts by the volts: I = P ÷ V. A 1200 W appliance on 120 V draws 10 A. This is exact for heaters and incandescent bulbs. Motors and electronics on AC can draw a little more because of power factor.
Does Ohm’s law work for AC circuits?
Yes for resistive loads such as heaters and incandescent bulbs, using the RMS voltage and current your meter shows. For motors, transformers and electronics, impedance and power factor also matter, so the results are an estimate.
Why does my multimeter show a different resistance than the calculator?
The calculator gives the resistance while the device is running. A multimeter measures it cold, with the power off, and heating elements and bulb filaments have much lower resistance when cold. On very low readings, also subtract the resistance of your test leads.
Can a multimeter measure watts?
No. A standard multimeter measures volts and amps separately. Multiply the two readings to get watts, or use a plug-in power meter for appliances.