How do I find watts from volts and amps?

Multiply them: watts = volts × amps. A device running at 120 V and drawing 5 A uses 600 W.

How do I find amps from watts?

Divide watts by volts: amps = watts ÷ volts. A 1,500 W heater on a 120 V circuit draws 12.5 A.

Ohm's law states that voltage equals current multiplied by resistance (V = I × R). It's the basic relationship this calculator, and most circuit analysis, builds on.

Why do I need to know resistance for a household appliance?

You usually don't directly — most people use this calculator to go from watts to amps or back. Resistance is included because it falls out of the same formulas and is occasionally useful, for example when working with a resistive heating element.

Does this work for AC power like household mains?

It's accurate for simple resistive loads (heaters, incandescent lighting) on AC or DC. For loads with a motor or power supply, the real current can differ because of power factor, which isn't accounted for here.

Electrical Calculator: Volts, Amps, Watts & Ohms

Enter any two of voltage (V), current (A), power (W) or resistance (Ω) and the calculator finds the other two using Ohm's law (V = I × R) and Watt's law (P = V × I). A 120 V circuit drawing 10 A is using 1,200 W.

Enter exactly two values — the calculator finds the other two.

Voltage

Current

Power

Resistance

Voltage

120 V

Current

10 A

Power

1,200 W

Resistance

12 Ω

Assumes a simple DC or resistive-load circuit. For AC loads with motors, transformers or electronics (power factor below 1), real current can be higher than this calculator shows for the same wattage.

Show the math

1
Find power with Watt's law
P = V × I = 120 V × 10 A = 1,200 W
Power is voltage multiplied by current.
= 1,200 W

Read the steps as text

Find power with Watt's law. P = V × I = 120 V × 10 A = 1,200 W Power is voltage multiplied by current.
Find resistance with Ohm's law. R = V ÷ I = 120 V ÷ 10 A = 12 Ω Resistance is voltage divided by current.

How the four values connect

Voltage, current, power and resistance describe the same electrical flow from four different angles, and just two of them are enough to work out the other two. Ohm's law relates voltage, current and resistance (V = I × R). Watt's law relates voltage, current and power (P = V × I). Combining the two gives four more useful shortcuts: P = I² × R, P = V² ÷ R, R = V² ÷ P and R = P ÷ I², which is how this calculator can solve for any missing pair.

Think of it as a wheel with four spokes: pick any two known values, and the other two follow automatically from these formulas — there's no need to remember which exact rearrangement applies to your two known values.

Everyday uses

This calculator is handy for checking an appliance's rated current from its wattage label (I = P ÷ V), sizing a fuse or breaker for a known load, sanity-checking whether an extension cord or power strip can handle a device, or working out a battery-powered circuit's resistance from its voltage and current draw.

Most household appliance labels state either watts or (volts and amps) but not both — this fills in whichever is missing, and adds resistance too, which is rarely printed but easy to derive.

A note on AC circuits

These formulas are exact for DC circuits and for simple resistive AC loads (heaters, incandescent bulbs, older resistive appliances). For AC loads with a motor, transformer or electronics — many pumps, air conditioners, and anything with a power supply — the relationship between voltage, current and real power also depends on the load's power factor, which this calculator doesn't account for. Treat the result as a close estimate rather than an exact reading for those loads, and check the appliance's nameplate current directly when it matters (like sizing a breaker).

Frequently asked questions

How do I find watts from volts and amps?: Multiply them: watts = volts × amps. A device running at 120 V and drawing 5 A uses 600 W.
How do I find amps from watts?: Divide watts by volts: amps = watts ÷ volts. A 1,500 W heater on a 120 V circuit draws 12.5 A.
What is Ohm's law?: Ohm's law states that voltage equals current multiplied by resistance (V = I × R). It's the basic relationship this calculator, and most circuit analysis, builds on.
Why do I need to know resistance for a household appliance?: You usually don't directly — most people use this calculator to go from watts to amps or back. Resistance is included because it falls out of the same formulas and is occasionally useful, for example when working with a resistive heating element.
Does this work for AC power like household mains?: It's accurate for simple resistive loads (heaters, incandescent lighting) on AC or DC. For loads with a motor or power supply, the real current can differ because of power factor, which isn't accounted for here.