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Ohm's Law Calculator

Solve for volts, amps, ohms or watts from any two known values.

Circuit type
V
A
Ω
W

Enter any two values. The rest are calculated.

Waiting for inputs

Enter any two values above and the remaining two are solved instantly.

What this assumes

  • Direct current, or alternating current at unity power factor.
  • Resistance is treated as constant and purely resistive.
  • For AC circuits with motors or electronics, apply the power factor separately.

Important

Circuit calculations do not replace a design review. Installed work must comply with the National Electrical Code as adopted and amended by your jurisdiction.

Understanding the result

What Ohm's law tells you

Ohm's law fixes the relationship between voltage, current and resistance: V = I × R. Combined with the power formula P = V × I, knowing any two of the four values determines the other two. It is the foundation of every load, conductor and protection calculation that follows.

Where it stops being exact

The law describes purely resistive DC behavior. On AC circuits with motors, transformers or switching power supplies, current and voltage fall out of phase and real power is less than volts times amps. In those cases you need power factor, and apparent power in VA rather than watts.

Power is the fourth variable

Combining P = V × I with V = I × R gives P = I²R and P = V²/R. These are how you calculate heat generated in a conductor, fuse sizing, and load wattage. A 1,500-watt space heater on 120V draws 12.5 amps (1,500 ÷ 120), which needs a 15-amp circuit minimum.

Series and parallel change the math

Resistors in series add: R_total = R1 + R2. In parallel, reciprocals add: 1/R_total = 1/R1 + 1/R2. Two 100-ohm resistors in parallel equal 50 ohms. This matters for understanding circuit loads, LED arrays, and troubleshooting voltage dividers.

Common questions

Can I use Ohm's law on a 240V AC circuit?
Yes, for resistive loads such as water heater elements and baseboard heat. For motors, compressors and electronics, the reactive component means you also need the power factor to get real power.
Why does my measured current differ from the calculation?
Real conductors have resistance, connections add resistance, and supply voltage sags under load. A measurement within a few percent of the calculation is normal; a large gap usually indicates a loose connection or an undersized conductor.
How do you calculate amps from watts and volts?
Amps = watts ÷ volts. A 1,500-watt heater on 120V draws 12.5 amps. The same heater on 240V draws 6.25 amps. This is the most common Ohm's law calculation in residential electrical work.
What is the formula for Ohm's law?
V = I × R, where V is voltage in volts, I is current in amps, and R is resistance in ohms. Rearranged: I = V ÷ R and R = V ÷ I. Combined with P = V × I, knowing any two of the four values (V, I, R, P) determines the other two.
How many watts is 15 amps at 120 volts?
15 amps × 120 volts = 1,800 watts. A 15-amp circuit at 80% continuous load capacity supports 1,440 watts (1,800 × 0.80). That is why a 1,500-watt space heater needs a dedicated 20-amp circuit.
How do you calculate resistance from voltage and current?
R = V ÷ I. A 120V circuit drawing 10 amps has a load resistance of 12 ohms. A 240V circuit drawing 20 amps has 12 ohms. Same resistance, but double the voltage quadruples the power (P = V²/R).
What is watts law?
Watts law states P = V × I — power in watts equals voltage times current. Combined with Ohm's law, you get P = I²R and P = V²/R. A 10-amp load at 120V dissipates 1,200 watts. At 240V, the same 10 amps dissipates 2,400 watts.
How many amps does a 60 watt light bulb draw?
A 60-watt bulb on 120V draws 0.5 amps (60 ÷ 120). Ten bulbs on one circuit draw 5 amps — well within a 15-amp breaker. LED equivalents draw 0.1 to 0.15 amps each, which is why you can run many more on the same circuit.
Does Ohm's law apply to batteries?
Yes. A 12V car battery delivering 10 amps to a starter sees a load of 1.2 ohms (12 ÷ 10). Battery internal resistance (typically 0.01 to 0.05 ohms) causes voltage sag under heavy load — which is why headlights dim during engine cranking.
How do you calculate power consumption?
Power (watts) = voltage × current. Energy (watt-hours) = power × time in hours. A 100-watt device running 10 hours consumes 1,000 watt-hours (1 kWh). At $0.15/kWh, that costs $0.15. Multiply by 730 hours/month for continuous loads.

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