Ohm’s Law Calculator

An Ohm’s law calculator uses V = I × R and P = V × I to find the two unknowns from any two known values. With 12 V across 220 Ω, I = 12 ÷ 220 = 54.55 mA and P = 12² ÷ 220 = 0.6545 W. For watts to amps, divide watts by volts: 1,500 W at 120 V draws 12.5 A.

How to use the Ohm’s law calculator

1. V, I, R, P tab: type any two of voltage, current, resistance and power, and leave the other two blank. Pick a unit for each field (mV, V, kV; µA, mA, A; mΩ, Ω, kΩ, MΩ; mW, W, kW).
2. Resistors tab: list resistor values separated by commas, choose series or parallel, and add a supply voltage to see the current, voltage drop and power for each resistor.
3. Watts ↔ amps tab: choose DC or single-phase AC, then convert watts to amps or amps to watts. For AC, enter the power factor.

The result shows all four quantities with sensible prefixes, the power wheel with the formulas used highlighted, and every step of the working.

Ohm’s law formula

Ohm’s law says that the current through a resistor is proportional to the voltage across it:

• V = I × R (voltage in volts, V)
• I = V ÷ R (current in amperes, A)
• R = V ÷ I (resistance in ohms, Ω)

Electrical power is P = V × I, measured in watts (W). These units are tied together by definition. In the SI tables published by NIST, a watt is a joule per second, a volt is a watt per ampere, and an ohm is a volt per ampere. So 1 V × 1 A = 1 W, and 1 V ÷ 1 A = 1 Ω.

The power wheel: all 12 formulas

Combine V = I × R with P = V × I and you can write each quantity three ways. The wheel in the calculator shows all of them; here they are as a table.

Choose the formula that uses the two values you know. If you know power and resistance, for example, use V = √(P × R) and I = √(P ÷ R).

SI prefixes for volts, amps, ohms and watts

Electronics uses small and large values all the time, so prefixes matter. Convert to base units before you multiply or divide.

Capital M means mega and small m means milli, a factor of a billion apart. In the resistor list you can also write 4k7 for 4.7 kΩ and 1R5 for 1.5 Ω, the way values are printed on schematics.

Watts to amps

For direct current (DC), and for AC loads with a power factor of 1 such as heaters, amps = watts ÷ volts. For single-phase AC with other loads, amps = watts ÷ (volts × power factor). Power factor (PF) is the ratio of real power, in watts, to apparent power, in volt-amperes. The U.S. Department of Energy uses the example of a load drawing 100 kW of real power and 142 kVA of apparent power, which has a power factor of 0.70. To go the other way, watts = volts × amps × PF.

A power factor below 1 raises the current for the same watts. Three-phase circuits use a different formula and are not covered here.

Series and parallel resistors

Resistors in series share one path, so the same current flows through each and the resistances add: R = R₁ + R₂ + …. Resistors in parallel share the same voltage, and their reciprocals add: 1/R = 1/R₁ + 1/R₂ + …. For two resistors in parallel, R = (R₁ × R₂) ÷ (R₁ + R₂). A series total is always larger than the largest resistor; a parallel total is always smaller than the smallest one.

Worked examples

Current and power in a resistor

A 12 V supply is connected across a 220 Ω resistor. I = V ÷ R = 12 ÷ 220 = 0.05455 A = 54.55 mA. P = V² ÷ R = 144 ÷ 220 = 0.6545 W, so a 1/4 W resistor would be too small and a 1 W part would be needed.

Resistor for an LED

An LED circuit runs from 5 V. Suppose the LED’s datasheet gives a forward voltage of 2 V and you want 20 mA. The resistor must drop 5 − 2 = 3 V, so R = 3 ÷ 0.02 = 150 Ω, and it dissipates P = 3 × 0.02 = 0.06 W.

Watts to amps for a heater and a motor

A 1,500 W space heater on 120 V: I = 1,500 ÷ 120 = 12.5 A. A 1 kW motor on 230 V single-phase with a power factor of 0.8: I = 1,000 ÷ (230 × 0.8) = 5.435 A, and its apparent power is 1,000 ÷ 0.8 = 1,250 VA.

Two resistors in parallel

100 Ω and 220 Ω in parallel give R = (100 × 220) ÷ (100 + 220) = 22,000 ÷ 320 = 68.75 Ω. On 12 V the total current is 12 ÷ 68.75 = 174.5 mA: 120 mA through the 100 Ω resistor and 54.55 mA through the 220 Ω one.

Common mistakes

• Mixing prefixes: 20 mA is 0.02 A, not 20 A. Convert first, or let the calculator do it.
• Using P = V × I with a voltage that is not across the part you are studying. In series circuits each resistor gets only part of the supply voltage.
• Adding parallel resistances directly instead of adding their reciprocals.
• Ignoring power factor for motors and electronic loads, which understates the current.
• Treating Ohm’s law as universal. It holds for resistors at a steady temperature; diodes, LEDs and lamps change their resistance with current.

A note on safety

This calculator is for learning, homework and low-voltage hobby projects. It is not a guide for household wiring, choosing breakers or repairing appliances. Mains voltage can cause serious injury or death, so follow your local electrical code and use a licensed electrician for that work.

Sources

• NIST SP 330, The International System of Units (SI), 2019 edition: tables of the watt (J/s), volt (W/A) and ohm (V/A), and the SI prefixes. Checked November 2026.
• NIST: Federal Register notice on the SI units used in the United States. Checked November 2026.
• U.S. Department of Energy fact sheet on power factor: power factor as real power divided by apparent power. Checked November 2026.

Ohm’s Law practice quiz: 10 questions

Test yourself with 10 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.

Frequently asked questions

What is Ohm’s law in simple terms?

The voltage across a resistor equals the current through it times its resistance, V = I × R. Double the voltage across a fixed resistor and the current doubles.

How many amps is 1,500 watts?

At 120 V it is 1,500 ÷ 120 = 12.5 A. At 230 V it is about 6.52 A. For a motor or other load with a power factor below 1, divide by the power factor as well.

How do I convert watts to amps?

Divide the watts by the volts for DC. For single-phase AC, divide the watts by volts times power factor. You need the voltage: watts alone cannot be turned into amps.

Does Ohm’s law work for AC?

Yes for purely resistive loads such as heaters, using RMS voltage and current. Circuits with motors, capacitors or coils need impedance and power factor, which is why the watts-to-amps tab asks for the power factor in AC mode.

Why is the parallel resistance smaller than each resistor?

Each resistor adds another path for current, so the circuit carries more current at the same voltage. More current for the same voltage means less total resistance.

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