Ohm's Law Calculator

Enter any two values and get the other two instantly. Type values like 4.7k, 20m or 3.3.

This free Ohm's law calculator finds voltage, current, resistance and power from any two known values. It doubles as a watts to amps calculator, an amps to watts calculator and a resistor power calculator — enter volts and watts to get amps, or volts and amps to get watts. Values like 4.7k, 20mA or 1M are understood directly.

Ohm's law formulas

You know	Voltage	Current	Resistance	Power
V and I	—	—	V ÷ I	V × I
V and R	—	V ÷ R	—	V² ÷ R
V and P	—	P ÷ V	V² ÷ P	—
I and R	I × R	—	—	I² × R
I and P	P ÷ I	—	P ÷ I²	—
R and P	√(P × R)	√(P ÷ R)	—	—

Worked example

A 12 V supply across a 240 Ω resistor: I = 12 ÷ 240 = 50 mA, and P = 12² ÷ 240 = 0.6 W. A 1/4 W resistor would overheat here; use a 1 W part.

Watts to amps conversion chart

Current drawn by a load at common supply voltages (amps = watts ÷ volts, resistive load).

Power	12 V	120 V	230 V	240 V
100 W	8.33 A	0.83 A	0.43 A	0.42 A
500 W	41.67 A	4.17 A	2.17 A	2.08 A
1,000 W	83.33 A	8.33 A	4.35 A	4.17 A
1,500 W	125 A	12.5 A	6.52 A	6.25 A
2,000 W	166.67 A	16.67 A	8.7 A	8.33 A
3,000 W	250 A	25 A	13.04 A	12.5 A

For motors and other AC loads with a power factor below 1, the real current is higher: amps = watts ÷ (volts × power factor). To choose the cable for that current, use the wire size calculator; to see what the load costs to run, use the electricity cost calculator.

Choosing a resistor power rating

As a rule of thumb, pick a resistor rated for at least twice the power it will dissipate. Running a resistor near its rating makes it hot, shifts its value and shortens its life. Common through-hole ratings are 1/8 W, 1/4 W, 1/2 W, 1 W, 2 W and 5 W; common SMD sizes are 0402 (1/16 W), 0603 (1/10 W), 0805 (1/8 W) and 1206 (1/4 W).

Frequently asked questions

How do I convert watts to amps?

Divide watts by volts: amps = watts ÷ volts. Enter the voltage and the power in the calculator above and it shows the current. For example, a 60 W device on 12 V draws 60 ÷ 12 = 5 A.

How many amps is 1500 watts?

At 120 V, 1500 W draws 12.5 A — which is why a 1500 W space heater is close to the limit of a 15 A circuit. At 230 V it draws 6.52 A and at 12 V it draws 125 A.

How many watts can a 15 amp circuit handle?

Multiply amps by volts: 15 A × 120 V = 1,800 W. For continuous loads (on for 3 hours or more), the US electrical code limits a circuit to 80% of its rating, so plan for about 1,440 W on a 15 A circuit and 1,920 W on a 20 A circuit.

What is Ohm's law?

Ohm's law states that the current through a conductor is proportional to the voltage across it: V = I × R. Combined with the power equation P = V × I, any two of voltage, current, resistance and power determine the other two.

How do I calculate current from voltage and resistance?

Divide voltage by resistance: I = V ÷ R. For example, 12 V across a 240 Ω resistor gives 12 ÷ 240 = 0.05 A, or 50 mA.

How much power does a resistor dissipate?

Use P = I² × R or P = V² ÷ R. A 100 Ω resistor with 5 V across it dissipates 25 ÷ 100 = 0.25 W, so a 1/4 W resistor would be running at its limit — use a 1/2 W part for margin.

Can I type values like 4.7k or 20mA?

Yes. The calculator understands engineering prefixes: p, n, u (or µ), m, k, M and G. "4.7k" is 4,700, "20m" is 0.02, and "4k7" is also read as 4.7k.

Does Ohm's law work for AC circuits?

For purely resistive loads like heaters and incandescent lamps, yes, using RMS values. For circuits with capacitors or inductors you need impedance (Z) instead of resistance, and real power also depends on the power factor.

Ohm's law formulas

Worked example

Watts to amps conversion chart

Choosing a resistor power rating

Frequently asked questions

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