Ohm's Law Calculator

An Ohm’s law calculator takes two numbers, voltage and current, and gives you resistance and power. Resistance is voltage divided by current, and power is voltage times current. For 12 V at 3 A, the resistance is 12 ÷ 3 = 4 ohms and the power is 12 × 3 = 36 W.

The formulas in plain words

Think of water in a hose. Voltage is the pressure pushing the water. Current is how much water flows past a point each second. Resistance is how narrow or kinked the hose is.

With the same pressure, a narrower hose lets less water through. In electrical terms, higher resistance gives lower current. So resistance is pressure divided by flow: R = V ÷ I.

Power is the work being done each second. It is the pressure times the flow: P = V × I. A circuit at 12 V carrying 3 A is using 36 watts.

Worked examples

Example 1, the default: 12 V and 3 A.

• Resistance: 12 ÷ 3 = 4 ohms
• Power: 12 × 3 = 36 W

Example 2, a household-style load: 120 V and 10 A.

• Resistance: 120 ÷ 10 = 12 ohms
• Power: 120 × 10 = 1,200 W

Example 3, a small electronics circuit: 9 V and 0.25 A (250 mA).

• Resistance: 9 ÷ 0.25 = 36 ohms
• Power: 9 × 0.25 = 2.25 W

Reference at 120 volts

Current	Resistance	Power
5 A	24 ohms	600 W
10 A	12 ohms	1,200 W
15 A	8 ohms	1,800 W
20 A	6 ohms	2,400 W

Finding the other combinations

The calculator starts from voltage and current. When you know a different pair, the same law works in other forms.

• Current from voltage and resistance: I = V ÷ R. At 12 V across 4 ohms, I is 3 A.
• Voltage from current and resistance: V = I × R. At 3 A through 4 ohms, V is 12 V.
• Power from current and resistance: P = I² × R. At 3 A and 4 ohms, P is 9 × 4 = 36 W.
• Power from voltage and resistance: P = V² ÷ R. At 12 V and 4 ohms, P is 144 ÷ 4 = 36 W.

All four of these land on the same 36 W as the worked example, which is a useful check on your arithmetic.

Common mistakes

• Mixing units. Milliamps and kilovolts have to be converted to amps and volts before you enter them.
• Applying the law to AC loads with motors or transformers without thinking about power factor. Ohm’s law as written fits resistive loads like heaters and incandescent bulbs best.
• Forgetting that a hot wire or element changes its resistance. A cold bulb filament measures lower than a lit one.
• Treating the power result as a safe load. A conductor and a breaker each have their own current limits, and a licensed electrician sizes them under your local code.
• Entering zero current and reading the 0 ohm result as real.

Working backward from a nameplate

Appliance labels often list watts and volts, and you can find the rest. A heater rated 1,500 W at 120 V draws:

• Current: 1,500 ÷ 120 = 12.5 A
• Resistance: 120 ÷ 12.5 = 9.6 ohms

Enter 120 V and 12.5 A in the calculator and you get the same 9.6 ohms and 1,500 W back. That round trip is a good check on a nameplate. A circuit has to carry that current with margin, and the breaker and wire sizes belong to a licensed electrician under your local code.

Series and parallel

Resistors in series add. Two 4 ohm resistors in series make 8 ohms, and 12 V across them draws 12 ÷ 8 = 1.5 A. In parallel the current has two paths, so two 4 ohm resistors make 2 ohms, and 12 V draws 6 A. Work out the equivalent resistance first, then use the calculator on the whole circuit.

Related calculators

To go from watts to current, use the watts to amps calculator, and the amps to watts calculator goes the other way. Long runs lose voltage, which the voltage drop calculator estimates. For choosing a conductor, see the wire size calculator. Electrical work is code-governed, so have a licensed electrician review the final design.