About the Ohm's Law Calculator
This Ohm's law calculator solves a simple resistive circuit from any two known quantities. Pick which pair you know — voltage and current, voltage and resistance, current and power, and so on — and the headline shows the two values you did not enter (voltage in volts, current in amps, resistance in ohms or power in watts), with each solved quantity listed separately below.
It is useful for electronics hobbyists sizing a resistor, students checking homework, technicians confirming a measured reading, and anyone who wants to know how much power a heating element or load dissipates. The power result tells you what wattage rating a resistor needs so it does not overheat.
The math assumes a purely resistive DC circuit (or AC with a resistive load, using RMS values). Reactive loads such as motors and capacitors need impedance and power factor, which the watts/amps/volts calculator handles.
How to use the ohm's law calculator
- 1Choose which two values you already know.
- 2Enter them in volts, amps, ohms or watts (use decimals for milliamps, e.g. 0.02 A = 20 mA).
- 3Read the two solved values in the headline and the breakdown below it.
- 4Use the power figure to pick a resistor wattage rating with a safety margin.
Formula and method
Ohm's law states that the voltage across a resistor equals the current through it multiplied by its resistance. Combined with the power law P = V × I, any two of the four quantities determine the other two: for example, knowing V and R gives I = V ÷ R and P = V² ÷ R, while knowing P and R gives V = √(P × R) and I = √(P ÷ R).
These relations hold exactly for resistive loads with DC or RMS AC values. For inductive or capacitive loads, resistance is replaced by impedance and real power also depends on the power factor.
- V
- Voltage in volts
- I
- Current in amperes
- R
- Resistance in ohms (Ω)
- P
- Power in watts
Worked examples
12 V across a 6 Ω resistor
I = V ÷ R = 12 ÷ 6 = 2 A, and P = V² ÷ R = 144 ÷ 6 = 24 W. A resistor in this circuit would need a rating well above 24 W.
20 mA through a 220 Ω resistor
V = I × R = 0.02 × 220 = 4.4 V across the resistor, and P = I² × R = 0.0004 × 220 = 0.088 W, so a 1/4 W resistor is ample.
1500 W heater on 120 V
I = P ÷ V = 1500 ÷ 120 = 12.5 A, and the element’s hot resistance is R = V² ÷ P = 14,400 ÷ 1500 = 9.6 Ω.
100 W into an 8 Ω speaker
V = √(P × R) = √800 ≈ 28.28 V RMS and I = √(P ÷ R) = √12.5 ≈ 3.54 A RMS, treating the speaker as a simple 8 Ω resistive load.
Frequently asked questions
What is Ohm's law?+
Ohm's law says voltage equals current times resistance (V = I × R). It means that for a fixed resistance, doubling the voltage doubles the current flowing through the circuit.
How do I calculate current from voltage and resistance?+
Divide voltage by resistance: I = V ÷ R. For example 9 V across a 1,000 Ω resistor gives 0.009 A, or 9 mA.
How do I work out what wattage resistor I need?+
Calculate power with P = I² × R or P = V² ÷ R, then choose a resistor rated for at least about twice that figure so it stays well within its temperature limit.
Does Ohm's law work for AC circuits?+
Yes for purely resistive loads using RMS voltage and current. For motors, transformers or capacitors you must use impedance (Z) instead of resistance and account for power factor.
What is the Ohm’s law triangle?+
It is a memory aid with V on top and I and R on the bottom. Cover the value you want: V = I × R, I = V ÷ R and R = V ÷ I.