Ohm's Law Calculator
Three Quantities, One Relationship
Voltage, current, and resistance are locked together by a fixed relationship in any simple circuit — know two of them and the third follows automatically. Georg Ohm published this relationship in 1827, and it remains the starting point for nearly every electrical calculation that comes after it, from sizing a resistor for an LED to troubleshooting a circuit that won't power on.
The Formula
This calculator solves for whichever quantity you're missing:
V = I × R
I = V / R
R = V / I
I = V / R
R = V / I
where V is voltage in volts, I is current in amps, and R is resistance in ohms.
A Worked Example
| Voltage (V) | Resistance (Ω) | Current (A) |
|---|---|---|
| 120 | 60 | 2.0 |
I = V / R = 120 ÷ 60 = 2.0 A
Where This Calculation Matters
- Component selection — choosing a resistor value to limit current through an LED or sensor to a safe level.
- Circuit troubleshooting — if measured current doesn't match the value predicted from voltage and resistance, that mismatch points to a fault such as a short or a bad connection.
- Load estimation — working out how much current a known resistance will draw from a given supply voltage, before wiring is installed.
- Classroom and lab work — verifying measured values against theoretical predictions in basic circuit experiments.
How to Use This Calculator
- Choose what you want to solve for: Voltage, Current, or Resistance.
- Enter the two known values in the fields that appear — for example, Current and Resistance when solving for Voltage.
- Select Calculate to see the result along with the formula substitution.
Related Calculations
Once you know voltage and current, the Wattage Calculator gives you power, and the Circuit Power Calculator extends the same relationship to AC circuits with a power factor.