Ohm's Law Calculator
Calculate voltage, current, and resistance using Ohm's Law (V=IR). Free online physics calculator.
About this calculator
Enter any two of voltage, current, resistance and power, and the calculator finds the other values using Ohm's Law and the power formulas.
Useful for electronics projects, circuit homework and wiring checks.
How to use it
- Choose what to calculate.
- Enter the two known values.
- Press Calculate.
The formulas
V = I × R, so I = V ÷ R and R = V ÷ I. Power: P = V × I = I² × R = V² ÷ R. Units: volts (V), amps (A), ohms (Ω), watts (W). In series circuits resistances add (R = R₁ + R₂); in parallel, 1/R = 1/R₁ + 1/R₂.
Worked example
2 A through 5 Ω
| Voltage | 2 × 5 = 10 V |
| Power | 10 × 2 = 20 W |
| LED resistor for 9 V supply, 2 V LED, 20 mA | (9 − 2) ÷ 0.02 = 350 Ω |
Tips and common mistakes
- Use a resistor with a power rating about twice the calculated watts.
- Convert mA to A by dividing by 1000.
- Check wire gauge for the current.
- Parallel resistance is always less than the smallest branch.
- Never work on mains wiring unless you are qualified.
Frequently asked questions
What is Ohm's Law?
Voltage equals current times resistance (V = IR).
How do I find power?
P = V × I.
What is a watt?
One joule per second.
How do resistors add in series?
They sum.
How do I pick an LED resistor?
(Supply − LED voltage) ÷ LED current.
Is it free?
Yes.