What is Ohm’s law?
Ohm’s law relates voltage, current and resistance: V = I × R. Voltage in volts equals current in amps times resistance in ohms. Together with the power formula P = V × I (in watts), knowing any two of the four values gives you the other two.
All the formulas
| To find | Formulas |
|---|---|
| Voltage V | V = I × R, V = P / I, V = √(P × R) |
| Current I | I = V / R, I = P / V, I = √(P / R) |
| Resistance R | R = V / I, R = V² / P, R = P / I² |
| Power P | P = V × I, P = V² / R, P = I² × R |
Examples
- A 9 V battery across 470 Ω: I = 9 ÷ 470 ≈ 19.15 mA.
- 20 mA through 150 Ω: V = 0.02 × 150 = 3 V.
- 2 A at 12 V: R = 12 ÷ 2 = 6 Ω.
- A USB device drawing 0.5 A at 5 V: P = 5 × 0.5 = 2.5 W.
- 10 W into an 8 Ω speaker: I = √(10 ÷ 8) ≈ 1.118 A.
Frequently asked questions
How do I enter units?
Voltage can be 5, 3.3, 3V3 or 500mV. Current can be 0.02, 20mA or 500uA. Resistance can be 220, 4.7k, 4k7 or 1M. Power can be 0.25, 1/4W or 250mW. A plain number means amps for current and watts for power.
Why does power matter?
A resistor turns electrical power into heat. Choose a resistor rated for at least twice the power it will dissipate: if the result is 0.1 W, use a 1/4 W part.
Does Ohm’s law work for every component?
No. It applies to components like resistors, where current is proportional to voltage. LEDs, diodes and motors don’t have a fixed resistance, so you can’t use a single R for them. For LEDs, use the LED resistor calculator.