Ohm's law calculator
Work out voltage, current, resistance or power from any two known values. Electronics hobbyists, students and technicians use it to size resistors and check circuit loads.
- Runs in your browser
- Free, no account needed
- Copy or download the result
Result
Example
Voltage (V): 12 Voltage unit: V Current (I): Current unit: A Resistance (R): 470 Resistance unit: Ω Power (P): Power unit: W
Voltage (V): 12 V Current (I): 25.5319 mA (0.0255319148936 A) Resistance (R): 470 Ω Power (P): 306.383 mW (0.306382978723 W) Formulas used: I = V / R, P = V² / R Prefixed values are rounded to 6 significant digits and base-unit values in parentheses to 12. This assumes a DC circuit or a purely resistive load.
How to use the ohm's law calculator
- Fill in exactly two of Voltage (V), Current (I), Resistance (R) and Power (P), and leave the other two blank.
- Set Voltage unit, Current unit, Resistance unit and Power unit, such as mA or kΩ.
- Press Run to see all four values and the formulas used.
- Press Copy result or Download to keep the results.
How it works
Ohm's law is V = I x R, and power is P = V x I. Combining them gives the other forms, such as P = V² / R and I = √(P / R).
Every entered value must be greater than zero. Results are shown with a suitable prefix, such as 25.5319 mA, and in base units in parentheses.
Prefixed results are rounded to 6 significant digits. The math assumes a DC circuit or a purely resistive load, not AC circuits with reactance.
Everything runs in your browser. Your input is not uploaded or stored.
Questions and answers
What is Ohm's law?
It says voltage equals current times resistance, V = I x R. If you know any two, you can find the third.
How do I pick a resistor for an LED?
Subtract the LED's forward voltage from the supply voltage, then divide by the LED current. Enter that voltage and current here to get the resistance and the power the resistor must handle.
How do I calculate power from voltage and resistance?
Use P = V² / R. A 12 V supply across 470 Ω uses about 306 mW.