About this calculator
Choose the pair you know and enter them with their units.
How to use it
- Choose the two values you know.
- Enter them with units.
- Read the other two.
The formula
V = I × R; P = V × I = I²R = V² ÷ R
- V
- voltage in volts
- I
- current in amperes
- R
- resistance in ohms
- P
- power in watts
Worked example
12 V across 6 Ω
- I = 12 ÷ 6 = 2 A.
- P = 12 × 2 = 24 W.
What the result means
Higher resistance means less current for the same voltage.
Assumptions
- Resistance does not change with current (ohmic conductors).
Limitations
- Bulb filaments, diodes and motors are not ohmic.
Frequently asked questions
What is Ohm’s law?
The current through a conductor is proportional to the voltage across it: V = IR.
How much current does a 1,000 W heater draw at 230 V?
About 4.35 A.
What is the resistance of a 60 W bulb at 230 V?
About 882 Ω when hot.
Last reviewed on 6 October 2026. Found a mistake? Tell us.