Workshop calculator
Ohm's law calculator: volts, amps, ohms and watts
Ohm's law (V = I × R) and the power law (P = V × I) connect the four basic electrical values. Knowing a component's resistance and the supply voltage tells you the current it should draw, and the other way round.
Fill in exactly two boxes.
V
A
Ω
W
How to use it in a diagnosis
- Enter exactly two values you know (for example 12 V and 0.8 Ω for a glow plug).
- The other two values are calculated.
- Compare the expected current with what you measure with a clamp meter.
Questions mechanics ask
What is Ohm's law?
Voltage equals current times resistance (V = I × R). From it: I = V ÷ R and R = V ÷ I. Power is P = V × I = I² × R = V² ÷ R.
How much current should a glow plug draw?
A plug measuring 0.8 Ω cold draws about 14 A at 11 V when it is first switched on; the current then falls as the plug heats up and its resistance rises. Compare the plugs with each other using a clamp meter.