Ohm's Law Calculator
Find voltage, current or resistance from the other two using Ohm's law, plus the power in watts.
New: please check your results.
For guidance only. This is not a substitute for a qualified engineer, and important designs should be checked by one.
- Current
- 2 A
- Power
- 24 W
What it does
Ohm's law links voltage, current and resistance. Choose which one to find, enter the other two, and the calculator works out the missing value and the power used.
How to use it
- Find
The value you want to work out. Enter the other two.
- Voltage
The voltage across the component, in volts. For UK mains use 230 V.
- Current
The current flowing through it, in amps.
- Resistance
The resistance, in ohms.
Common uses
- Choosing a resistor for an LED or circuit.
- Checking what current a heater or element draws.
- Checking voltage drop on a cable run.
- Converting between watts, amps and volts.
The formula
V = I × RI = V ÷ RR = V ÷ IP = V × IWhere:
- V
- voltage in volts
- I
- current in amps
- R
- resistance in ohms (Ω)
- P
- power in watts
Results are shown to 4 significant figures.
Worked example
To find the current, divide voltage by resistance:
- Current: 12 V ÷ 6 Ω = 2 A
- Power: 12 V × 2 A = 24 W
Questions people ask
How do I use the Ohm's law triangle?
Cover the letter you want to find. If the other two sit side by side, multiply them (V = I × R). If one is above the other, divide the top by the bottom (I = V ÷ R, R = V ÷ I).
What is Ohm's law?
Voltage equals current multiplied by resistance (V = I × R). Rearranged, it gives current (I = V ÷ R) or resistance (R = V ÷ I) from the other two.
Does it work for mains electricity?
For simple resistive loads such as heaters and kettles, yes, using the normal mains voltage. Motors, transformers and electronics behave differently, so treat results for those as a rough guide only.
Last checked 24 October 2026