Fill in exactly two of the four fields below, then press Calculate to work out the other two with Ohm's law and the power law.
These results assume a pure resistive load in direct current. For any real electrical installation, consult a qualified electrician.
Ohm's law for UK mains and appliances
UK sockets deliver 230V at 50Hz through a ring main protected by a consumer unit. Every plug also carries its own fuse, typically 3A for lamps and small electronics, 5A for some kitchen appliances, and 13A for kettles, toasters, or heaters under BS 1363. This calculator lets you check the current an appliance draws once you know its power rating, or work out the resistance behind a simple DC load, filling in any two of voltage, current, resistance, and power.
The twelve derived formulas
| Known values | Voltage (U) | Current (I) | Resistance (R) | Power (P) |
|---|---|---|---|---|
| U and I | - | - | U / I | U x I |
| U and R | - | U / R | - | U^2 / R |
| U and P | - | P / U | U^2 / P | - |
| I and R | I x R | - | - | I^2 x R |
| I and P | P / I | - | P / I^2 | - |
| R and P | sqrt(P x R) | sqrt(P / R) | - | - |
What this calculator cannot do
These formulas only hold for direct current running through a pure resistor. Alternating current circuits with a phase shift between voltage and current need impedance and reactive power, which are not covered here. Components such as diodes, LEDs, or transistors do not have a fixed resistance either, so their real-world behaviour will differ from what this tool predicts.
A worked example
A UK kettle rated at 2.3kW plugged into a 230V socket draws I = P / U = 2300 / 230 = 10A, comfortably inside its 13A fuse. Enter voltage and power above to check any appliance the same way, or use the Electrical Load Calculator to see how several appliances add up on one circuit breaker. To convert the result into other physics units, the Physics Unit Converter covers power, energy, and more.