Volts / Amps / Watts / Ohms converter
Pick what you know, drag the sliders, and the other two values update instantly — Ohm's law and the power law, done for you.
What you know
Supply voltage — 230V for a UK domestic single-phase circuit.
Current flowing through the circuit.
The full picture
Single-phase, resistive-load maths only. Doesn't account for power factor, three-phase supplies or voltage drop. Not a substitute for a design calculation to BS 7671 where one's required.
Ohm's law and the power law are the two equations that sit behind almost every quick sum on a job — sizing a cable, checking a nameplate rating, working out what a load will draw, or sanity-checking a reading that looks off. Volts, amps, watts and ohms: know any two and the other two fall straight out. That's what the converter above does. Here's the maths behind it.
The two equations
Ohm's law: volts = amps × ohms (V = IR). The power law: watts = volts × amps (P = VI). Combine them and you can get from any pair of volts/amps/watts/ohms to the other two — swap V = IR into P = VI and you get P = I²R and P = V²/R, which is where the watts-and-volts mode above pulls its current and resistance from.
Where this actually gets used
A nameplate gives you watts and volts — the converter gives you the current draw, which is what you actually need for cable sizing, breaker selection and checking a circuit isn't overloaded. A multimeter gives you volts and resistance — the converter gives you the current that would flow and the power that would be dissipated, useful for spotting a fault before you energise something. Volts and amps off a clamp meter or an appliance rating plate give you watts (handy for load calculations) and the effective resistance of whatever's connected.
A worked example
A 230V immersion heater is rated at 3kW. Switch to "watts & volts", enter 3000W and 230V, and the converter shows roughly 13.04A of current draw and about 17.63Ω of resistance. That current figure is what tells you the circuit needs a minimum 16A (typically 20A in practice) breaker and cable sized to suit — not a number you'd want to be guessing at.
What this isn't
This is single-phase, resistive-load maths — it doesn't account for power factor on inductive or capacitive loads, three-phase calculations, voltage drop over cable runs, or diversity across multiple circuits. Use it for quick sanity checks and back-of-van working, not as a substitute for a proper design calculation to BS 7671 where one's required.
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