Guidance only — always verify against current standards and manufacturer instructions.
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This calculator solves Ohm's law and the power equation together: enter any two of voltage, current, resistance and power, and it works out the remaining values. It is the quick mental-arithmetic checker every electrician reaches for — what current does a 3kW load draw at 230V, what resistance should that element read, what power is dissipated across this resistance?
The relationships are the foundations of every other electrical calculation: V = I × R links voltage, current and resistance, and P = V × I links power to both. The tool picks whichever pair you have entered and derives the rest, so you never need to rearrange the formula in your head on site.
V = I × R, P = V × I
V is voltage in volts, I is current in amps, R is resistance in ohms and P is power in watts. From any two known values the others follow by rearrangement: current is voltage divided by resistance, power is voltage times current, resistance is voltage divided by current, and combinations such as I = √(P / R) cover the remaining pairs. The calculator applies the appropriate rearrangement automatically based on which two fields you fill in.
Fill in exactly two of the four fields — voltage, current, resistance or power. Common site pairs are voltage and power (appliance ratings) or voltage and resistance (element checks).
The tool calculates the remaining two quantities from the pair you entered and shows all four together.
Use the output to confirm expectations: a 3kW element at 230V should draw about 13A and read about 17.6Ω cold — big departures from calculated values point to a faulty component.
These are DC/resistive relationships. For resistive loads — heating elements, immersion heaters, incandescent lamps — they hold directly on AC too. For motors, drivers and anything with a power factor below 1, real current draw is higher than P / V suggests; use the kW to amps calculator with a power factor for those.
Measured resistance of heating elements is taken cold and rises when hot, so a cold reading slightly below the calculated operating resistance is normal. Extreme readings — open circuit or near zero — are the fault indicators.
Keep units consistent: volts, amps, ohms and watts. Enter kilowatt ratings as watts (3kW = 3000W) or the derived values will be out by a factor of a thousand.
Ohm's law states that the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance: I = V / R. Combined with the power equation P = V × I, any two of the four quantities determine the other two.
Current is power divided by voltage: 3000W / 230V ≈ 13A. That is why a 3kW heater sits at the top end of what a 13A plug and socket can supply, and why larger fixed loads are wired on dedicated circuits.
From its rating: R = V² / P. A 3kW immersion element at 230V should measure around 230² / 3000 ≈ 17.6Ω. A cold reading somewhat lower is normal since resistance rises with temperature; open circuit or a few ohms indicates a failed element.
For purely resistive loads, yes — heaters and elements behave the same on AC as DC. For inductive or electronic loads, impedance and power factor come into play and the simple V = I × R and P = V × I relationships understate the current. Use a power-factor-aware calculation for motors and similar loads.
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