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Free calculator

Use this calculator to

  • Solve any two of voltage, current, resistance and power for the other two
  • Check the current a resistive load draws from a 230 V or 110 V supply
  • Confirm a load stays within the 13 A limit of a standard UK fused plug
  • Sanity-check meter readings against nameplate ratings during fault-finding

Ohm's Law Calculator

Enter any 2 of 4 values (V, I, R, P) to calculate the remaining two.

V
A
Ω
W
Results

Formulas

  • V = I × R
  • I = V / R
  • R = V / I
  • P = V × I = I² × R = V² / R

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Common scenarios

Select one to run it in the calculator above.

For business

Why this matters for businesses

Ohm's Law is the keystone of every low-voltage diagnosis. Voltage, current and resistance are locked into a single equation, and power follows from any two of them. For a maintenance team chasing an intermittent fault on a control circuit, a heating element, or a sensor loop, working the equation in both directions is what separates a clean diagnosis from an afternoon of swapping parts at random. The measurement at the terminals tells you which leg of the triangle has moved, and Ohm's Law tells you what that means for the rest of the circuit.

For a building services team running a BMS, the same calculation underpins every loop. A 4 to 20 mA sensor loop assumes a defined supply voltage and a defined loop resistance, and a drift in either tells you something specific about the wiring or the field device. Knowing the expected current at a given resistance is the difference between a quick fix at the terminal block and a rip-and-replace of the wrong instrument, which on a multi-storey commercial site is the difference between a one-hour callout and a half-day disruption to tenants.

For an in-house engineering function supporting operations, Ohm's Law is also the documentation that survives a change of contractor. Annotating a wiring diagram with the expected voltage, current and resistance at each node turns a generic schematic into a fault-finding aid, and it gives a new engineer joining the team a fighting chance of diagnosing a problem on the first visit. That kind of institutional knowledge is what keeps operating costs flat as headcount turns over, and the maths to underwrite it has not changed since 1827.

Common questions

What is Ohm's law and how do I use this calculator?

Ohm's law links voltage, current and resistance: V = I × R. Combined with the power relationships P = V × I = I² × R = V² / R, knowing any two of the four quantities fixes the other two. Enter the two values you know, leave the others blank, and the calculator solves the remaining pair. It works for DC circuits and for AC circuits feeding purely resistive loads.

How much current does a 3 kW appliance draw at 230 V?

Current equals power divided by voltage: 3000 W / 230 V is just over 13 A. That is why roughly 3 kW is the practical ceiling for anything running from a standard UK 13 A fused plug, and why larger loads such as commercial kitchen equipment or EV chargers need their own dedicated circuit sized by an electrician rather than a plug top.

Does Ohm's law work for AC circuits?

For purely resistive loads such as heaters and incandescent lamps, yes: use the rms voltage and current and the relationships hold. Motors, drives, fluorescent ballasts and most electronic equipment also draw reactive current, so their behaviour is governed by impedance and power factor rather than simple resistance. For those loads, use a power factor or three-phase calculator to get from amps to real kilowatts.

What is the difference between watts and volt-amperes?

Watts measure real power, the rate at which energy is actually converted to heat, light or motion. Volt-amperes measure apparent power, the simple product of rms voltage and current. For the resistive loads this calculator covers they are equal. For reactive loads, real power is the apparent power multiplied by the power factor, which is why supplies and generators are rated in kVA rather than kW.

Why do my measured values not match the calculation exactly?

Real components are not ideal. Resistance changes with temperature: a heating element's cold resistance can be noticeably lower than its operating value, so the switch-on current is higher than the steady-state figure. Supply voltage also varies within the permitted UK tolerance of 230 V plus 10% minus 6%, so the same load draws slightly different current through the day. Treat calculated figures as nominal values for sizing and sanity checks.

Related calculators

The conversions that procurement and engineering use alongside this one.

Ohm's Law Calculator | Purely Energy