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Free calculator

Use this calculator to

  • Solve any two-known-values combination of power, voltage, current and resistance in one pass
  • Check the current a 230 V appliance draws before picking a fuse or MCB rating
  • Work out the resistance of a heating element from its nameplate wattage
  • Sanity-check nameplate data on plant and portable appliances during an energy audit

Watts / Volts / Amps / Ohms Calculator

Enter any 2 of 4 electrical values to calculate the other two.

W
V
A
Ω
Results

Formulas

  • R = V / I = V² / P = P / I²
  • I = V / R = P / V = √(P / R)
  • V = I × R = P / I = √(P × R)
  • P = V × I = V² / R = I² × 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 and the power equations sit behind every electrical decision on a commercial site, from cable sizing to transformer specification to fault-level calculations. Solving any of V = IR, P = VI, P = I squared R or P = V squared over R from two known values is the daily work of the engineer carrying out a supply audit. Doing it in one place, with consistent units, removes the small arithmetic errors that propagate through a design and end up overstated on the bill of materials or understated on the loss calculation.

On a multi-site portfolio, the same calculations get repeated thousands of times across different supply voltages (230V single-phase, 400V three-phase, 11kV HV, 33kV at very large sites). The risk is not in the maths, it is in the consistency of the assumptions: which voltage to use, how to handle power factor on AC, how to size for diversity. A single working tool that handles all four permutations gives engineers a reliable starting point and keeps the audit traceable when it gets reviewed at board level.

Purely Energy supports clients with portfolio-wide audits as part of contract renewal and as standalone capacity reviews. We combine the electrical engineering work (cable sizing, supply capacity, transformer loading) with the commercial procurement work (MIC, MEC, NCC, tariff selection) into a single recommendation that finance can approve. For Industrial & Commercial clients consuming 10 GWh per year and above, that integrated approach typically pays for itself inside the first contract year.

Common questions

How do I calculate watts, volts, amps and ohms from two known values?

Enter any two of the four values and the calculator solves the other two using Ohm's law and the power law. The core relationships are V = I x R and P = V x I. Every other combination is derived from those, for example P = V squared / R when you know voltage and resistance, or I = the square root of P / R when you know power and resistance. The result panel always shows all four values together, so you can sanity-check the answer against the equipment nameplate.

What is the relationship between Ohm's law and the power formula?

Ohm's law (V = I x R) links voltage, current and resistance. The power law (P = V x I) adds power to the picture. Substituting one into the other produces the full wheel of twelve formulas: P = I squared x R, P = V squared / R, I = the square root of P / R, and so on. They are not separate rules, just the same two relationships rearranged, which is why knowing any two of the four quantities is always enough to find the other two.

Why is a UK plug limited to about 3 kW?

A standard BS 1363 plug takes a fuse of at most 13 A. At the UK nominal voltage of 230 V, the power law gives 230 V x 13 A = 2,990 W, which is why kettles, heaters and similar appliances top out at about 3 kW. Anything that needs more power, such as a commercial oven or an electric shower, has to be hard-wired on its own dedicated circuit with an appropriately rated breaker and cable.

Does this calculator work for 230 V AC mains circuits?

It is exact for DC and for purely resistive AC loads such as immersion heaters, kettles and panel heaters, where the power factor is effectively 1. For motors, drives, fluorescent ballasts and electronic power supplies, the current drawn is higher than P / V because the power factor is below 1. For those loads, use the watts to amps calculator's AC tab, which divides by power factor as well as voltage.

How do I find the resistance of a heating element from its wattage?

Use R = V squared / P. A 3 kW element designed for 230 V works out at 230 x 230 / 3000, which is about 17.6 ohms. A multimeter reading on a cold nichrome element will be close to this figure, which makes the formula a quick health check: a much higher reading suggests a failing element or poor connection, and an open circuit means the element has burned out.

Related calculators

The conversions that procurement and engineering use alongside this one.

Watts / Volts / Amps / Ohms Calculator | Purely Energy