For guidance only. These calculators are planning aids for licensed electrical workers and inspectors. All results must be independently verified against the current editions of AS/NZS 3000:2018, AS/NZS 3008.1.2, and other applicable standards before being relied on for any installation. CertMate accepts no responsibility or liability for any work carried out, decisions made, or certification issued in reliance on these results. The certifying electrical worker remains solely responsible for compliance with the Electricity (Safety) Regulations 2010 and all applicable New Zealand law.
The four-field shortcut
Ohm's Law and the power formula together only really need four numbers - voltage, current, resistance, power - and any two of them pin down the other two. Most of the time you're solving the same handful of variations of the same two formulas, so rather than remembering which arrangement to use for which pair, this just takes whatever two you've got and fills in the rest.
It's the calculation that comes up constantly and rarely gets its own tool - checking an appliance's rated current from its wattage, working out the resistance of an element from its rated power, or sanity-checking a reading against what the nameplate says it should be.
Worked examples
Hot water cylinder element current
A 3kW hot water cylinder element on a 230V supply - a very common check when you're confirming the element draws what the nameplate claims, or sizing a circuit around it.
Element resistance from a multimeter reading
You've measured a heating element's resistance directly with a multimeter and want to know what current and wattage that corresponds to on the mains supply, without doing the arithmetic by hand on site.
Nameplate sanity check
An appliance nameplate gives both voltage and wattage. Cross-checking the current that implies against what you're actually measuring on the circuit is a quick way to catch a faulty element or a wiring fault before it becomes a bigger problem.
Frequently asked questions
What formulas does this calculator use?
Depending on which two values you enter, it uses whichever pairing of V = IR, W = VI, or the combined forms (W = I²R, W = V²/R) applies. The calculator shows you exactly which formula it used for your specific inputs.
Why do I need exactly two values, not one or three?
Two known values fully define the other two under Ohm's Law and the power relationships - one value alone is ambiguous, and three values would either be redundant or, if they don't agree with each other, contradictory. Two is the sweet spot that gives a single, unambiguous answer.
Does this work for AC circuits with power factor?
This calculator is for straightforward DC-style Ohm's Law relationships. For AC loads where power factor matters - motors, most real-world three-phase loads - use the Three-Phase Converter calculator instead, which handles kW, kVA and power factor together.
Related calculators
CertMate creates your CoCs, ESCs and RoIs in minutes - no more juggling paper on site.
Start Free Trial