Transformer Size Calculator
Estimate required transformer kVA from real load, power factor and spare capacity margin.
Interactive Electrical Calculator
Transformer Size Calculator
Estimate required transformer kVA from real load, power factor and spare capacity margin.
Result
Enter values and calculate.
About the Transformer Size Calculator
Use the Transformer Size Calculator below to work through transformer sizing and turns-ratio calculations in seconds, right in your browser. Estimate required transformer kVA from real load, power factor and spare capacity margin. Turns ratio is what actually sets a transformer's voltage transformation, and getting it wrong is a common cause of an installation that delivers the wrong output voltage.
Electricians and engineers selecting or troubleshooting step-up and step-down transformers typically reach for a tool like this to double-check a number before it goes into a real installation.
How the calculation works
Behind the scenes, the calculation follows:
kVA = (kW / PF) × (1 + margin).
Where each input maps to a field in the calculator above:
- Load in kW.
- Power Factor.
- Spare Capacity in %.
You can adjust any of these values as many times as you like; the result updates on every change with nothing sent off your device.
A worked example
For example, start with 50 kW for load, 0.9 for power factor and 20 % for spare capacity. That's a reasonable starting point to explore from: nudge one value up or down and watch how much the result shifts before you commit to a final design figure.
When to use the Transformer Size Calculator
Typical reasons people reach for the Transformer Size Calculator:
- Sizing a transformer for a known kVA or kW load.
- Working out the turns ratio needed to step a voltage up or down to a target value.
- Checking a transformer nameplate ratio against a measured voltage.
- Planning a transformer replacement or upgrade for growing load.
Good practice: Transformer kVA rating should be checked against total connected load with an appropriate safety margin, not the bare minimum needed today.
Also worth knowing: Turns ratio equals the ratio of primary to secondary voltage under no-load conditions — real loaded voltage will sag slightly due to internal impedance.
Frequently asked questions
What does the Transformer Size Calculator calculate?
Estimate required transformer kVA from real load, power factor and spare capacity margin.
Can I rely on this number for a real installation?
The formula above is applied exactly to the values you enter, so the math itself is precise. Real-world results can still differ due to component tolerances, temperature, installation method, equipment efficiency and local code requirements, so treat the output as a strong planning estimate and verify final designs against manufacturer data and the electrical code that applies to your installation.
Is this calculator free to use?
Yes. The Transformer Size Calculator is completely free, with no account, download or installation required.
What happens if I get this wrong?
Turns ratio is what actually sets a transformer's voltage transformation, and getting it wrong is a common cause of an installation that delivers the wrong output voltage.
Can I use this on a phone, or does it need a desktop?
The Transformer Size Calculator works on any modern browser, phone or desktop, and recalculates as soon as you press Calculate. If a result looks off, double-check that every field is filled in with the unit shown next to it (mixing up units, such as W with kW, is the most common cause of an unexpected answer) and re-enter the values.