Transformer Calculator
Turns ratio, currents and VA rating for a transformer.
About this tool
Enter the primary and secondary voltages, the load current and the expected efficiency. The tool shows the turns ratio, output VA, primary current, input power and heat loss, and the secondary turns for a given primary winding.
Transformer basics
An ideal transformer's voltages are in the ratio of its turns, Vs รท Vp = Ns รท Np, and currents in the inverse ratio, so power in equals power out. A 230 V to 12 V transformer has a turns ratio of about 19.2:1; with 1,000 turns on the primary, the secondary needs about 52. Supplying 2 A at 12 V is 24 VA, so at 90% efficiency it draws about 26.7 VA, around 116 mA from the 230 V mains, and turns about 2.7 W into heat. Transformers are rated in VA rather than watts because winding heating depends on current, whatever the load's power factor. Allow 20โ25% headroom when choosing a rating, more for rectifier and capacitor loads, which draw current in peaks. Honest limits: these are ideal relationships. Real secondaries read above nominal at light load (regulation), small transformers are often only 70โ85% efficient, and the turns per volt depend on the core size and frequency. Mains voltage is dangerous, so this is not electrical advice. The watts to amps calculator and the power factor calculator are related.
Frequently asked questions
How do I calculate a transformer's turns ratio?
Divide the primary voltage by the secondary voltage. 230 V to 12 V is about 19.2:1.
How do I calculate primary current?
Primary current = secondary VA รท efficiency รท primary voltage.
Why are transformers rated in VA?
Heating depends on current, which flows whatever the load's power factor, so VA is the true limit.
What size transformer do I need?
Add up the load's VA and add 20 to 25% headroom.
Is my data sent anywhere?
No. Everything is calculated in your browser, and nothing is stored or sent.