Transformer Calculator
Turns ratio, winding currents and core size from the voltages and rating.
Ideal-transformer relations plus the EMF equation. Regulation, leakage inductance, inrush current and thermal design all matter in a real build and none of them are here.
How it works
A transformer conserves power, so what goes up in voltage comes down in current in the same proportion. The turns ratio sets both:
Vp ÷ Vs = Np ÷ Ns = Is ÷ Ip
The step that connects a paper design to a physical core is the EMF equation:
V = 4.44 × f × N × B × A
Frequency, turns, peak flux density and core cross-section. Rearranged, it gives turns per volt — the single number a transformer winder works from. At 50 Hz with 1.2 T in a 10 cm² core, that is about 3.8 turns per volt, so a 230 V primary needs roughly 870 turns.
Two consequences follow immediately. Higher frequency needs fewer turns and less iron, which is why a switch-mode supply running at 100 kHz uses a transformer the size of a thimble where a 50 Hz one needs a brick. And running a 60 Hz transformer on 50 Hz pushes flux density up by 20%, which can drive the core into saturation — at which point magnetising current rises steeply and the thing overheats with no load on it at all.
Impedance transforms as the square of the turns ratio. A 4:1 transformer presents a 16:1 impedance ratio, which is the entire basis of impedance matching in audio and RF work.
Real transformers are not ideal. Copper loss rises with the square of load current and iron loss stays roughly constant whenever it is energised, so efficiency peaks somewhere around half to three-quarter load and falls away either side.
Common questions
How many turns per volt?
From V = 4.44 f N B A. At 50 Hz, 1.2 T and a 10 cm² core it is about 3.8. Winders often use the shortcut 42 ÷ core area in square centimetres for 50 Hz mains transformers, which is the same equation with typical values substituted.
Can I run a 60 Hz transformer at 50 Hz?
Only with care. Flux density scales inversely with frequency, so 50 Hz operation raises it by 20% and may saturate the core. The other direction — a 50 Hz transformer on 60 Hz — is safe, and it will run cooler.
Why is the primary current higher than the ratio suggests?
Losses. Copper resistance and core hysteresis both have to be supplied from the primary, so the real primary current is the ideal one divided by efficiency, plus a magnetising component that flows whether or not there is a load.
What does impedance ratio mean here?
It is the turns ratio squared. A transformer with a 4:1 turns ratio makes an 8 Ω speaker look like 128 Ω to the amplifier — which is exactly what an output transformer in a valve amplifier is for.