Transformer Calculator

Turns ratio, secondary voltage and current, with power conserved.

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

Primary voltage

Primary turns

Secondary turns

Primary current

Optional — gives the secondary current.

This describes an ideal transformer. Real ones lose a few percent to core hysteresis, eddy currents and winding resistance, so the secondary delivers slightly less than this and runs warm. The figures are a design starting point, not a specification.

Secondary voltage (V)

Step-down · turns ratio 0.1

Secondary voltage23 V

Turns ratio0.1

TypeStep-down

Secondary current20 A

Apparent power460 VA

Voltage scales with the turns ratio and current scales inversely, because power is conserved. This steps voltage down by 10× and changes the current by exactly the reciprocal. A transformer is not a source of energy — whatever it gains in volts it gives up in amps.

How the Transformer Calculator works

Voltage scales with the turns ratio and current scales inversely, because power is conserved. Whatever a transformer gains in volts it gives up in amps — it is not a source of energy, which is the point people most often need reminding of.

Also known as: turns ratio calculator · step up transformer calculator · transformer voltage calculator · transformer formula

Nothing is created

Voltage scales with the turns ratio; current scales with its reciprocal. A 10:1 step-down takes 230 V at 2 A and delivers 23 V at 20 A.

Both sides carry 460 VA. A transformer trades volts for amps at a fixed product. It is not a source of energy, which is the point most worth taking from this page.

Why the grid uses them

Transmission losses are I²R — proportional to the square of current. Stepping voltage up by a hundred cuts current by a hundred and losses by ten thousand.

That is the whole reason for high-voltage transmission lines, and the reason transformers matter at all: they make it possible to generate at one voltage, transmit at another and consume at a third.

Real units lose a few percent to core hysteresis, eddy currents and winding resistance, so the secondary delivers slightly less than the ideal figure and the unit runs warm. Mains transformers are typically 95–99% efficient.

Where to go next

The Transformer question rarely arrives on its own. These are the ones that usually come with it:

Power Factor Calculator — Real against apparent power, and the current it costs.
Voltage Divider Calculator — Output voltage, current and the power each resistor dissipates.
Power Calculator — Watts, kilowatts and horsepower, with a running cost.
Speed Distance Time Calculator — Any one of the three from the other two, in four units.

Sources

Frequently asked questions

What is the transformer formula?

Vs/Vp = Ns/Np. A 1,000:100 turns ratio on 230 V gives 23 V out.

What happens to the current?

It changes by the reciprocal of the voltage ratio. Stepping voltage down by ten multiplies the available current by ten, keeping the power the same.

What is the difference between step-up and step-down?

Whether the secondary has more turns than the primary or fewer. More turns raises voltage and lowers current; fewer does the reverse.

How efficient are real transformers?

Typically 95–99% for mains units. Losses come from core hysteresis, eddy currents and winding resistance, so the secondary delivers slightly less than the ideal figure and the unit runs warm.

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