Last updated: 2026-06-15

Transformer turns ratio

A transformer changes voltage by winding two coils around a shared magnetic core. The ratio of the number of turns sets how much the voltage changes. For an ideal transformer the turns ratio, the voltage ratio and the inverse current ratio are all the same:

This calculator takes the primary and secondary turns, the primary voltage and the primary current, and returns the turns ratio, the secondary voltage and the secondary current.

Step up or step down

The turns ratio $n = N_p/N_s$ tells you the type of transformer:

Voltage and current always move in opposite directions, because an ideal transformer conserves power.

Formula

Worked example

A transformer has 1000 turns on the primary and 100 on the secondary, with 120 V and 0.5 A on the primary:

The output power $V_s I_s = 12 \times 5 = 60\ \text{W}$ matches the input power $V_p I_p = 120 \times 0.5 = 60\ \text{W}$, as it must.

Power conservation

Because the secondary power equals the primary power in an ideal transformer, stepping the voltage down by a factor of ten raises the current by the same factor. This trade-off is the basis of the electric grid: power is stepped up to very high voltage for long-distance transmission — which keeps the current, and therefore the resistive line losses, low — and stepped back down for use in homes and businesses.

Limitations

The relations above describe an ideal, loss-free transformer. Real transformers lose a few percent of the power to resistance in the copper windings and to hysteresis and eddy currents in the core, so the secondary voltage under load is slightly lower than the ideal value. They also work only with alternating current; a steady DC input produces no changing magnetic field and therefore no output.