Transformer Turns Ratio Calculator
Turns ratio from the two voltages, with the secondary winding count and the current on each side.
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The formula
V_p ÷ V_s = N_p ÷ N_s = I_s ÷ I_p
One chain of equalities holds the whole device together. Voltage follows the turns directly; current follows them inversely, because the power on each side has to match. An ideal transformer transforms the ratio and nothing else.
What to watch out for
Worth knowing before winding anything:
- Power is conserved, so current inverts. Step voltage down by ten and the secondary carries ten times the current — which is why the low-voltage winding uses much thicker wire.
- Real transformers lose a little. Expect 95–99% efficiency. The secondary voltage also sags under load, typically a few per cent, so unloaded measurements read high.
- Turns per volt is set by the core. The ratio tells you the relationship between the windings, but not the absolute number of turns — that depends on core area and frequency, and too few turns will saturate the core.
Frequently Asked Questions
How do I calculate transformer turns ratio?
Divide the primary voltage by the secondary voltage. 230 V to 12 V is a ratio of 19.17:1, so 1,000 primary turns needs about 52 on the secondary.
Does a transformer change power?
No — it trades voltage for current. Ignoring losses, the power out equals the power in, which is why a step-down transformer can supply more current than it draws.
Why is the low-voltage winding made of thicker wire?
Because it carries more current. A 19:1 step-down means the secondary handles 19 times the primary current, and the conductor has to be sized for it.