Transformer Turns Ratio Calculator

Turns ratio, secondary voltage and current for a mains or signal transformer.

Example: 1000:50 turns steps down 230 V to 11.5 V, with an impedance ratio of 0.0025.

Inputs

Primary turns

Secondary turns

Primary voltage(V)

Secondary current (optional)(A)

Primary impedance (optional)(Ω)

For impedance matching — the source impedance seen at the primary.

Efficiency

Result

Secondary voltage

11.5 V

Turns ratio: 20 : 1
Impedance ratio: 400 : 1
Primary current: 105.3 mA
Power delivered: 23 W
Losses: 1.211 W

Current transforms inversely to voltage: stepping down the voltage by 20× raises the current by the same factor. Power is conserved apart from the 1.211 W lost to core and copper.
A transformer only works on changing current. Apply DC and the primary is just a low-resistance winding across your supply — it draws destructive current and delivers nothing to the secondary.
Turns ratio sets the voltage relationship, but the core sets what the transformer can actually deliver. Too few primary turns for the applied voltage and frequency saturates the core, and a saturated transformer draws enormous magnetising current while transferring almost no power.

Secondary voltage against turns ratio

Secondary turns	Ratio	Secondary voltage	Impedance ratio
50	0.05	11.5 V	0.0025
100	0.1	23 V	0.01
250	0.25	57.5 V	0.0625
500	0.5	115 V	0.25
1000	1	230 V	1
2000	2	460 V	4

Formula

V_s/V_p = N_s/N_p    I_s/I_p = N_p/N_s    Z_s/Z_p = (N_s/N_p)²

Worked example

1000:50 turns steps down 230 V to 11.5 V, with an impedance ratio of 0.0025.

V_s = V_p × N_s / N_p
230 V × 50 / 1000
11.5 V
I_p = I_s × N_s / N_p / η
2 A × 0.05 / 0.95
105.3 mA
Z_s / Z_p = (N_s / N_p)²
(0.05)²
0.0025
Squared, not linear — the reason impedance matching uses transformers.

Frequently asked questions

How do I calculate transformer secondary voltage?

Multiply the primary voltage by the turns ratio: V_s = V_p × N_s / N_p. A 1000-turn primary and 50-turn secondary on 230 V gives 11.5 V. Current goes the other way, so the secondary carries proportionally more.

Why does impedance transform as the square of the turns ratio?

Because voltage scales with the ratio and current scales inversely, and impedance is their quotient — so the two effects multiply. A 1:8 turns ratio gives a 1:64 impedance change, which is why modest ratios make effective matching transformers.

Can a transformer work on DC?

No. It relies on a changing magnetic field to induce voltage in the secondary. Applied DC leaves the primary as a low-resistance winding straight across the supply, drawing destructive current while delivering nothing.

What limits how much power a transformer can deliver?

The core and the wire, not the turns ratio. Too few primary turns for the applied voltage and frequency saturates the core, at which point magnetising current soars and power transfer collapses. Wire gauge sets the thermal limit on top of that.

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