Transformer Turns Ratio Calculator

Turns ratio, secondary voltage, current ratio, impedance ratio and volts per turn — solved whichever way round you have the numbers, with the losses and regulation the ideal formula leaves out.

Last updated 22 Sep 2026

Transformer turns ratio

Turns ⇄ voltage, current and impedance

What are you solving for?The field the page works out is locked and filled in for you.

Primary turns, NpCount them, or work back from the volts per turn your core gives.

Secondary turns, NsCalculated in the third mode.

Primary voltage (V)230 V in India, the Gulf and Europe; 120 V in North America.

Secondary voltage, no load (V)The open-circuit voltage. A small transformer measures several per cent above this figure with nothing connected.

Rating (VA)Used only for the two full-load currents. The same VA passes through both windings.

Voltage regulation at full load (%)How far the secondary sags from no load to full load. Enter 0 for the ideal case. A small mains transformer is often 5–15%; a large distribution transformer is a few per cent.

Load impedance on the secondary (Ω)For the impedance-matching view — 8 Ω is a loudspeaker. Enter 0 to leave it out. It does not change the voltages or the rated currents.

The ideal transformer: the same flux links both windings, so the volts per turn are the same on each side and the voltages divide as the turns do. The dots mark the winding polarity — both at the same end here, so primary and secondary voltages are in phase. The dots on the wires show the full-load currents, which go the other way: the winding with more turns carries less current.

9.583Example

Np = 1,150 turns, Ns = 120 turns, 230 V on the primary, rated 50 VA

The ideal transformer

V_p ÷ V_s = N_p ÷ N_s = n I_s ÷ I_p = n Z_p ÷ Z_s = n² volts per turn = V_p ÷ N_p = V_s ÷ N_s

n: turns ratio, primary turns divided by secondary turns
N_p, N_s: turns on the primary and the secondary winding
V_p, V_s: RMS voltages across those windings, no load
I_p, I_s: RMS currents in them; the same VA passes through both
Z_p: what a secondary impedance Z_s looks like from the primary

Worked example

Np = 1,150 turns, Ns = 120 turns, 230 V on the primary, rated 50 VA
n = N_p ÷ N_s = 1,150 ÷ 120 = 9.583
V_s = 230 ÷ 9.583 = 24.00 V with nothing connected
Volts per turn = 230 ÷ 1,150 = 0.2000 V, and 120 × 0.2000 = 24.00 V — the same answer from the winder's side
At 50 VA the currents are 50 ÷ 230 = 217.4 mA on the primary and 50 ÷ 24 = 2.083 A on the secondary
Impedance ratio n² = 91.84, so the 8 Ω load entered looks like 734.7 Ω from the primary
At 8% regulation the secondary falls to 22.08 V at full load

Common 230 V transformer ratios

Secondary turns are for a core giving 0.2 V per turn, which is what a 1,150-turn primary on 230 V works out at. Volts per turn is a property of the core and the frequency, not of the ratio, so one core gives every one of these windings.
Ratio	n	n² (impedance ratio)	Secondary turns at 0.2 V/turn	Current at 50 VA
230 V : 6 V	38.333	1,469.4	30 turns	8.333 A
230 V : 9 V	25.556	653.1	45 turns	5.556 A
230 V : 12 V	19.167	367.4	60 turns	4.167 A
230 V : 15 V	15.333	235.1	75 turns	3.333 A
230 V : 18 V	12.778	163.3	90 turns	2.778 A
230 V : 24 V	9.583	91.8	120 turns	2.083 A
230 V : 48 V	4.792	23.0	240 turns	1.042 A

Turns, volts, amps and ohms

A transformer has exactly one job: it changes the ratio of volts to amps without changing their product. The physics behind it is a single sentence — the same alternating flux threads every turn of every winding, so every turn develops the same voltage. That is why the voltages come out in the ratio of the turns, and it is also why the useful number for anybody actually winding one is the volts per turn: fix that from the core area and the frequency, and both windings follow from it.

The currents go the other way. An ideal transformer neither stores energy nor loses it, so the volt-amperes going in equal the volt-amperes coming out: V_pI_p = V_sI_s. A step-down transformer that divides the voltage by ten multiplies the current by ten. Both windings of the example above are rated 50 VA, and that is 217.4 mA on a 230 V primary but 2.083 A on a 24 V secondary — the secondary wants wire several gauges heavier even though it is the low-voltage side.

Impedance transforms by n². Put Z on the secondary and the primary sees n²Z, because the voltage is multiplied by n and the current divided by n. This is the whole basis of matching transformers: to present a 5 kΩ load to a valve output stage from an 8 Ω loudspeaker you need n = √(5,000 ÷ 8) = 25.00, so about 25 primary turns for every secondary turn. The same arithmetic sizes an RF matching transformer or a current-sense transformer.

What the ideal model leaves out. Four things, and they all make the real thing worse than the arithmetic. Magnetising current flows in the primary with the secondary open-circuit, because the core needs ampere-turns to make its flux; it is what a transformer draws when it is doing nothing. Core loss — hysteresis and eddy currents — heats the iron whenever the transformer is energised, load or no load. Copper loss is I²R in both windings and rises with the square of the load. Leakage reactance is the flux that links one winding and not the other. The last two together are what produce voltage regulation: the secondary sags as it is loaded. This page takes regulation as a percentage you enter and applies it to the no-load voltage, which is the linear first-order model and is good to a fraction of a per cent at the single-digit regulations real transformers show. Small mains transformers are poor at this — 5% to 15% is normal, and a 12 V winding can measure 14 V with nothing on it. At the 8% in the example the 24 V secondary is really 22.08 V under full load. Textbooks more often define regulation against the full-load voltage instead, which would give 22.22 V; the difference is smaller than the tolerance on the figure itself.

For a switching converter’s transformer, where the turns come from the volt-second product rather than from the mains voltage, use the SMPS transformer calculator. For the rating side of a mains or distribution transformer — full-load current, short-circuit current and the breaker that goes with them — use the transformer kVA and current calculator, then the cable size calculator and the MCB size calculator.

Frequently asked questions

How do I calculate the turns ratio of a transformer?

Divide the primary turns by the secondary turns, or the primary voltage by the secondary voltage — they are the same number. With 1,150 primary turns and 120 secondary turns the ratio is 9.583:1, so 230 V in gives 24 V out.

Does a transformer turns ratio change the current too?

Yes, in the opposite direction. The current ratio is the inverse of the voltage ratio, so a 10:1 step-down transformer carries ten times as much current in its secondary as in its primary. The volt-amperes are the same on both sides.

What is the impedance ratio of a transformer?

The square of the turns ratio. A load of Z ohms on the secondary looks like n²Z from the primary. With n = 9.583 the impedance ratio is 91.8, so an 8 Ω secondary load appears as 734.7 Ω on the primary.

Why is my transformer’s output voltage higher than the rating?

Because the rating is the voltage at full load and you are measuring it at no load. The difference is the voltage regulation — the drop across the winding resistance and the leakage reactance once current flows. Small transformers commonly measure 5–15% high with nothing connected.

What are volts per turn, and why do winders use them?

The voltage each single turn develops, which is the same for every winding on the same core: V ÷ N. It is set by the core’s cross-sectional area, the flux density and the frequency. Fix it once and every winding on that core is just a multiplication — the example above is 0.2 V per turn, so a 12 V winding is 60 turns.

Can I use a 50 Hz transformer on 60 Hz?

Almost always yes, and it will run cooler: the flux density falls in proportion to the frequency, so the core is further from saturation. The reverse — a 60 Hz transformer on 50 Hz at the same voltage — pushes the flux density up by 20% and can saturate it.

Related calculators

References

Chapman S J. Electric Machinery Fundamentals, 5th ed. McGraw-Hill, 2012. Chapter 2, Transformers — the ideal transformer, the turns ratio, impedance transformation and voltage regulation; chapter 6, Induction Motors — synchronous speed, slip, rotor frequency and induced torque.
IEC 60076-1:2011, Power transformers — Part 1: General. Table 1 tolerances: voltage ratio at the principal tapping, the lesser of ±0.5% of the declared ratio and ±1/10 of the actual percentage impedance; short-circuit impedance at the principal tapping ±7.5% of a declared value of 10% or more and ±10% below that. Definitions 3.4.6 rated power, 3.4.4 rated voltage ratio, 3.7.1 short-circuit impedance, 3.6.2 no-load current.
Hughes E, Hiley J, Brown K, Smith I M. Electrical and Electronic Technology, 12th ed. Pearson 2016: three-phase star and delta relationships, the power triangle, and the polyphase induction motor.