Transformer Turns Ratio Calculator

Understanding the transformer turns ratio calculator — concept and principle

This calculator gives the turns ratio of an ideal transformer from its primary and secondary voltages, the full-load primary and secondary currents for a given VA rating, and the impedance ratio between the two sides. It is used to choose a mains transformer for a power supply, check a nameplate, match impedances in audio or RF circuits, and estimate fuse sizes.

Voltage drops by the turns ratio and current rises by the same ratio, so 60 VA goes in and 60 VA comes out (turns not to scale).

How it is calculated

Turns ratio: a = N_p / N_s = V_p / V_s, shown as a : 1.
Primary current: I_p = S / V_p, with S in VA.
Secondary current: I_s = S / V_s.
Impedance ratio: Z_p / Z_s = a² = (V_p / V_s)².

The principle

Both windings share the same changing magnetic flux in the core. By Faraday's law, each turn has the same induced voltage, so the voltage across a winding is proportional to its number of turns: V_p/V_s = N_p/N_s. In an ideal transformer no power is lost, so the power going in equals the power coming out: V_p · I_p = V_s · I_s. Stepping the voltage down therefore steps the current up by the same ratio. Because impedance is voltage divided by current, and voltage is multiplied by a while current is divided by a, a load Z on the secondary looks like a² · Z from the primary side.

Worked example

Inputs: V_p = 230 V, V_s = 12 V, S = 60 VA.
Turns ratio: 230 ÷ 12 = 19.1667 : 1.
Primary current: 60 ÷ 230 = 0.26087 A.
Secondary current: 60 ÷ 12 = 5 A.
Impedance ratio: 19.1667² = 367.36, so a 2.4 Ω load on the secondary looks like about 882 Ω from the mains side.

How to read the result

The currents are the full-load values the windings are rated for; continuous loads should stay at or below them. The primary current also guides fuse selection, although transformers draw a large inrush when switched on, so slow-blow fuses are usual. A ratio above 1 is a step-down transformer; below 1 is step-up.

Common mistakes

Assuming the secondary voltage stays constant: small transformers can read 10–20% higher than rated at no load, falling to the rated value at full load.
Using DC. A transformer needs a changing flux and does not work on steady DC.
Rectifying 12 V AC and expecting 12 V DC. With a capacitor filter, the DC level approaches the peak (about 12 × √2 ≈ 17 V) minus diode drops.

Keep in mind: These are ideal-transformer relations. Real units have copper and core losses, leakage inductance and regulation, and must not be operated outside their voltage, frequency and VA ratings. Mains wiring should be done by a qualified person.

Frequently asked questions

What does the VA rating mean?

It is the apparent power the transformer can supply continuously without overheating, limited by winding current and core heating.

Can I run a transformer backwards?

An ideal transformer works both ways, but real windings are built for their rated voltage and insulation, so check the manufacturer's guidance.

Why do I need the impedance ratio?

It matches loads such as loudspeakers to amplifiers and is used to refer secondary impedances to the primary in circuit analysis.

Sources: IEC 61558-1, Safety of transformers, reactors, power supply units and combinations thereof · D. Halliday, R. Resnick and J. Walker, Fundamentals of Physics, Wiley, chapter on electromagnetic induction and transformers

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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