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Transformer Turns Ratio Calculator

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A transformer's whole behaviour follows from one number: how many turns one winding has against the other. Voltage scales with it, current scales inversely with it, and impedance scales with its square. Enter either two voltages or two winding counts, add the load, and this gives all three along with what the primary actually draws once the losses are allowed for.

How is the ratio known?
V
V
VA
%

Small mains transformers run 80–90%; large ones exceed 97%.

How to use the transformer turns ratio calculator

  1. 1Choose whether you know the two voltages or the two turns counts.
  2. 2Enter the primary voltage, then either the secondary voltage or the winding counts.
  3. 3State the load as a VA rating, a secondary current or a load impedance — whichever the datasheet or the job gives you.
  4. 4Set the efficiency: 80 to 90% is usual for a small mains transformer, and above 97% for a large one.

Examples

A 230 V to 12 V supply

Input
230 V primary, 12 V secondary, 60 VA, 90% efficient
Result
Ratio 19.17:1, secondary 5 A, primary 0.29 A

The primary current includes the 10% loss, so it is higher than the ideal 0.26 A.

Matching a speaker to a valve amplifier

Input
Turns 25:1 with an 8 Ω speaker
Result
Impedance ratio 625:1 — the primary sees 5 kΩ

Impedance goes as the square of the ratio, which is why output transformers are specified by impedance rather than volts.

Reading a winding count

Input
1150 primary turns, 60 secondary turns, 230 V in
Result
12 V out, 0.2 volts per turn

Volts per turn is the same on both windings, so it tells you what one extra turn is worth if you are rewinding.

About the transformer turns ratio calculator

Voltage up means current down, and that is not a design choice

Both windings sit on the same magnetic circuit and link the same flux, and the voltage induced in a winding is proportional to how many turns do that linking. So the voltage ratio is the turns ratio. The current relation is not an independent fact but a consequence of energy conservation: an ideal transformer neither stores nor dissipates power, so what goes in must come out, and if the voltage was multiplied by n the current must be divided by n.

This is why a transformer cannot be a source of power, however the ratio is arranged. A step-up transformer that doubles voltage halves the current available at the same instant, and the product — the volt-amps — is unchanged. It is also why a step-down transformer's secondary is wound in thick wire and its primary in thin: the low-voltage side is the high-current side.

The squared relation, which is the useful one

Impedance transforms as the square of the turns ratio, because the primary sees n times the voltage across 1/n times the current. That squaring is what makes transformers matching devices as much as voltage-changing ones. A valve output stage that wants to see 5 kΩ and an 8 Ω loudspeaker are 625 to 1 apart in impedance, which is a turns ratio of 25 to 1 — and choosing that transformer is choosing the impedance, not the voltage.

The same relation is why the grid transmits at hundreds of kilovolts. Line loss is I squared R, so cutting the current by a factor of a hundred through a 100:1 step-up cuts the loss by ten thousand. The power arriving is the same; the heat left in the cable on the way is four orders of magnitude smaller.

What the ideal model leaves out, and the safety part

Real windings have resistance, which turns some of the power into heat and makes the output voltage sag under load. Real cores have hysteresis and eddy-current losses that appear as heat whether or not anything is connected, and leakage inductance because not every field line links both windings. The efficiency figure here rolls all of those into one number to get the primary current right; it does not tell you where the heat is going or how hot the transformer will get, which is what the manufacturer's temperature rise specification is for.

A VA rating is a thermal limit rather than an electrical one. Exceed it and nothing fails immediately — the transformer simply runs hotter than the insulation is rated for, and its life falls sharply. Intermittent loads can exceed it briefly for the same reason, though how briefly depends on the mass of the core.

Mains-voltage work is genuinely dangerous. These are figures for design and diagnosis, not instructions: wiring on the primary side of anything connected to a mains supply is work for someone qualified to do it, under the wiring rules that apply where you are.

Frequently asked questions

Why does the secondary voltage measure higher than the rating with nothing connected?
That is regulation, and it is normal. The rating on the label is the voltage at full rated load, where the resistance of the windings and the leakage inductance are dropping a few percent. Take the load away and those drops disappear, so a 12 V secondary commonly reads 13 or 14 V open circuit — more on a small transformer, since regulation is worse the smaller the core. If a rectifier and reservoir capacitor follow the secondary, remember it charges towards the peak of that unloaded voltage, which is another factor of about 1.4.
Is a VA rating the same as watts?
Only for a purely resistive load. VA is volts times amps regardless of whether the two are in phase; watts is the part of that which does real work. A motor, a fluorescent ballast or a switch-mode power supply draws current that is out of phase or badly distorted, so a load consuming 60 W can pull the current of 90 VA or more. The winding heats according to the current it carries, so it is VA that has to be met — sizing a transformer by the wattage of the load is how one ends up running hot.
Can I run a 60 Hz transformer on a 50 Hz supply?
Usually not safely, though the reverse is generally fine. The flux in the core for a given applied voltage is inversely proportional to frequency, so dropping from 60 Hz to 50 Hz raises the peak flux by about 20%. A core designed with little margin then saturates on each half cycle, magnetising current rises sharply, and the transformer runs hot and buzzes even with nothing connected to the secondary. Nothing in a turns ratio reveals this, because saturation is a property of the core and its cross-sectional area rather than of the windings.
What does an isolation transformer do if the ratio is 1:1?
It breaks the electrical connection between the supply and the load while passing the power magnetically. With a 1:1 ratio no voltage is changed, but neither secondary conductor is tied to the supply's neutral or earth, so touching one of them and an earthed object does not complete a circuit through you. That is the entire point, and it is why bench isolation transformers exist for servicing equipment where the chassis may sit at mains potential. Note that the isolation is between windings, not a substitute for an RCD, and it does nothing about touching both secondary conductors at once.
How do I work out the turns for a winding I want to add?
Use volts per turn, which this reports when you enter a winding count. It is the same figure for every winding on the core, because they all link the same flux — so if a transformer is 0.2 V per turn, a 9 V auxiliary winding needs 45 turns whatever the primary is doing. In practice add a few percent for the resistive drop under load, and check there is physical room in the window for the wire gauge the current requires before committing.

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