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Transformer Turns Ratio Calculator

Find the output voltage and current of a transformer from its winding turns.

How it is calculated

For an ideal transformer the voltages follow the turns: Vs = Vp × Ns / Np, where Vp is the primary voltage in volts, Np the primary turns and Ns the secondary turns. The turns ratio is shown as Np / Ns : 1. If you enter the primary current Ip in amperes, the secondary current is Is = Ip × Np / Ns, because the power in equals the power out.

Turns must be positive and the voltage cannot be negative. The current line is left empty when no primary current is given. An ideal transformer has no losses, so real secondary voltage under load is slightly lower and real currents slightly differ. Alternating current is required, since a transformer does not work on steady DC.

Worked example

Primary voltage 230 V, 1000 primary turns, 52 secondary turns, primary current 0.5 A. Vs = 230 × 52 / 1000 = 11.96 V. Turns ratio = 1000 / 52 ≈ 19.23 : 1. Secondary current = 0.5 × 1000 / 52 ≈ 9.62 A. The tool shows 11.96 V, 19.23 : 1 and 9.62 A: the voltage steps down and the current steps up by the same factor.

Try this example →

Common mistakes

  • Swapping primary and secondary turns, which turns a step-down into a step-up.
  • Entering the secondary current in the primary-current field; the field expects the primary current, and the secondary one is calculated.
  • Expecting the ratio to give a higher current and a higher voltage together; power is conserved, so one goes up as the other goes down.
  • Ignoring losses and the maximum VA rating of a real transformer, and trying to draw the calculated current from it.

About this calculator

This calculator finds a transformer's secondary voltage and current from its turns ratio: Vs/Vp = Ns/Np, and by conservation of power, Is/Ip = Np/Ns. Given the primary voltage, the primary and secondary turns counts and, optionally, the primary current, it returns the induced secondary voltage, the turns ratio and the secondary current under the ideal-transformer assumption.

Electrical engineers, electricians, and electronics students use it when selecting a step-up or step-down transformer for a power supply, choosing tap settings on a distribution transformer, or checking a design against a datasheet's rated voltages. It's a quick way to confirm whether a transformer will deliver the voltage a circuit needs.

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Comments (1)

  • Editorial team

    A good cross-check is power: secondary voltage times secondary current should land close to primary voltage times primary current, which is about 115 W in the 230 V example. Compare that figure with the VA rating on the transformer nameplate before connecting a load. Real units lose some power as heat, so the primary draws slightly more than the ideal result. If your measured secondary voltage differs noticeably, post your turns and readings here.

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