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Transformer Turns Ratio Calculator

Find a transformer's secondary voltage and turns ratio.

How it is calculated

For an ideal transformer the voltage follows the turns:

Vs = Vp × Ns / Np and turns ratio = Np / Ns

Vp is the primary (input) voltage in volts, Np and Ns are the numbers of turns on the primary and secondary windings. The ratio is written as primary over secondary, so a value above 1 means a step-down transformer and below 1 a step-up. Turn counts must be above zero and the voltage cannot be negative. The calculation ignores losses, magnetising current and load, so a real unloaded or loaded output is slightly different. It works for AC; a transformer does not transform steady DC.

Worked example

A mains transformer with 1150 primary turns and 60 secondary turns on a 230 V supply.

  1. Ns / Np = 60 / 1150 ≈ 0.0522
  2. Vs = 230 × 0.0522 = 12.00 V
  3. Turns ratio = 1150 / 60 ≈ 19.17

The secondary gives 12.00 V, and the ratio of 19.17 (to 1) shows it is a step-down unit.

Try this example →

Common mistakes

  • Swapping the primary and secondary turns: the output is then inverted, giving a step-up where you wanted step-down.
  • Reading the displayed ratio as secondary over primary: it is primary over secondary, so 19.17 means the primary has 19.17 times more turns.
  • Treating the result as the voltage under load: it is the ideal value, and real windings drop a little voltage when current flows.
  • Entering a DC supply voltage: transformers only work with alternating current, so the numbers do not apply to a steady DC source.

About this calculator

This calculator finds a transformer's secondary voltage and turns ratio using Vs = Vp·(Ns/Np), the ideal-transformer relationship stating that voltage scales directly with the ratio of secondary to primary winding turns. A transformer with more secondary turns than primary turns steps voltage up (used, for example, at power plants to send electricity efficiently over long transmission lines), while fewer secondary turns steps voltage down (used in the transformers that reduce grid voltage to safe levels for homes and electronics).

Electrical engineers use this when designing power distribution equipment and specifying transformers for a given voltage conversion; electricians use it to verify transformer ratings match a job's requirements; and students studying electromagnetism or power systems use it as a foundational worked example. Hobbyists building audio equipment or small power supplies use the same relationship to pick or wind transformers for a target voltage. Enter the primary voltage and the winding turns to get the secondary voltage and ratio.

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