What is the turns ratio of a transformer?

The turns ratio (a) is the ratio of primary winding turns (N1) to secondary winding turns (N2): a = N1 / N2. It directly equals the voltage ratio (V1 / V2) and the inverse of the current ratio (I2 / I1) for an ideal transformer.

How is secondary voltage calculated from the turns ratio?

V2 = V1 / a, where a = N1 / N2. Equivalently, V2 = V1 × (N2 / N1). If a > 1 the transformer is step-down; if a < 1 it is step-up.

How is secondary current related to the turns ratio?

For an ideal transformer, power is conserved: V1 × I1 = V2 × I2. Therefore I2 = I1 × (N1 / N2) = I1 × a. A step-down transformer increases current proportionally as it reduces voltage.

What is a step-up versus a step-down transformer?

A step-up transformer has more secondary turns than primary turns (N2 > N1, a 1), reducing the output voltage. An isolation transformer has equal turns (a = 1).

Does this calculator handle three-phase transformers?

This tool calculates the per-phase turns ratio applicable to any winding. For three-phase systems, apply the same ratio to each phase; the line voltage relationships depend additionally on the winding configuration (delta or wye), which is outside the scope of this calculator.

What are the assumptions of an ideal transformer?

An ideal transformer assumes zero winding resistance, no core losses, perfect magnetic coupling, and no leakage flux. Real transformers deviate slightly due to copper losses, iron losses, and leakage inductance, but the ideal equations provide accurate first-order results for design and verification.

Transformer Turns Ratio Calculator

Enter primary and secondary turns (or voltages) to instantly compute the turns ratio, output voltage, output current, and winding type.

Winding Parameters

Input Mode

Turns Count Voltage Ratio

Primary Turns (N1)

Secondary Turns (N2)

Primary Voltage V1 (V)

Primary Current I1 (A)

Enter to calculate secondary current (optional).

Results

Enter winding parameters and click Calculate.

Summary

This calculator determines the turns ratio of a transformer and computes secondary voltage and current based on primary winding parameters. Enter the number of turns in each winding or the primary and secondary voltages, along with the primary current or load, and the tool applies the ideal transformer equations to give you the turns ratio, secondary voltage, and reflected current. Use it for design verification, troubleshooting, or educational purposes.

How it works

Select the input mode: Turns Count or Voltage Ratio.
Enter the primary turns (N1) and secondary turns (N2), or primary voltage and secondary voltage.
Enter the primary voltage and primary current.
Click Calculate to compute the turns ratio (a = N1/N2), secondary voltage, and secondary current.
Review the step-up/step-down classification and the full results breakdown.
Use the Reset button to clear all fields and start over.

Use cases

Verify winding data when designing a custom power transformer.
Determine secondary voltage from a known turns ratio and primary supply.
Calculate secondary current from primary current and turns ratio for load sizing.
Identify whether a transformer is step-up or step-down before installation.
Troubleshoot transformer output that does not match expected voltage.
Teach or learn the ideal transformer equations in electrical engineering courses.
Cross-check nameplate data against measured winding turns on reclaimed transformers.
Size secondary-side fusing by knowing the expected secondary current.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu