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Electromagnetism · AC

Ideal Transformer Calculator

Use the turns ratio to calculate secondary voltage and current, compare primary and secondary power, and classify an ideal transformer.

Formula shown Runs locally Reviewed Aug 11, 2026
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Equation guide

Ideal lossless transformer driven by alternating current

Voltage and turns ratio

Vₛ/Vₚ = Nₛ/Nₚ

Inverse current ratio

Iₛ/Iₚ = Nₚ/Nₛ

Ideal power

VₚIₚ = VₛIₛ
AC transformers

Connect turns, voltage, current, and power

Use RMS voltage and current for sinusoidal AC. Both winding turn counts must be positive whole numbers. Other inputs and displayed results use at most three decimal places. The calculation assumes an ideal lossless transformer.

Vs/Vp = Ns/Np

Enter the primary operating values and both whole-number winding turn counts.

Ideal transformer under load with primary and secondary windings, AC source, currents, voltages, turn counts, and a secondary load
Original SVG by BillC, Wikimedia Commons, used without modification under CC BY-SA 3.0.

Using the model

Symbols, assumptions and limitations

Interpretation notes

The model assumes sinusoidal AC, positive whole-number winding turn counts, perfect magnetic coupling, no winding resistance, no leakage flux, no core losses, and no saturation. Voltage follows the turns ratio, current follows its inverse, and input power equals output power.

Worked examples

See the method in practice

01

Step-down supply

A 230 V primary with 1000 turns and a 100-turn secondary ideally produces 23 V. A 0.5 A primary corresponds to 5 A secondary current.

02

Step-up transformer

If the secondary has five times as many turns, its voltage is five times larger while its current is five times smaller.

Questions

Frequently asked

Why does current change inversely with voltage?

In the ideal model power is conserved. Increasing voltage by a given factor therefore decreases current by the same factor.

Does a transformer work with DC?

A steady DC current does not continuously change magnetic flux and therefore cannot sustain a secondary emf. Transformers require changing current, normally AC.

Why do real input and output powers differ?

Real transformers lose energy through winding resistance, eddy currents, magnetic hysteresis, leakage flux, and auxiliary loads.

Sources & review

Equations and examples are checked against the references below. Results are educational and should be independently verified for safety-critical work.

OpenStax University Physics — Transformers

Written by the STEM Hub editorial team · Reviewed August 11, 2026 · Review methodology