Ideal Transformer Calculator — Turns Ratio, Voltage & Current
Compute secondary voltage, secondary current, and apparent power for an ideal transformer given primary voltage, primary turns, and secondary turns.
V
A
Step-down transformer — secondary voltage is lower than primary
- 1
Turns ratio a = N₁ / N₂
100 ÷ 20 = 5a > 1 means step-down; a < 1 means step-up. - 2
Secondary voltage V₂ = V₁ × (N₂ / N₁)
230 × 20 ÷ 100 = 46
How does this calculator work?
V₂ = V₁ × N₂/N₁ and I₂ = I₁ × N₁/N₂ for an ideal transformer. Enter primary voltage (V), primary turns (N₁), and secondary turns (N₂) to get the secondary voltage. Optionally enter primary current for secondary current and apparent power. The model assumes 100% efficiency — real transformers typically achieve 95–99%.
Formula
How this is calculated
An ideal transformer transfers electrical energy between two magnetically coupled coils (windings) with no losses: no core losses, no winding resistance, and 100% magnetic coupling. Its behaviour is governed by two equations derived from Faraday's law. The voltage equation V₁/V₂ = N₁/N₂ says that voltages scale directly with the turns ratio: a primary winding of N₁ turns carrying voltage V₁ induces a secondary voltage V₂ = V₁ × N₂/N₁. If N₂ < N₁ the transformer steps voltage down; if N₂ > N₁ it steps it up.
Because an ideal transformer conserves power (P_in = P_out), and P = V × I, the current must scale inversely with voltage: I₂ = I₁ × N₁/N₂. A step-down transformer that halves the voltage doubles the current. Entering primary current I₁ in the optional field unlocks the secondary current and the apparent power (VA).
Real transformers deviate from this ideal: copper losses in the windings (I²R heating), core losses (hysteresis and eddy currents), and leakage flux all reduce efficiency. High-quality power transformers reach 95–99% efficiency; small signal transformers are typically 85–95%. The ideal transformer model is an accurate approximation for ratio and impedance-matching calculations at mid-frequency and modest current levels.
Frequently asked questions
The turns ratio a = N₁/N₂ is the single number that sets both the voltage and current transformation. Voltage scales by 1/a on the secondary and current scales by a — so a turns ratio of 5:1 halves the secondary current but produces a secondary voltage one-fifth of the primary.
Real transformers lose energy to copper losses (resistance heating in the windings, proportional to I²R), core losses in the iron (hysteresis as the magnetic domains realign each cycle, plus eddy-current circulation losses), and small amounts of flux that do not couple both windings (leakage flux). These are modelled by adding series resistance and a shunt branch to the ideal transformer circuit.
Yes for the turns-ratio relationships — V₂ = V₁ × N₂/N₁ holds at any frequency for an ideal model. However, real audio and RF transformers have frequency-dependent behaviour: inductance limits low-frequency response (causing bass roll-off), and capacitance between windings limits high-frequency response. Impedance matching (Z₁/Z₂ = (N₁/N₂)²) is also a key use in these applications.
Also known as
TG we-Calculate Editorial Team. (2026). Ideal Transformer Calculator — Turns Ratio, Voltage & Current [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/ideal-transformer-calculator
TG we-Calculate Editorial Team. "Ideal Transformer Calculator — Turns Ratio, Voltage & Current." TG we-Calculate. 2026. https://we-calculate.com/calculator/ideal-transformer-calculator.
TG we-Calculate Editorial Team, "Ideal Transformer Calculator — Turns Ratio, Voltage & Current," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/ideal-transformer-calculator
@misc{wecalculate_ideal_transformer_calculator, title = {Ideal Transformer Calculator — Turns Ratio, Voltage & Current}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/ideal-transformer-calculator}}, year = {2026}, note = {TG we-Calculate} }
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