Transformer Calculator
Secondary voltage, current and turns from a transformer's ratio — with the power check that shows why current goes up as voltage comes down.
A transformer trades voltage for current at constant power.
How the transformer calculator works
A transformer trades voltage for current at constant power. The voltage ratio equals the turns ratio, V₁ ÷ V₂ = N₁ ÷ N₂, while the current ratio is the inverse — step the voltage down by ten and the current available goes up by ten, minus losses.
That inverse relationship is why the grid transmits at hundreds of thousands of volts. Power lost as heat in a cable is I²R, so it goes with the square of the current; raising the voltage a hundredfold cuts the current a hundredfold and the losses ten-thousandfold. Transformers only work on alternating current, because a changing magnetic field is what induces the secondary voltage.
Formula: V₁ ÷ V₂ = N₁ ÷ N₂ = I₂ ÷ I₁
Worked examples
| Inputs | Secondary voltage | Note |
|---|---|---|
| 240 V through a 1000:50 transformer | 12 V | 12 V out, step-down |
| A step-up transformer | 120 V | 120 V out |
| An isolation transformer | 240 V | same voltage, isolated |
FAQFrequently asked questions
How does a transformer change voltage?
The voltage ratio equals the turns ratio. A 1000:50 winding on 240 V gives 12 V.
Why does the current go up when the voltage comes down?
Because power is conserved. Step voltage down by twenty and the available current rises by twenty, less losses.
Why does the grid use high voltage?
Cable losses are I²R, so they scale with the square of current. Raising voltage a hundredfold cuts losses ten-thousandfold.
Do transformers work on DC?
No. A steady current produces a steady magnetic field, which induces nothing. They need alternating current.
What is an isolation transformer?
One with equal turns — no voltage change, but the two circuits share no direct connection, which is a safety property.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Physical Laboratory — the UK's national measurement institute
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.