Transformer Turns Ratio Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/5/2026
A transformer's secondary voltage is given by V2 = V1 × (N2 ÷ N1), and the current by I2 = I1 × (N1 ÷ N2), where N1 and N2 are the numbers of turns in the primary and secondary windings. A transformer with 1000 turns on the primary and 100 on the secondary, fed with 230V, delivers 23V on the secondary.
Explanation
A transformer exploits the phenomenon of electromagnetic induction to change an alternating voltage with no direct electrical contact between the two circuits: the alternating current in the primary winding creates a varying magnetic field in the shared core, which in turn induces a voltage in the secondary winding — the same electromagnetic principle behind Ohm's law's companion circuit laws, formalized more directly in our Ohm's law calculator. The ratio between primary and secondary voltage is directly proportional to the ratio of the number of turns in each winding — a simple geometric ratio, independent of the properties of the current itself. This calculator assumes an ideal transformer, with no energy loss whatsoever (neither through Joule heating in the windings nor through magnetic losses in the core): in that case, electrical power is fully conserved between primary and secondary, which forces the current to vary in the OPPOSITE direction of the voltage — a transformer that steps down the voltage mechanically increases the available current in the same proportion, and vice versa. This is the principle that allows electricity to be transported at very high voltage (and therefore low current, to limit Joule losses in the lines) over long distances, before being stepped down in successive stages down to household voltage.
Example: a step-down transformer from 230V to 23V
Inputs
Primary voltage: 230 V. Primary current: 1 A. Primary turns: 1000. Secondary turns: 100.
Calculation
V2 = 230 × (100 ÷ 1000) = 23 V. I2 = 1 × (1000 ÷ 100) = 10 A.
Result
This transformer delivers 23 V on the secondary, with 10 A available — ten times more than on the primary, in the opposite direction of the voltage.
Frequently asked questions
Why does the current increase when the voltage decreases in a transformer?
Because an ideal transformer conserves total electrical power (P = V × I, the same relationship at the heart of our Ohm's law calculator) between its primary and secondary, neither creating nor losing any. If the voltage is divided by ten on the secondary, the current must necessarily be multiplied by ten so that the product of voltage and current, and therefore the power, stays the same on both sides.
Does a real transformer actually behave exactly like this?
A real transformer always has some losses, mainly through Joule heating in the resistance of the windings and through magnetic losses in the core (eddy currents, hysteresis) — the power actually available on the secondary is therefore always slightly lower than on the primary. Industrial-quality transformers nonetheless reach very high efficiencies, often above 95%, which makes the ideal-transformer approximation useful as a first estimate.
Why does electricity transmission use very high voltages?
Because Joule losses in a power line are proportional to the SQUARE of the current flowing through it, but not to the voltage. By sharply raising the transmission voltage (and therefore proportionally reducing the current, for the same power transmitted), line losses are considerably reduced — which is why the electrical grid uses step-up transformers near power plants, then successive step-down transformers as the electricity gets closer to end users.
Is electromagnetic induction related to the electrostatic force between charges?
They are two distinct phenomena within electromagnetism: induction (used by a transformer) involves a changing magnetic field creating a voltage, while the electrostatic force described by our Coulomb's law calculator is the static attraction or repulsion between charged particles, with no motion or changing field required. Both are part of the same broader theory of electromagnetism, but they describe different physical situations.