Transformer Calculator

Written byOleh Soloninupdated 25 September 2026

Primary voltage, V

Secondary voltage, V

Apparent power, VA

Equal to the wattage for a heater or an incandescent lamp; for anything else, watts divided by the power factor.

Primary turns

0 if the number of turns is unknown.

Result

Turns ratio18.3333
Typestep-down
Primary current, A0.455
Secondary current, A8.333
Secondary turnsenter the primary turns
Working
k = 220 ÷ 12 = 18.3333; I₁ = 100 ÷ 220 = 0.455 A; I₂ = 100 ÷ 12 = 8.333 A

Turns set the voltage, the core sets the power

Works out the turns ratio from the winding voltages, the primary and secondary currents at a given power, and the secondary turns from the primary's.

In an ideal transformer the winding voltages stand in the same ratio as their turns: V₁ ÷ V₂ = N₁ ÷ N₂ = k. Currents go the other way: a step-down transformer's secondary delivers that many times more current. At 220 → 12 V the ratio is 18.33, and 100 VA means 0.455 A in the primary and 8.333 A in the secondary.

A real transformer loses some power in its windings and core, and its secondary voltage sags a little under load. The calculator models an ideal one, so its currents are a lower bound.

How to use

Enter the primary and secondary voltages and the load power.
If you know the primary turns, enter them and the calculator gives the secondary turns.
Read the turns ratio, the transformer type, the winding currents and the turns per volt.

Winding currents for different transformers

Each row is a separate run of the calculator.

All rows come from the calculator on this page.
Voltages	Power	Ratio	Primary current	Secondary current	Type
220 → 12 V	100 VA	18.33	0.455 A	8.333 A	step-down
220 → 12 V	50 VA	18.33	0.227 A	4.167 A	step-down
220 → 24 V	100 VA	9.17	0.455 A	4.167 A	step-down
220 → 36 V	200 VA	6.11	0.909 A	5.556 A	step-down
12 → 220 V	60 VA	0.0545	5 A	0.273 A	step-up
220 → 220 V	100 VA	1	0.455 A	0.455 A	1:1, no voltage change

Turns per volt

A 220 V primary of 1,100 turns has 5 turns per volt, so a 12 V secondary needs 60 turns. With 880 turns it is 4 turns per volt and 96 turns for 24 V. How many turns the primary needs depends on the core, which the calculator does not choose.

What this calculator shows

The turns ratio and what kind of transformer it makes
Primary and secondary winding currents
Secondary turns when the primary turns are known

What to keep in mind

The transformer is ideal: winding and core losses are not modelled.
Power is apparent power in volt-amperes; for a resistive load it equals watts.

FAQs

How do I find the turns ratio?

Divide the primary voltage by the secondary: 220 ÷ 12 = 18.33. It also equals the ratio of the turns.

How do I work out the secondary current?

Divide the power by the secondary voltage: 100 VA at 12 V is 8.333 A.

How many turns does the secondary need?

The primary turns divided by the ratio: 1,100 ÷ 18.33 = 60.

How does a step-up transformer differ from a step-down one?

By the ratio: above one for step-down, below one for step-up. A 12 → 220 V transformer with a ratio of 0.0545 at 60 VA draws 5 A from the 12 V side and delivers 0.273 A.

What is a 220-to-220 transformer for?

A ratio of 1 only means the two windings have the same voltage. What makes a transformer an isolating one is its construction: windings electrically separated so the secondary has no connection to the mains. A 220-to-220 V isolating transformer is used to separate a device from the supply.

How do I work out the core area?

The calculator does not: rules of the form A = k × √P are empirical, and k depends on the steel, the frequency and the construction. Core area and turns per volt come from core data or the maker's datasheet.

Worked example

A 220 → 12 V transformer

Input: 220 V, 12 V, 100 VA, 1,100 turns

Output: Ratio 18.33, secondary current 8.333 A, 60 turns

Note: That is 5 turns per volt.

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