Transformer Turns Ratio Calculator
Find turns ratio and secondary turns of a transformer.
Results update as you type.
How it works
Transformers change voltage by the ratio of their winding turns, and this calculator finds that turns ratio and the secondary winding count. You enter the primary and secondary voltages and the number of primary turns, and it returns the turns ratio and the required secondary turns. The governing relationship is that in an ideal transformer, the ratio of primary voltage to secondary voltage equals the ratio of primary turns to secondary turns. So the turns ratio is simply the primary voltage divided by the secondary voltage, and knowing the primary turns lets you find how many secondary turns produce the desired output voltage. A step-down transformer has more primary than secondary turns and reduces voltage (like 240 volts to 12 volts), while a step-up transformer has more secondary turns and raises voltage. This is fundamental to power distribution, where transformers step voltage up for efficient long-distance transmission and back down for safe use, and to electronics, where they provide the right voltage for a device. The same ratio also inversely relates current: a step-down transformer that lowers voltage raises current proportionally, since power is conserved (minus small losses). This calculator assumes an ideal transformer; real ones have small losses from winding resistance and core effects. Use it to design or understand a transformer's windings for a target voltage. Enter the voltages and primary turns to find the turns ratio and secondary turns.
Vp / Vs = Np / Ns
In an ideal transformer, the ratio of primary to secondary voltage equals the ratio of primary to secondary turns. The turns ratio is the primary voltage divided by the secondary voltage, and the secondary turns equal the primary turns times the secondary voltage divided by the primary voltage.
Source:Electrical engineering
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter primary voltage, secondary voltage and primary turns.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Turns ratio (Np:Ns) updates instantly as you type — no button to press.
Frequently asked questions
What is a transformer turns ratio?
The turns ratio is the ratio of the number of turns in the primary winding to the secondary winding, which equals the ratio of primary to secondary voltage. A ratio greater than one is a step-down transformer (lower output voltage), and less than one is a step-up transformer (higher output voltage). It determines how the transformer changes voltage.
How do you calculate secondary turns?
Multiply the primary turns by the secondary voltage and divide by the primary voltage, since voltage and turns are proportional. For 1000 primary turns stepping 240 volts down to 12 volts, the secondary needs 1000 times 12 divided by 240, which is 50 turns. The calculator computes this from your inputs.
What is the difference between step-up and step-down transformers?
A step-up transformer has more turns on the secondary than the primary, raising the output voltage above the input. A step-down transformer has fewer secondary turns, lowering the output voltage. Power grids step voltage up for efficient transmission and down for safe distribution, while device adapters step mains voltage down to what electronics need.
How does the turns ratio affect current?
Because a transformer conserves power (apart from small losses), voltage and current change in opposite directions. A step-down transformer that reduces voltage increases current by the same ratio, and a step-up transformer that raises voltage decreases current. So the secondary current equals the primary current times the turns ratio, inversely to voltage.
Does this account for transformer losses?
No. This calculator assumes an ideal transformer where voltage is exactly proportional to turns. Real transformers have small losses from winding resistance, core hysteresis, and eddy currents, so the actual output voltage under load is slightly lower than ideal. For most design purposes the ideal turns-ratio relationship is an accurate and useful starting point.