Transformer Calculator
A transformer trades voltage for current in proportion to its winding turns, conserving power. This calculator applies the ideal transformer relations: the voltage ratio equals the turns ratio, and power in equals power out. Enter the primary voltage, the primary and secondary turns, and the apparent power, and it returns the turns ratio, the secondary voltage, and the primary and secondary currents. The model is lossless, so real efficiency will be slightly lower. Use it for quick design checks and to understand step-up and step-down behaviour.
Transformer formula
Turns ratio = primary turns / secondary turns
Secondary voltage = primary voltage * secondary turns / primary turns
Primary current = power / primary voltage
Secondary current = power / secondary voltage
These are the ideal transformer equations from Faraday's law and conservation of power. No empirical constant is involved; the only assumption is a lossless core, which slightly overstates real secondary current.
Step-up and step-down
- A turns ratio above one steps voltage down and current up.
- A turns ratio below one steps voltage up and current down.
- Power is conserved in the ideal model, so voltage gain costs current and vice versa.
- Real transformers lose energy to copper resistance and core hysteresis.
- Transformers are rated in volt-amperes, the apparent power used for the current figures here.
Transformer: frequently asked questions
What is the transformer turns ratio?
The turns ratio is the number of primary turns divided by the number of secondary turns. For an ideal transformer the voltage ratio equals the turns ratio: primary voltage divided by secondary voltage equals primary turns divided by secondary turns. A ratio above one steps voltage down; below one steps it up.
How is secondary voltage calculated?
Secondary voltage equals primary voltage multiplied by secondary turns divided by primary turns. So a 120-volt primary with 240 primary turns and 24 secondary turns gives 120 times 24 divided by 240, which is 12 volts.
How do the currents relate?
In an ideal transformer power in equals power out, so current scales inversely with voltage. Primary current equals power divided by primary voltage; secondary current equals power divided by secondary voltage. Stepping voltage down raises the available current in the same proportion.
Does this account for losses?
No. It models an ideal lossless transformer where output power equals input power. Real transformers have copper and core losses, so efficiency is below 100 percent and the secondary current is slightly lower than the ideal figure. Use the ideal result as a close upper bound.
What is apparent power here?
The power figure you enter is treated as apparent power in volt-amperes for current calculations, which is how transformers are rated. If you work in watts at unity power factor the numbers coincide; otherwise use volt-amperes for the winding currents.
Official sources
- National Institute of Standards and Technology: SI electrical units, the volt and ampere.
- U.S. Department of Energy: Distribution Transformers.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.