Transformer Calculator
Enter what you know about the primary and secondary: voltages, turns, current and efficiency. The missing values are filled in.
Give both voltages, both turns counts, or one voltage with both turns counts. Leave the rest empty. Alternating current only.
Worked examples
Bell transformer
230 V primary, 12 V secondary
19.17 to 1 step-down
230 / 12.
Winding data
230 V, 1,000 primary turns, 52 secondary turns
11.96 V
230 x 52 / 1000.
About Transformer Calculator
A transformer changes an alternating voltage using two coils on a shared iron core. The ratio of the voltages equals the ratio of the numbers of turns, so a primary with 1,000 turns and a secondary with 52 turns turns 230 volts into about 12. Voltage and current trade against each other, so a step-down transformer gives a lower voltage and a higher current. This calculator fills in whichever values are missing.
Give the two voltages to get the ratio, give the turns to get the voltage, or give one voltage and both turns counts. Enter a current on either side and an efficiency, and the page gives the current on the other side, the power in and out and the heat lost. Real mains transformers are usually 90 to 98% efficient, with small ones noticeably lower.
How to use Transformer Calculator
- Enter the primary and secondary voltages if you know them.
- Optionally enter the primary and secondary turns.
- Enter the primary current or the secondary current, and the efficiency.
- Read the turns ratio, the missing values, the currents and the power lost.
Common uses
- Checking a mains to low voltage transformer for a project
- Winding a coil with a chosen output voltage
- Working out the primary current for a given load
- Learning the transformer equations for exams
Frequently asked questions
What is the transformer turns ratio?
It is the number of primary turns divided by the number of secondary turns, which equals the primary voltage divided by the secondary voltage. A ratio above 1 steps the voltage down.
How is current related to the voltage?
Power in roughly equals power out, so the current changes in the opposite way to the voltage. A transformer that halves the voltage doubles the current, less the losses.
Can a transformer work on DC?
No. A transformer needs a changing magnetic field, so it works only on alternating current. A steady DC supply does not induce a voltage in the secondary and can burn out the primary.
What is volts per turn?
It is the voltage divided by the number of turns, which is the same on both coils. It is a handy check when you wind your own transformer.