Transformer Turns Ratio Calculator
Determine a transformer's turns ratio from its primary and secondary voltages in seconds. Use this when designing power supplies, selecting step-up or step-down transformers, or verifying transformer specifications.
About this calculator
A transformer transfers electrical energy between circuits via electromagnetic induction, and the ratio of primary to secondary turns determines how voltage is stepped up or down. The turns ratio is defined as: n = V_primary / V_secondary. This ratio equals the ratio of the number of wire turns on the primary coil to those on the secondary coil. A ratio greater than 1 indicates a step-down transformer (output voltage is lower), while a ratio less than 1 indicates a step-up transformer. For an ideal transformer, the voltage ratio and the turns ratio are identical: N_primary / N_secondary = V_primary / V_secondary. The turns ratio also inversely governs current: a step-down transformer that halves voltage will double current on the secondary side, conserving power (P = V × I).
How to use
Imagine a transformer with a primary voltage of 240 V and a secondary voltage of 12 V. Step 1 — Enter 240 as the primary voltage. Step 2 — Enter 12 as the secondary voltage. Step 3 — The calculator computes: n = 240 / 12 = 20. This means the transformer has a turns ratio of 20:1 — for every 20 turns on the primary coil there is 1 turn on the secondary coil. This confirms it is a step-down transformer reducing mains voltage to a safe 12 V output suitable for low-voltage electronics.
Frequently asked questions
What does a transformer turns ratio of 10:1 mean in practice?
A turns ratio of 10:1 means the primary coil has ten times as many wire turns as the secondary coil. For an ideal transformer this directly means the secondary voltage is one-tenth of the primary voltage — so 2,400 V in would produce 240 V out. Current on the secondary side would be ten times higher than on the primary side, since power is conserved. This type of configuration is a step-down transformer commonly used in industrial and residential distribution systems.
How do I use the turns ratio to find the secondary voltage of a transformer?
Rearrange the turns ratio formula to solve for secondary voltage: V_secondary = V_primary / n, where n is the turns ratio. For example, with a primary voltage of 120 V and a turns ratio of 4, the secondary voltage is 120 / 4 = 30 V. If you know both voltages, you can verify the turns ratio using n = V_primary / V_secondary. This calculator performs either direction of the calculation for you.
Why is the transformer turns ratio important when selecting a transformer for a power supply?
The turns ratio determines whether the transformer will step voltage up or down to the level your circuit requires, making it the primary selection criterion. Choosing the wrong ratio can damage components by supplying too high a voltage or cause poor performance by supplying too little. The ratio also affects the impedance matching between source and load, which is critical in audio and RF applications. Always verify the turns ratio against both the input supply voltage and the required output voltage before purchasing or winding a transformer.