Transformer Turns Ratio Calculator
Enter the primary voltage, primary current, and winding turns to instantly calculate the turns ratio, secondary voltage, secondary current, and power for an ideal transformer.
For educational and reference use only. Always verify results before use in real-world designs or safety-critical applications. For more information, see Calculation Assumptions and Disclaimer.
How to Use This Calculator
Fill in all four fields and the results update instantly:
Primary Voltage (Vp) — the AC voltage applied to the primary winding, in volts.
Primary Current (Ip) — the current drawn by the primary winding, in amps.
Primary Turns (Np) — the number of wire turns on the primary winding. Must be a positive whole number.
Secondary Turns (Ns) — the number of wire turns on the secondary winding. Must be a positive whole number.
Use the decimal places selector to control output precision from 2 to 4 significant decimal places.
Formula
Turns Ratio
Secondary Voltage
Secondary Current — current behaves inversely to voltage to conserve power:
Power — in an ideal transformer, input and output power are equal:
Where: Vp = primary voltage (V), Ip = primary current (A), Np = primary turns, Ns = secondary turns, Vs = secondary voltage (V), Is = secondary current (A).
Example
Given: Vp = 230 V, Ip = 0.5 A, Np = 1000, Ns = 100
Turns Ratio = 100 / 1000 = 0.1 → expressed as 100:1000 (or simplified 1:10)
Secondary Voltage = 230 × 0.1 = 23 V
Secondary Current = 0.5 × (1000 / 100) = 5 A
Power = 230 × 0.5 = 115 W
This is a step-down transformer — secondary voltage (23 V) is lower than primary voltage (230 V), and secondary current (5 A) is proportionally higher than primary current (0.5 A).
Frequently Asked Questions
What is the transformer turns ratio?
The turns ratio is the ratio of the number of secondary winding turns (Ns) to primary winding turns (Np). It determines how much the voltage is stepped up or stepped down. A turns ratio greater than 1 means step-up; less than 1 means step-down.
How is secondary voltage calculated from the turns ratio?
Secondary voltage (Vs) equals the primary voltage (Vp) multiplied by the turns ratio (Ns / Np). For example, if Vp = 230 V and the turns ratio is 0.1, then Vs = 23 V.
Why does current behave inversely to voltage in a transformer?
In an ideal transformer, power is conserved: Vp × Ip = Vs × Is. Since voltage is multiplied by the turns ratio, current must be divided by it to keep power constant. A step-down transformer that halves voltage will double the available current.
What is an isolation transformer?
An isolation transformer has equal primary and secondary turns (Np = Ns), giving a 1:1 turns ratio. The voltage and current remain the same, but the secondary winding is electrically isolated from the primary — useful for safety and noise reduction.
Do these calculations apply to real transformers?
These formulas assume an ideal transformer with 100% efficiency — no core losses, winding resistance, or leakage flux. Real transformers are typically 95–99% efficient, so actual secondary voltage and current will be slightly lower than calculated.