Transformer Turns Ratio Calculator
By Santaji GadepatilinUpdated October 27, 2026
A transformer changes voltage in proportion to the number of turns on its windings, and changes current the other way. This guide to the transformer turns ratio and secondary voltage explains the relationship, and turns ratio is one of the standard transformer routine tests.
How to use the transformer turns ratio calculator
Enter the primary voltage, primary turns and secondary turns, plus the secondary load current. You get the turns ratio, secondary voltage and primary current. Fill any consistent set — for example, both voltages and primary turns to find secondary turns.
For more background, read Isolation Transformer Working and Uses.
The ideal transformer equations
Vp ÷ Vs = Np ÷ Ns = Is ÷ Ip
Example
230 V primary, 1,000 turns; 52 secondary turns:
- Ratio = 1,000 ÷ 52 = 19.23
- Secondary voltage = 230 ÷ 19.23 = 11.96 V
- With 5 A secondary load, the primary draws 5 ÷ 19.23 = 0.26 A
Turns per volt
Small transformers are often designed by turns per volt, which depends on core area and flux density: turns/V ≈ 1 ÷ (4.44 × f × B × A). For a 50 Hz core with 10 cm² area at 1.2 T, that is about 3.75 turns per volt.
Three-phase ratios
For three-phase transformers, the voltage ratio uses line voltages, and the vector group affects the turns ratio:
| Connection | Line voltage ratio |
|---|---|
| Yy or Dd | Np ÷ Ns |
| Dy (delta primary, star secondary) | Np ÷ (√3 × Ns) |
| Yd (star primary, delta secondary) | √3 × Np ÷ Ns |
A Dyn11 11 kV/433 V distribution transformer has a phase-winding turns ratio of 11,000 ÷ 250 = 44.
Real transformers
On load, the secondary voltage drops slightly because of winding resistance and leakage reactance (regulation, typically 2–5%). Transformers are often wound with a few percent extra turns to compensate.
Frequently asked questions
What is the transformer turns ratio?
The ratio of primary turns to secondary turns, Np ÷ Ns. For an ideal transformer it equals the voltage ratio Vp ÷ Vs and the inverse of the current ratio.
How do I calculate secondary voltage?
Vs = Vp × Ns ÷ Np. A 230 V transformer with 1,000 primary and 52 secondary turns gives about 12 V.
Why does current change the opposite way to voltage?
Power in equals power out (ignoring losses), so if voltage is stepped down by 20 times, current is stepped up by 20 times.
How is the turns ratio tested on site?
With a transformer turns ratio (TTR) tester. The measured ratio should be within ±0.5% of the nameplate ratio on each tap.