Case 1
- Primary voltage Vp (V)
- 120V
- Primary turns Np
- 100
- Secondary turns Ns
- 10
12.000V
Open with these valuesTransformer Turns Ratio Calculator
12.000V
Result: 12.000 VVoltage follows the turns in direct proportion: Vs = Vp × Ns ÷ Np. Only the ratio matters — 100:10 and 1000:100 both step down 10:1. Current goes the other way: Vp × Ip = Vs × Is, so a tenfold voltage drop buys roughly ten times the secondary current.
The numbers at a glance
Held fixed: Primary voltage Vp (V) 120.000 V, Primary turns Np 100.
| Secondary turns Ns | Result (V) |
|---|---|
| 3 | 3.000 |
| 5 | 6.000 |
| 8 | 9.000 |
| 10Your value | 12.000 |
| 13 | 15.000 |
| 15 | 18.000 |
| 18 | 21.000 |
| 20 | 24.000 |
Worked examples
Case 2
- Primary voltage Vp (V)
- 230V
- Primary turns Np
- 1000
- Secondary turns Ns
- 50
11.500V
Open with these valuesCase 3
- Primary voltage Vp (V)
- 120V
- Primary turns Np
- 10
- Secondary turns Ns
- 100
1,200.000V
Open with these valuesHow it's calculated
Vs = Vp × Ns ÷ Np
- StepDivide the secondary turns by the primary turns — that is the ratio.
- StepMultiply the primary voltage by that ratio.
- ResultMore secondary turns steps up, fewer steps down, equal leaves it alone.
Reference table
| Vp, Np, Ns | What it does | Vs |
|---|---|---|
| 120, 100, 10 | 10:1 step-down | 12 |
| 120, 10, 100 | 1:10 step-up | 1200 |
| 240, 100, 100 | 1:1 isolation | 240 |
| 12, 2, 1 | 2:1 step-down | 6 |
| 230, 1000, 50 | 20:1 step-down | 11.5 |
Questions
How do I calculate a transformer's secondary voltage?
Multiply the primary voltage by the turns ratio Ns ÷ Np. For 120 V with 100 primary and 10 secondary turns that is 120 × 0.1 = 12 V. The voltage scales in direct proportion to the turns.
What is the difference between step-up and step-down?
More secondary turns than primary steps the voltage up; fewer steps it down. With equal turns the voltage is unchanged — an isolation transformer.
Does the exact number of turns matter, or just the ratio?
Only the ratio Ns ÷ Np sets the voltage: 100:10 and 1000:100 both give the same 10:1 step-down. Absolute counts affect core size, magnetising current and frequency response instead.
What happens to the current and the power?
Power is conserved in an ideal transformer, so current changes inversely to voltage: Ip × Vp = Is × Vs. Stepping voltage down by ten raises the available secondary current by about ten.
Is this an ideal transformer calculation?
Yes. The formula assumes perfect magnetic coupling and no winding resistance, leakage or core losses. Real transformers run a few percent lower under load.
Sources and last check
Information, not professional advice.