Use the ideal transformer equation Vp/Vs = Np/Ns = Is/Ip to find the missing voltage or number of turns, or the turns ratio from two voltages. Add a load current or VA rating to get primary and secondary currents, power and the reflected load impedance.
Last updated:
V
V
A
Optional: gives primary current and VA
VA
If set, overrides the load current
%
100 for an ideal transformer; small units are often 80–90%
Enter your values to see the result.
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How to use the Transformer Turns Ratio Calculator
Choose what to solve for: a voltage, a number of turns or just the turns ratio.
Enter the known voltages and turns.
Optionally enter the load current or VA rating and the efficiency.
Read the result with the turns ratio and transformer type.
Check the currents and reflected impedance in the results and table.
Ideal transformer equations
Find
Formula
Example
Turns ratio
a = Np ÷ Ns = Vp ÷ Vs
230 V → 12 V: a = 19.17
Secondary voltage
Vs = Vp × Ns ÷ Np
230 × 60 ÷ 1,150 = 12 V
Secondary turns
Ns = Np × Vs ÷ Vp
920 × 9 ÷ 230 = 36 turns
Primary current
Ip = Vs × Is ÷ (η × Vp)
12 V × 2 A ÷ 230 V = 0.104 A
Max secondary current
Is = VA ÷ Vs
40 VA ÷ 24 V = 1.67 A
Reflected impedance
Zp = a² × Zs
25² × 8 Ω = 5 kΩ
Common transformer ratios
Application
Voltages
Turns ratio
Type
Low-voltage supply (EU/UK)
230 V → 12 V
19.17 : 1
Step-down
Doorbell / thermostat (US)
120 V → 24 V
5 : 1
Step-down
Halogen lighting (US)
120 V → 12 V
10 : 1
Step-down
Travel converter
120 V → 230 V
1 : 1.917
Step-up
Isolation transformer
230 V → 230 V
1 : 1
Isolation
Distribution transformer
11 kV → 400 V
27.5 : 1
Step-down
Results assume an ideal transformer: no winding resistance, no leakage flux and no magnetising current. Real transformers have regulation (voltage drop under load) and efficiency losses; working on mains-voltage equipment should be left to qualified people.
Frequently asked questions
What is the transformer turns ratio formula?
For an ideal transformer, Vp ÷ Vs = Np ÷ Ns = Is ÷ Ip. The turns ratio a = Np ÷ Ns sets how much the voltage is stepped down (a > 1) or up (a < 1), and the current changes by the inverse ratio.
How do I calculate the secondary voltage?
Vs = Vp × Ns ÷ Np. A transformer with 1,150 primary turns and 60 secondary turns on 230 V gives 230 × 60 ÷ 1,150 = 12 V (no-load, ideal).
How do I calculate primary current?
Power in equals power out in an ideal transformer, so Ip = Vs × Is ÷ Vp. A 12 V, 2 A load on a 230 V primary draws 24 VA ÷ 230 V ≈ 0.104 A. With 90% efficiency the primary current is about 0.116 A.
What does the VA rating mean?
VA (volt-amperes) is the apparent power the transformer can deliver continuously without overheating. The maximum secondary current is VA ÷ Vs: a 40 VA, 24 V transformer can supply about 1.67 A.
Why is the measured secondary voltage higher than calculated?
Small transformers are wound so that they reach their rated voltage at full load. With no load, the output can be 10–20% (or more) higher because there is no voltage drop across the winding resistance. This calculator gives the ideal ratio.
How does a transformer change impedance?
Impedance transforms by the square of the turns ratio: Zp = a² × Zs. An audio transformer with a 25:1 ratio makes an 8 Ω speaker look like 25² × 8 = 5,000 Ω to the amplifier.
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