Transformer turns ratio calculator
Turns, voltages and currents of an ideal transformer.
Fill in the fields and the result will appear here automatically.
Relate RMS voltages and currents in an ideal transformer to its turns ratio. One mode accepts integer N1,N2; the other accepts desired U2. The ratio N2/N1 may be fractional and must not be rounded to a whole turn count. Loss, magnetising current and voltage regulation are omitted; the W result assumes a resistive load with cosφ=1.
How it works
Formula and logic
k=N2/N1=U2/U1, U2=U1 k, I2=I1/k. With RMS inputs and cosφ=1, P=U1 I1=U2 I2. In general AC, UI is apparent power in VA and active P=UIcosφ. U1,U2>0 and I1≥0; entered turn counts are positive safe integers, at most 9007199254740991 as a numeric limit.
Example
500/100 turns, 220 V and 2 A give 44 V, 10 A and 440 W in the ideal resistive model. 220→12 V gives k=12/220≈0.054545: this is not a turn count to round upwards. I1=0 gives zero model I2 and P; actual no-load current is excluded.
Fields and units
- What to find — list option
- Primary turns N1 — turns
- Secondary turns N2 — turns
- Primary RMS voltage — V
- Desired secondary RMS voltage — V
- Primary RMS current — A
How to use
- — Choose known turns or desired secondary voltage and fill only visible inputs.
- — Use consistent RMS voltage and current values.
- — W assumes cosφ=1; a different load needs its power factor.
- — For actual winding, choose integer N1,N2 and recheck their ratio plus magnetic and thermal design.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- OpenStax: ideal transformer with resistive load and RMS OpenStax: RMS and active power with cosφ
- Limitation
- Ideal RMS with cosφ=1 for W; no loss, saturation or certified isolation/current ratings.
FAQ
Why does the current rise when the voltage drops?
Current ratio is inverse turns ratio: k=0.2 gives I2=5 I1. This transforms the entered operating point; it does not certify allowable winding current.
How far is this from a real transformer?
Loss, magnetising current, loaded regulation and saturation are absent. There is no universal percentage error; actual core, frequency and load data are needed.
Can the turns come out fractional?
A ratio can be fractional, such as 12/220. Whole turn counts are separate inputs. Rounding the ratio up to an integer is wrong: choose integer N1,N2 and calculate voltage again.
Does this apply to an autotransformer?
The ideal voltage relationship can apply, but an autotransformer does not provide galvanic isolation. Even 1:1 states only a ratio; it does not establish construction or insulation rating.