Transformer Turns Ratio Calculator
This transformer turns ratio calculator finds the turns ratio of a transformer from the primary and secondary voltages. Enter both and the transformer turns ratio calculator returns the ratio as n : 1. The transformer ratio is the same for the turns and the voltages, so it tells you at once whether the transformer steps up or down.
Results
The Transformer Turns Ratio Calculator is part of our AC & Three Phase collection. Browse more ac & three phase to size, convert, and check your work alongside this tool.
What the transformer turns ratio calculator shows
A transformer changes voltage in proportion to the ratio of turns on its two windings. This transformer turns ratio calculator divides the primary voltage by the secondary voltage to give that ratio, since the voltage ratio equals the turns ratio. The transformer ratio tells you the step at a glance: above one is a step down, below one is a step up.
How to calculate transformer ratio
To calculate a transformer ratio, divide the primary voltage by the secondary voltage:
The turns ratio equals the voltage ratio because each turn links the same magnetic flux. The current ratio is the inverse, so a step down transformer that halves the voltage doubles the available current. This is the core relation the transformer voltage ratio calculator uses.
Transformer ratio example
A transformer with 240 V on the primary and 12 V on the secondary. The transformer ratio is:
So it is a 20 to 1 step down transformer, with 20 primary turns for each secondary turn. Change the secondary voltage and the transformer turns ratio calculator updates the ratio at once.
How to use the transformer turns ratio calculator
Enter the primary voltage and the secondary voltage in the same unit, then the transformer turns ratio calculator returns the ratio as n : 1. Use the rated voltages from the transformer nameplate. Because the ratio is dimensionless, the units cancel, so any consistent voltage unit works. To calculate a transformer ratio the other way, a known turns ratio and one voltage give the other.
Reading the transformer turns ratio calculator result
A ratio above one means the secondary voltage is lower than the primary, a step down; below one is a step up; and exactly one is an isolation transformer that changes nothing but the connection. The current ratio is the inverse of the turns ratio, so stepping the voltage down steps the current up by the same factor, since power is conserved apart from small losses. Use the ratio to check a nameplate or to work out an unknown winding voltage.
When to use the turns ratio calculator
- Checking a nameplate with the turns ratio calculator
- Working out an unknown secondary voltage from the ratio
- Confirming a step up or step down design
- Finding the current ratio from the turns ratio
Common transformer turns ratio calculator mistakes
- Swapping primary and secondary. Primary goes on top; swapping them inverts the ratio.
- Forgetting the current inverts. A step down in voltage is a step up in current.
- Using loaded voltages. Use the rated no load voltages, since losses drop the secondary a little under load.
Tips for accurate results
- Read the rated voltages from the nameplate.
- Remember the current ratio is the inverse of the turns ratio.
- Above one is step-down, below one is step-up.
How to calculate the turns ratio of a transformer
To calculate the turns ratio of a transformer, you only need the two voltages, because an ideal transformer holds the voltage per turn equal on both windings. Divide the primary voltage by the secondary and you have the ratio of turns, Np to Ns, which is why a transformer calculator that works in turns and one that works in voltages give the same number. If you know the turns instead, the ratio is Np divided by Ns directly, and the secondary voltage is the primary divided by that ratio. Real transformers lose a little to winding resistance and core losses, so the measured secondary sits slightly below the ideal, but for sizing and checking a nameplate the voltage ratio is accurate.
Transformer voltage ratio calculator and current
A transformer voltage ratio calculator tells only half the story; the current is the other half. Because a transformer conserves power apart from small losses, whatever it does to the voltage it does the inverse to the current: a 20 to 1 step down that drops 240 V to 12 V raises the available current by about 20 times. That is why the thin high voltage primary and the thick low voltage secondary look so different. Sizing the windings and the fuse on each side needs the current, so pair the ratio with a current calculation to find the current from the power on each side.
Transformer ratio reference
Turns ratio for common primary and secondary voltages, from the transformer turns ratio calculator:
| Primary | Secondary | Turns ratio | Type |
|---|---|---|---|
| 240 V | 12 V | 20 : 1 | Step-down |
| 240 V | 24 V | 10 : 1 | Step-down |
| 120 V | 240 V | 0.5 : 1 | Step-up |
| 230 V | 230 V | 1 : 1 | Isolation |
Transformer Turns Ratio Calculator FAQ
How do you calculate a transformer turns ratio?
Divide the primary voltage by the secondary voltage. The turns ratio equals the voltage ratio, so the transformer turns ratio calculator returns Vp divided by Vs as n : 1.
Is the turns ratio the same as the voltage ratio?
Yes. Each turn links the same flux, so the turns ratio equals the voltage ratio. The current ratio is the inverse of it.
What does a ratio above one mean?
A ratio above one is a step down transformer, with the secondary voltage lower than the primary. Below one is a step up, and exactly one is isolation.
How does current change with the ratio?
The current ratio is the inverse of the turns ratio, so a step down that halves the voltage roughly doubles the available current, since power is conserved.