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Transformer Turns Ratio Calculator

Calculate the turns ratio of any transformer and instantly find secondary voltage, current, or the number of winding turns needed for a given design.

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Quick answer

The transformer turns ratio (N) is the ratio of primary turns (N1) to secondary turns (N2), equal to N1/N2. It also equals the ratio of primary voltage to secondary voltage: N = V1/V2. Because power is conserved in an ideal transformer, current scales inversely: I2 = I1 × (N1/N2). A turns ratio greater than 1 means a step-down transformer; less than 1 means step-up.

Formula & method

N = N₁ / N₂ = V₁ / V₂ = I₂ / I₁
  • N — Turns ratio (dimensionless)
  • N₁ — Number of primary winding turns
  • N₂ — Number of secondary winding turns
  • V₁ — Primary voltage (V)
  • V₂ — Secondary voltage (V)
  • I₁ — Primary current (A)
  • I₂ — Secondary current (A)

Core turns ratio identity — the ratio of primary to secondary turns equals the ratio of voltages and the inverse ratio of currents.

V₂ = V₁ / N
  • V₁ — Primary voltage (V)
  • V₂ — Secondary voltage (V)
  • N — Turns ratio N₁/N₂

Secondary voltage from primary voltage and turns ratio.

I₂ = I₁ × N
  • I₁ — Primary current (A)
  • I₂ — Secondary current (A)
  • N — Turns ratio N₁/N₂

Secondary current from primary current and turns ratio (ideal transformer, lossless).

Examples

Example 1: Mains to 12 V step-down transformer
Input
V1 = 240 V, V2 = 12 V
Result
Turns ratio N = 20 : 1
Why
N = V1 / V2 = 240 / 12 = 20. This is a step-down transformer: the secondary has 20 times fewer turns than the primary, reducing the voltage from 240 V to 12 V.
Example 2: Step-up transformer for distribution
Input
V1 = 120 V, V2 = 480 V
Result
Turns ratio N = 0.25 (i.e., 1 : 4)
Why
N = V1 / V2 = 120 / 480 = 0.25. A ratio below 1 indicates a step-up transformer. The secondary voltage is four times higher than the primary voltage.
Example 3: Winding turns and secondary current
Input
N1 = 500 turns, N2 = 50 turns, I1 = 2 A
Result
N = 10 : 1; I2 = 20 A
Why
Turns ratio N = N1 / N2 = 500 / 50 = 10. Secondary current I2 = I1 × N = 2 × 10 = 20 A. Stepping down the voltage by 10× multiplies the available current by 10× (ideal transformer).
Example 4: 110 V to 220 V isolation transformer current
Input
V1 = 110 V, V2 = 220 V, I1 = 4 A
Result
N = 0.5 (1 : 2); I2 = 2 A
Why
N = V1 / V2 = 110 / 220 = 0.5. Secondary current I2 = I1 × N = 4 × 0.5 = 2 A. Doubling the voltage halves the current, keeping power constant at P = 440 W in an ideal transformer.

Frequently asked questions

What is a transformer turns ratio?

The turns ratio is the number of primary winding turns divided by the number of secondary winding turns (N1/N2). It determines how much a transformer steps voltage up or down. A ratio of 10:1 means the primary has 10 turns for every 1 secondary turn, reducing voltage by a factor of 10.

How does the turns ratio affect current?

In an ideal transformer, power is conserved (P = V × I). So if the turns ratio steps voltage down by a factor of N, the secondary current increases by the same factor N. Conversely, a step-up transformer that doubles voltage will halve the available current.

What is the difference between a step-up and step-down transformer?

A step-down transformer has a turns ratio N > 1 (more primary turns than secondary turns), which reduces voltage. A step-up transformer has N < 1 (fewer primary turns), which increases voltage. Both obey the same N = V1/V2 = I2/I1 relationship.

Can I calculate turns ratio from voltage alone?

Yes. For an ideal transformer, N = V1 / V2, so knowing both voltages is sufficient. If you also know the primary current or one winding's turn count, you can derive the secondary current and exact turn counts as well.

What does a 1:1 turns ratio mean?

A 1:1 turns ratio means N = 1; primary and secondary have equal turns. Voltage and current are unchanged. This type is called an isolation transformer and is used to electrically isolate circuits for safety without changing voltage levels.

Why does the turns ratio formula assume an ideal transformer?

Real transformers have copper resistance losses, core hysteresis, eddy current losses, and leakage inductance. The ideal model (N = V1/V2 = I2/I1) ignores these, giving a close approximation for well-designed transformers with efficiencies typically above 95%.

Sources & references

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