Transformer Ratio Calculator
Calculate turns ratio, secondary voltage, secondary current, and impedance transformation for any transformer.
📖 What is a Transformer Ratio?
A transformer is an electromagnetic device that transfers electrical energy between two circuits through mutual induction. It consists of two coils (windings) — the primary and secondary — wound around a shared magnetic core. The turns ratio is the relationship between the number of turns (loops of wire) in these two windings, and it determines how the transformer changes voltage and current.
The turns ratio n = N₁/N₂ governs three fundamental transformations: voltage (V₂ = V₁ / n), current (I₂ = I₁ × n × η, where η is efficiency), and impedance (Z₁ = n² × Z₂). Understanding this ratio is essential when designing power supplies, selecting isolation transformers, matching audio amplifiers to speakers, and working with measurement transformers (CTs and VTs).
There are two categories based on function: a step-down transformer (n > 1) reduces voltage — used in household power adapters and distribution transformers that bring grid voltages down to usable levels. A step-up transformer (n < 1) increases voltage — used in power generation to raise generator output to transmission-line voltages. Both obey the same core equations and the law of energy conservation.
Real transformers have winding resistance, core losses (eddy currents and hysteresis), and leakage inductance. Efficiency is typically 95–99% for well-designed power transformers and 85–95% for smaller general-purpose units. This calculator lets you set a custom efficiency to reflect real-world performance.
📝 Transformer Formulas
n = N₁ / N₂ = V₁ / V₂
Secondary Voltage:
V₂ = V₁ × (N₂ / N₁) = V₁ / n
Secondary Current (with efficiency):
I₂ = I₁ × n × η
Apparent Power (VA):
S = V₁ × I₁ = V₂ × I₂ / η
Impedance Ratio:
Z₁ = n² × Z₂
Where: N₁/N₂ = primary/secondary turns | V₁/V₂ = primary/secondary voltage | I₁/I₂ = primary/secondary current | η = efficiency (0–1) | Z = impedance (Ω)
✍️ How to Use This Calculator
- Select the mode: find secondary voltage, calculate turns ratio for a desired voltage, or find reflected impedance.
- Enter the known values — primary voltage, turns count, and optionally primary current and efficiency.
- Click Calculate to see the secondary voltage, current, apparent power, and transformer type (step-up/step-down).
- For the Impedance mode, enter turns and load impedance to find the reflected primary impedance — useful for audio amplifier matching.
📄 Example Calculations
V₁ = 230 V, N₁ = 1000 turns, N₂ = ?
Required V₂ = 12 V
n = 230 / 12 = 19.17
N₂ = N₁ / n = 1000 / 19.17 ≈ 52 turns
Example 2 — Audio impedance matching:
Tube amplifier output impedance: Z₁ = 5000 Ω
Speaker impedance: Z₂ = 8 Ω
Required turns ratio: n = √(Z₁/Z₂) = √(5000/8) = √625 = 25:1
So N₁/N₂ = 25 — for every 25 primary turns, 1 secondary turn.