Transformer Turns Ratio Calculator
Enter a primary voltage and the turns on each winding and get the secondary voltage in volts — the single number that tells you how far a transformer steps a voltage up or down.
Only the ratio matters
The secondary voltage depends on Ns ÷ Np, not the absolute turn counts. A 100:10 winding behaves exactly like a 1000:100 one.
What is a transformer turns ratio calculator?
One ratio sets the secondary voltage
A transformer turns ratio calculator turns three inputs — the primary voltage and the number of turns on the primary and secondary windings — into a single number: the secondary voltage. A transformer scales voltage in direct proportion to the ratio of turns on its two windings, so Vs ÷ Vp equals Ns ÷ Np. Tapping more turns on the secondary side gives a higher voltage; fewer turns gives a lower one. That makes the turns ratio the simplest way to step mains voltage down to a safe level, step a low voltage up for transmission, or isolate two circuits while keeping the voltage the same.
Enter the primary voltage in volts and the turns on each winding to get the secondary voltage instantly.
One short formula: scale the primary voltage by the ratio of secondary turns to primary turns.
Vs = Vp × (Ns ÷ Np)The ratio Ns ÷ Np is the turns ratio, and the secondary voltage is exactly that ratio of the primary voltage. Keep the primary voltage in volts and the secondary comes out in volts. The turns are plain counts, so they cancel into a pure ratio — what matters is how many times more or fewer turns the secondary has, not the absolute numbers. A ratio above 1 steps the voltage up, a ratio below 1 steps it down, and a ratio of exactly 1 passes it through unchanged.
Suppose a 120 V supply feeds a transformer with 100 turns on the primary and 10 turns on the secondary.
Note the voltage and turns
Primary voltage is 120 V, with Np = 100 turns and Ns = 10 turns — the secondary has ten times fewer turns.
Find the turns ratio
Ns ÷ Np = 10 ÷ 100 = 0.1, a tenth of the primary.
Read the secondary voltage
120 × 0.1 = 12 V — a 10:1 step-down delivers 12 V on the secondary.
The single secondary figure tells a clear story about how your transformer reshapes the voltage. The key insight is that the secondary voltage is purely the primary scaled by the turns ratio Ns ÷ Np, so it depends on the proportion of turns, not their absolute size. When the secondary has more turns than the primary the ratio is above 1 and the voltage steps up toward the windings' proportion; when it has fewer turns the ratio is below 1 and the voltage steps down; when the counts are equal the secondary matches the primary, which is exactly what an isolation transformer does. One companion fact shapes every real design: in an ideal transformer power is conserved, so the current changes inversely to the voltage. Step the voltage down by ten and the available secondary current rises by about ten, keeping the power on both windings roughly equal. Read your result as the ideal, lossless value, then expect a real transformer to deliver a few percent less under load because of winding resistance and core losses.
The formula is exact for an ideal transformer, but a couple of practical points are worth keeping in mind.
Ideal model, real losses, and AC only
The formula assumes an ideal, lossless transformer with perfect magnetic coupling and no winding resistance, leakage, or core losses — a real transformer runs a few percent lower under load. Transformers also work only with alternating current, since a changing magnetic field is what couples the windings. The current changes inversely to the voltage, so a step-down winding carries more secondary current and must be wound with thicker wire to handle it safely.