About this tool
Transformer Turns Ratio Calculator is a free, in-browser tool for ideal transformer design. Enter the primary and secondary voltages to get the turns ratio; optionally add the primary turns to get the secondary turns, or the primary current to get the secondary current. It also labels the transformer as step-up, step-down or isolation. Nothing is uploaded — all math is local.
For an ideal transformer the turns ratio equals the voltage ratio: N = Vp ⁄ Vs = Np ⁄ Ns = Is ⁄ Ip. Voltage scales with the number of turns while current scales inversely, because power in equals power out. So the secondary turns are Ns = Np ⁄ N and the secondary current is Is = Ip × Vp ⁄ Vs. The ratio is shown in the familiar N:1 (or 1:N) form.
Use it to spec a transformer, wind a custom one, or reason about a power supply. A ratio greater than one is step-down (fewer secondary turns, lower voltage, higher current); less than one is step-up; exactly one is an isolation transformer.
Frequently asked questions
How is the turns ratio defined?
It is the primary-to-secondary ratio: N = Vp ⁄ Vs = Np ⁄ Ns. A 120 V primary and 15 V secondary give a ratio of 8, shown as 8:1 — eight primary turns for every secondary turn.
Why does current go up when voltage goes down?
An ideal transformer conserves power, so Vp · Ip = Vs · Is. If the secondary voltage is lower, the secondary current must be proportionally higher: Is = Ip × Vp ⁄ Vs. Real transformers lose a little to resistance and core losses.
What do step-up, step-down and isolation mean?
Step-down (ratio > 1) lowers voltage and raises current; step-up (ratio < 1) does the opposite; isolation (ratio = 1, 1:1) keeps voltage the same and is used to galvanically separate two circuits for safety.
Are the turns and current fields required?
No. Only the two voltages are needed for the turns ratio and type. Primary turns and primary current are optional; fill them in to also compute the secondary turns and secondary current. All figures stay in your browser.
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