What is step-up vs step-down?

Step-down has primary voltage higher than secondary (ratio > 1), like 240 V → 12 V. Step-up has secondary higher (ratio < 1), like 120 V → 240 V. The current changes the opposite way.

Is the turns ratio the same as the voltage ratio?

Yes, for an ideal transformer. Np/Ns = Vp/Vs exactly. Real transformers have small leakage and resistance, so the measured voltage ratio differs slightly from the turns ratio.

Does the calculator assume an ideal transformer?

Yes. It assumes 100% efficiency and perfect coupling. Actual secondary voltage and current are a few percent lower under load due to losses; treat the result as a design estimate.

Transformer Turns Ratio Calculator

Enter primary and secondary voltage to get the turns ratio (Np/Ns), whether it steps up or down, and the secondary current for an ideal transformer.

✓ Turns ratio Np:Ns

✓ Step-up / step-down

✓ Secondary current

✓ Vp/Vs = Is/Ip

✓ 100% Free

Turns ratio — Quick answer

Turns ratio equals the voltage ratio and the inverse of the current ratio.

n = Np/Ns = Vp/Vs = Is/Ip

Worked example: 240 V → 12 V gives 20:1 step-down.

Examples

Vp → Vs	Ratio	Type
240 → 12	20:1	step-down
120 → 240	1:2	step-up
480 → 120	4:1	step-down

Ideal transformer: power in = power out, so current inverts.

⚡ Transformer Turns Ratio Calculator

Enter the voltages (and optionally the primary current).

Primary voltage Vp (V)

Secondary voltage Vs (V)

Primary current Ip (A, optional)

Turns ratio (Np:Ns)

—

Ratio n (Vp/Vs)

—

Type

—

Secondary current Is

—

ℹ️ Ideal transformer: Np/Ns = Vp/Vs = Is/Ip. Is = Ip × Vp/Vs. Real units have small losses.

A transformer's turns ratio sets how it changes voltage: n = Np/Ns = Vp/Vs. Because power is conserved in an ideal transformer, current changes the opposite way, so Is/Ip = Vp/Vs too. This calculator gives the ratio, the step-up/down type, and the secondary current.

Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: ideal-transformer relations, recomputed in code.

The relationships

Ideal transformer

n = Np/Ns = Vp/Vs = Is/Ip · Vp·Ip = Vs·Is (power conserved)

If the primary voltage is higher, the transformer steps voltage down (n > 1) and steps current up; if the secondary is higher, it steps up (n < 1) and current down. The turns ratio equals the voltage ratio exactly in the ideal case. Reducing the ratio to whole numbers gives the familiar form like 20:1 or 1:2.

Worked examples

240 V → 12 V:

20:1 step-down

n = 240 ÷ 12 = 20 → 20 primary turns per 1 secondary turn

120 V → 240 V:

1:2 step-up

n = 120 ÷ 240 = 0.5 → secondary has twice the turns

480 V → 120 V at 10 A primary:

4:1 · Is = 40 A

n = 4 · Is = 10 × 4 = 40 A (current steps up as voltage steps down)

Power checks out: 480 V × 10 A = 4,800 VA on the primary, and 120 V × 40 A = 4,800 VA on the secondary. In a real transformer, small losses make the secondary figures a few percent lower under load.

Frequently Asked Questions

What is the turns ratio formula?⌄

n = Np/Ns = Vp/Vs = Is/Ip. 240 V → 12 V = 20:1.

How do I find the secondary current?⌄

Is = Ip × Vp/Vs. 480 V → 120 V, 10 A → 40 A.

Step-up vs step-down?⌄

Step-down: Vp > Vs (n > 1). Step-up: Vs > Vp (n < 1).

Turns ratio = voltage ratio?⌄

Yes, ideally — Np/Ns = Vp/Vs. Real units differ slightly.

Is it an ideal transformer?⌄

Yes — 100% efficient. Real secondary values run a few % lower under load.

Turns ratio — Quick answer