Quick answer
A transformer's secondary voltage is V_s = V_p × N_s ÷ N_p. With 240 V on 1,000 primary turns and 50 secondary turns, the secondary is 12 V, a turns ratio of 20 : 1 and a step-down transformer.
Enter the primary voltage and the primary and secondary turns. The calculator returns secondary voltage and ratio.
Related tools: Three-Phase Power Calculator: Real, apparent and reactive power of a balanced three-phase load. Gear Ratio Calculator: Gear ratio, output speed and torque multiplication. Power Converter: Convert watts, kilowatts, horsepower and BTU per hour.
How to use the Transformer Turns Ratio Calculator
- Enter primary voltage (V) (above 0).
- Enter primary turns (above 0).
- Enter secondary turns (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Primary voltage | A number, in V | > 0 |
| Primary turns | A number | > 0 |
| Secondary turns | A number | > 0 |
Formula and method
Formula
V_s = V_p × N_s ÷ N_p
IEEE-754 double precision; shown to the decimals indicated.
Ideal transformer.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Primary voltage = 240; Primary turns = 1000; Secondary turns = 50.
| Result | Value |
|---|---|
| Secondary voltage | 12 V |
| Turns ratio (Np:Ns) | 20 |
| Type | Step-down |
Example 2
Inputs: Primary voltage = 120; Primary turns = 100; Secondary turns = 200.
| Result | Value |
|---|---|
| Secondary voltage | 240 V |
| Turns ratio (Np:Ns) | 0.5 |
| Type | Step-up |
The first line is the headline result (secondary voltage). Below it the transformer Turns Ratio Calculator lists turns ratio (Np:Ns) and type, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Current moves the other way
An ideal transformer conserves power, so a 20:1 step-down raises the current by 20: if the secondary delivers 5 A at 12 V (60 W), the primary draws 0.25 A at 240 V.
Real transformers
Real transformers have winding resistance, core losses and leakage, so efficiencies of 95 to 99% are typical and the output voltage sags under load. Transformers work only with alternating current.
How the result changes with primary voltage
Holding every other input at the first worked example, this table changes only primary voltage. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Primary voltage | Secondary voltage | Turns ratio (Np:Ns) | Type |
|---|---|---|---|
| 120 | 6 V | 20 | Step-down |
| 240 | 12 V | 20 | Step-down |
| 480 | 24 V | 20 | Step-down |
| 960 | 48 V | 20 | Step-down |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Primary voltage: entering "0" shows "Primary voltage must be greater than 0." beside that field and no result.
- Primary turns: entering "0" shows "Primary turns must be greater than 0." beside that field and no result.
Choosing between the Transformer Turns Ratio Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Three-Phase Power Calculator](/engineering/three-phase-power-calculator/) | line-to-line voltage, line current and power factor | Real power |
| [Gear Ratio Calculator](/engineering/gear-ratio-calculator/) | driving gear teeth and driven gear teeth | Gear ratio |
| [Power Converter](/converters/power-converter/) | value, from and to | Horsepower (mechanical) → Watt (W) |
| [Power-to-Weight Ratio Calculator](/engineering/power-to-weight-ratio/) | power, power unit and weight | Power-to-weight |
| [Transmission Gear Ratio Calculator](/engineering/transmission-gear-ratio-calculator/) | transmission gear ratio, final drive ratio and tire diameter | Vehicle speed |
Background: structural and wiring estimates
Structural and electrical results are only as good as their assumptions. Bending stress is the bending moment divided by the section modulus, and an allowable stress is the yield strength divided by a safety factor that depends on the governing design code and the type of load. For a single-phase or DC run the voltage drop uses the round-trip length: V = 2 × L × R per unit length × I.
Recommended limits, such as keeping branch-circuit voltage drop around 3%, differ between sources and code editions. These pages are estimates: follow the applicable code and a qualified engineer or electrician for real work.
Assumptions and limitations
- Real transformers have losses
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for primary voltage and primary turns to confirm this.
Frequently asked questions
What is a step-up transformer?
One with more secondary turns than primary.
Can I use DC?
No, it needs a changing flux.
What is turns per volt?
Primary turns divided by primary voltage.
What does the Transformer Turns Ratio Calculator calculate?
Secondary voltage from primary voltage and turns. It returns Secondary voltage, Turns ratio (Np:Ns) and Type.
What formula does the Transformer Turns Ratio Calculator use?
V_s = V_p × N_s ÷ N_p. Ideal transformer.
What do I need to enter in the transformer Turns Ratio Calculator?
You enter Primary voltage (V), Primary turns and Secondary turns.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Real transformers have losses.
What is an example of a transformer Turns Ratio result?
With primary voltage 240, primary turns 1000 and secondary turns 50, the secondary voltage is 12 V.
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Real transformers have losses.
Related calculators
- Three-Phase Power Calculator — Real, apparent and reactive power of a balanced three-phase load.
- Gear Ratio Calculator — Gear ratio, output speed and torque multiplication.
- Power Converter — Convert watts, kilowatts, horsepower and BTU per hour.
- Power-to-Weight Ratio Calculator — Power per unit weight of a vehicle.
- Transmission Gear Ratio Calculator — Vehicle speed from engine rpm, gear ratio, final drive and tire size.
- Ratio Calculator — Simplify a ratio of two whole numbers.
Sources
- OpenStax, University Physics Volume 2 (Rice University): electricity, circuits and thermodynamics
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.