Skip to content
Free calculators and practical guides for smarter home projects.
DIY Cost Tools

DIY Cost Tools

Transformer Turns Ratio Calculator

Calculate ideal transformer voltage and current relationships from primary and secondary turns.

Use the units shown beside each field. Decimal and scientific notation are accepted for non-integer fields; blanks, commas, fractions and non-finite entries are rejected.

Primary-to-secondary turns ratio10:1
Ideal secondary voltage12 V
Ideal secondary current10 A
Estimated secondary output power114 W

The displayed results apply to the default example. Enable JavaScript to calculate other inputs.

How to use this calculator

  1. Replace the example values with measurements or ratings for the exact vehicle, circuit, component or system.
  2. Keep every value in the labeled unit and use manufacturer data where available.
  3. Select Calculate, then review the result together with the limitations and safety notes.

Formula and method

Np/Ns = Vp/Vs = Is/Ip for an ideal transformer. Output-power estimate applies the entered efficiency to primary apparent watts.

Calculations run in your browser. Results are hidden whenever an input changes or validation fails, preventing an older result from being mistaken for a new calculation.

Worked example

Primary turns = 1000; Secondary turns = 100; Primary RMS voltage = 120 V; Primary RMS current = 1 A; Efficiency = 95 %. The calculator returns Primary-to-secondary turns ratio = 10:1; Ideal secondary voltage = 12 V; Ideal secondary current = 10 A; Estimated secondary output power = 114 W.

The worked example uses the same defaults shown in the interface, allowing the formula and displayed result to be checked together.

Limitations and safety

Real transformers have regulation, magnetizing current, winding resistance, core loss, frequency and insulation limits. Do not design mains equipment from this page alone.

This is an educational planning model. Confirm ratings, tolerances, code requirements and safe procedures with manufacturers and qualified professionals before purchasing, modifying, wiring or operating equipment.

Frequently asked questions

Why does current change opposite to voltage?

In the ideal model power is conserved, so stepping voltage down increases the available current in inverse proportion.

Why does the calculator reject my entry?

Each field shows its supported range. Whole-number fields reject decimals, and all fields reject blanks, words, comma-formatted numbers, fractions and non-finite values.

How should I use the displayed precision?

Keep extra digits while checking the calculation, then round to the precision supported by the measurements, component tolerances and published ratings.

Review notes and reference

Reviewed October 4, 2026. The formula and default example were checked together. The script was tested for defaults, input boundaries, invalid formats, condition-specific errors and recovery after an error. Reference: OSHA electrical safe-work requirements. Explanations and examples are original.

Related calculators

Electrical calculators · Ohm’s Law Calculator · Energy Use Calculator · Search all calculators