Transformer Ratio Calculator: Vs / Vp = Ns / Np

The voltage and turns of an ideal transformer.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose which quantity you want to find.

Enter the other values in the units shown.

The calculator gives the answer with the formula, the substituted numbers, and the answer in other units.

Formulas used

  • V_s = V_p\,\frac{N_s}{N_p}
  • N_s = N_p\,\frac{V_s}{V_p}

Worked example

With the default numbers in the calculator above:

  1. The formula, arranged to find the secondary voltage: secondary voltage = (primary voltage x turns on the secondary) / (turns on the primary)
  2. Put the values in (every value in its SI unit): secondary voltage = (230 x 100) / (1000)
  3. The answer: secondary voltage: 23 V
  4. The same answer in other units: 0.023 kV

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: What do you want to find? Turns on the secondary; Secondary voltage (leave it as it is if this is what you are finding) 23 V; Primary voltage (leave it as it is if this is what you are finding) 230 V; Turns on the primary (leave it as it is if this is what you are finding) 1000.

  1. The formula, arranged to find the turns on the secondary: turns on the secondary = (secondary voltage x turns on the primary) / (primary voltage)
  2. Put the values in (every value in its SI unit): turns on the secondary = (23 x 1000) / (230)
  3. The answer: turns on the secondary: 100

Common questions

What is the transformer turns ratio?

The turns on the secondary divided by the turns on the primary, which equals the secondary voltage divided by the primary voltage.

How do I find the secondary voltage?

Multiply the primary voltage by the secondary turns and divide by the primary turns. 230 V with 1000 turns to 100 turns gives 23 V.

What happens to the current?

It changes in the opposite ratio. A step-down transformer gives more current than it takes.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any physics or electrical text (transformers).

Need a calculator we do not have?

Tell us what you would like to calculate. We read every request and build the most asked-for ones.

We store your request (and your email if you give it) to decide what to build. We do not store the numbers you type into calculators.

Embed this calculator on your own page

Copy this code into your page. The calculator runs inside the frame and links back to Drafting Central.

Need this drawn, designed or checked?

Drafting Central is a mechanical engineering and drafting business working with customers across Australia and around the world. A calculator gives you the numbers; we can turn them into drawings and designs.

Need help with a real project?

Talk to Drafting Central →

← All engineering calculators