Transformer Voltage and Turns Calculator

Calculate missing transformer parameter: input any 3 of primary/secondary voltage (Vp, Vs) or turns (Np, Ns). Ideal for step-down/up design. Free, no signup.

Instantly solve for any missing value—primary voltage (Vp), secondary voltage (Vs), primary turns (Np), or secondary turns (Ns)—using the ideal transformer equation.

Just enter any three known quantities, and the tool calculates the fourth in real time—no manual algebra, no guesswork.

Formula & Working Principle

Fundamental Transformer Equation

All calculations are based on the ideal transformer model, governed by the turns ratio law:

Vp / Vs = Np / Ns

Where:

• Vp = Primary voltage (AC input, in volts)
• Vs = Secondary voltage (AC output, in volts)
• Np = Number of turns in the primary winding
• Ns = Number of turns in the secondary winding

Key Assumptions

This calculator assumes an ideal transformer, meaning:

• No core losses (hysteresis or eddy currents)
• No copper losses (winding resistance = 0)
• No leakage flux (perfect magnetic coupling)
• No magnetizing current

These simplifications are standard in design-phase estimation, where quick parametric validation is more critical than real-world efficiency modeling. For detailed performance analysis, use finite-element simulation tools.

How It Works

The equation can be rearranged to solve for any unknown:

• Vp = Vs * (Np / Ns)
• Vs = Vp * (Ns / Np)
• Np = Ns * (Vp / Vs)
• Ns = Np * (Vs / Vp)

Our tool automates this logic—you just input three values.

Practical Example

Scenario: You’re designing a step-down transformer to convert a 480 V industrial supply to 120 V for control circuitry. The primary winding will have 800 turns. How many secondary turns are needed?

Given:

• Vp = 480 V
• Vs = 120 V
• Np = 800

Calculation:

Ns = Np * (Vs / Vp) = 800 * (120 / 480) = 800 * 0.25 = 200

Result: 200 secondary turns are required.

This exact calculation takes one second in the tool—just enter the three known values and get the answer instantly.

Common Applications

• Step-Down Transformer Design
  Convert 480 V to 120 V for control circuits. Input Vp, Vs, Np → get Ns.
• Step-Up Transformer Sizing
  Build a 12 V to 240 V inverter stage. Input Vp, Vs, Ns → get Np.
• Isolation Transformer Specification
  Verify 1:1 ratio (e.g., Vp = Vs = 230 V → Np = Ns).
• Rapid Prototyping & BOM Estimation
  Estimate wire length and layer count from calculated turns for mechanical integration.
• Educational Validation
  Students and instructors verify homework or lab results instantly.

Who Is This For?

This tool is designed for professionals who need fast, reliable transformer parameter estimation during the conceptual or specification phase:

• Electrical Engineers – Designing power supplies, motor drives, or distribution systems
• Power System Designers – Specifying transformers for industrial or utility applications
• Industrial Equipment Architects – Integrating custom magnetics into machinery
• R&D Engineers – Prototyping new energy conversion topologies
• Technical Educators & Students – Teaching or learning transformer fundamentals

Note: This tool is not intended for final production validation, where thermal, efficiency, and regulatory compliance must be modeled with advanced simulation software.

Frequently Asked Questions

What is the formula for transformer winding?
The core formula is Vp / Vs = Np / Ns. This relates input/output voltages to the number of turns in each coil.

How do you calculate transformer turns?
Enter any three of: primary voltage, secondary voltage, primary turns, or secondary turns. The calculator solves for the missing value using the ideal transformer equation.

Can a transformer go from 120V to 240V?
Yes—that’s a step-up transformer with a 1:2 turns ratio. For example, 100 primary turns require 200 secondary turns.

How to find secondary turns from voltage?
Use Ns = Np * (Vs / Vp). Our calculator does this automatically when you input Vp, Vs, and Np.

What does “ideal transformer” mean?
An ideal transformer has no losses (no resistance, no core loss, perfect coupling). It’s used for theoretical design estimates, not real-world efficiency prediction.

Is this calculator accurate for real transformers?
It’s accurate for initial sizing and ratio validation. Real transformers require derating for losses, regulation, and temperature—but this tool gives you the correct starting point.

Do I need to sign up to use it?
No. The calculator is free, requires no email, and works instantly on any device.