Utility July 13, 2026 · 8 Min Read

Transformer Calculator – Guide & Formulas

Solve step-up and step-down AC transformer characteristics including primary/secondary turns, voltages, and load currents.

Model mutual electrical induction. Input primary source characteristics alongside primary and secondary winding turns to resolve step-ratios, output voltage levels, and terminal load currents.

Key Takeaway

Use the free Transformer Calculator to solve step-up and step-down ac transformer characteristics including primary/secondary turns, voltages, and load currents. Get instant results with step-by-step explanations.

How to Use the Transformer Calculator

  1. Input the primary AC side supply voltage (Vp).
  2. Input the primary terminal load current rating (Ip) in Amps.
  3. Input the number of primary physical windings (Np).
  4. Input the number of secondary physical windings (Ns).
  5. View terminal output voltages, secondary currents, and conversion direction status.

The Formula

Secondary Voltage (Vs) = Vp * (Ns / Np) | Secondary Current (Is) = Ip * (Np / Ns)

Variable Definitions

  • Vp / Vs: Primary and secondary side RMS voltages respectively
  • Np / Ns: Total counts of physical primary and secondary side wire loops
  • Ip / Is: Primary and secondary circuit load currents in amperes

Analyzing Step-Down Voltage Transformer

Calculate secondary specs for a 120V system with 500 primary turns, 100 secondary turns, and a primary current of 2 Amps.

  1. Step 1: Determine step-ratio multiplier: Ns / Np = 100 / 500 = 0.20.
  2. Step 2: Solve secondary voltage: Vs = 120V × 0.20 = 24 Volts.
  3. Step 3: Solve secondary current: Is = 2A / 0.20 = 10 Amps.
  4. Step 4: Check conservation of power: Primary VA = 120V × 2A = 240 VA. Secondary VA = 24V × 10A = 240 VA.

Frequently Asked Questions

What is a Step-Down vs Step-Up transformer?

Step-Down transformers decrease voltage (Ns < Np) while increasing output current. Step-Up transformers increase output voltage (Ns > Np) while decreasing current.

Why are actual transformers not 100% efficient?

Real-world transformers experience losses due to copper wire resistance (heat), eddy currents in the iron core, and magnetic flux leakage.