Transformer Calculator
Calculate voltage, current, power, and impedance transformations with adjustable turns ratio and efficiency. Includes step-up/step-down indicator, VA sizing, and impedance matching—the most comprehensive transformer calculator online.
How to Use the Transformer Calculator
Choose Mode
Select Voltage, Current, Power, or Impedance tab based on what you need to calculate.
Enter Values
Input primary values and the turns ratio (N₁:N₂). For Power mode, set efficiency (typically 95-99%). Unit dropdowns handle V/kV, mA/A/kA, Ω/kΩ.
Read Complete Results
Get secondary values plus turns ratio, step-up/step-down indicator, VA rating, and all derived quantities in one display.
Transformer Formulas
Example Calculations
Standard industrial to residential. If I₁ = 10A, then I₂ = 40A (current steps up).
For running EU/UK equipment in the US. I₂ is halved.
Losses as heat. A 10 kVA transformer handles this load.
Matches tube amp output (~5kΩ) to 8Ω speaker for maximum power transfer.
A standard pole-mounted utility transformer. Typically 25-167 kVA.
Standard Transformer Sizes
| kVA Rating | 240V Max Current | 480V Max Current | Common Use | Approx. Weight |
|---|---|---|---|---|
| 15 kVA | 62.5A | 31.3A | Small commercial | ~150 lbs |
| 25 kVA | 104.2A | 52.1A | Residential pole-mount | ~250 lbs |
| 50 kVA | 208.3A | 104.2A | Small building | ~400 lbs |
| 75 kVA | 312.5A | 156.3A | Multi-unit residential | ~500 lbs |
| 100 kVA | 416.7A | 208.3A | Medium commercial | ~700 lbs |
| 500 kVA | 2,083A | 1,042A | Large commercial | ~3,000 lbs |
| 1,000 kVA | 4,167A | 2,083A | Industrial | ~6,000 lbs |
| 2,500 kVA | 10,417A | 5,208A | Substation | ~15,000 lbs |
Transformer Efficiency by Size
| Size | Typical Efficiency | Losses at Full Load | Notes |
|---|---|---|---|
| Wall-wart (5-20W) | 60-85% | 3-8W | Linear regulators, worst efficiency |
| Small (50-500VA) | 90-95% | 2.5-25W | Bench supplies, audio transformers |
| Medium (1-50 kVA) | 95-98% | 50W-1kW | Commercial distribution |
| Large (100-2500 kVA) | 98-99% | 1-25kW | Industrial distribution |
| Utility (5-500 MVA) | 99.0-99.7% | 15-1500kW | Transmission substations |
⚠️ Transformer Safety
- Never exceed VA rating: Overloaded transformers overheat, degrade winding insulation, and can catch fire. Sustained overloading shortens transformer life exponentially—even 10% overload can reduce life by 50%.
- Harmonic derating: Non-linear loads (servers, VFDs, LED drivers) create harmonics that cause additional heating. Derate by 10-30% for harmonic-rich loads, or use K-rated transformers (K-4, K-13, K-20).
- Ventilation: Dry-type transformers need adequate ventilation. Minimum 12 inches clearance on all sides. Never install in enclosed spaces without forced air cooling.
- High-voltage hazard: Transformer secondaries can produce lethal voltages. Step-up transformers are especially dangerous—even a small 120V to 480V step-up can kill. Always use appropriate protective equipment and lock-out/tag-out procedures.
Understanding Transformer Design
Transformers are among the most efficient machines ever invented, with large units achieving 99.7% efficiency—meaning only 0.3% of energy is lost as heat. This remarkable efficiency is why AC power won the "War of Currents" in the 1890s: transformers allow voltage to be stepped up for efficient long-distance transmission and stepped down for safe local use. DC cannot be transformed without first converting to AC.
The turns ratio is the fundamental design parameter. A transformer with a 10:1 ratio steps 4,800V down to 480V, or steps 48V up to 480V, depending on which winding receives input. The same ratio simultaneously transforms current inversely (a 10:1 step-down in voltage means a 1:10 step-up in current) and impedance by the square of the ratio (a 10:1 ratio reflects impedance 100:1).
Impedance transformation is a powerful property that goes beyond simple voltage conversion. In audio engineering, output transformers match the high impedance of vacuum tube amplifiers (2-10 kΩ) to the low impedance of speakers (4-16 Ω). In RF engineering, impedance matching transformers (baluns) ensure maximum power transfer between antennas and transmission lines. In power electronics, impedance matching optimizes solar inverter connections to the grid.
For related electrical calculations, try our Watts to Volts Calculator for basic voltage calculations, Amps to VA Calculator for transformer VA sizing from current, AC Wattage Calculator to determine the real power, apparent power, and reactive power of AC loads connected to your transformer's secondary, or our Big Number Calculator for precise arithmetic with very large numbers.