How to use this calculator
- Primary and secondary voltage, enter both sides of the transformer. For an inverter that's 12V (battery) to 220V (mains). For a UPS charging winding it's the reverse, 220V down to 12-15V. For a plain voltage converter (running a 110V appliance off 220V mains) it's 220V to 110V.
- Power rating, the VA (or watts) you need the transformer to handle continuously.
- Core steel quality, ordinary hot-rolled laminations if that's what you have, CRGO if you're buying good-quality cores.
- Current density, lower settings mean thicker wire and a cooler-running transformer, higher settings mean a smaller, cheaper winding that runs hotter.
- Optionally enter your core center-limb width (measure it with calipers on the lamination you actually have) to get a lamination sheet count.
The formulas behind it
Core area follows the standard trade formula used for EI-lamination transformers: area (cm²) = K × √(VA), where K depends on the steel grade, better steel needs a smaller core for the same power. From there, turns per volt comes straight from the transformer EMF equation, 10⁸ ÷ (4.44 × frequency × flux density × core area), multiply that by your primary and secondary voltage to get each winding's turn count. The secondary gets a small extra margin (7% by default) to compensate for voltage drop once the transformer is actually under load.
Wire size works backwards from current: primary and secondary current come from power ÷ voltage, then dividing by your chosen current density (amps per mm²) gives the wire's cross-sectional area, which converts to a diameter and rounds up to the nearest standard SWG, rounding up rather than down is the safe direction, since undersized wire is what actually causes a rewound transformer to overheat.
Inverter, UPS, or just a voltage converter?
These are the same calculation with different numbers, not three different tools. An invertertransformer steps 12V battery voltage up to 220V AC. A UPS charging transformer does the opposite, stepping 220V mains down to the 12-15V needed to charge the battery (most UPS units use two separate windings, one for each direction). A plain voltage converter, for running a 110V import appliance off 220V mains, is the same step-down math, just at a different ratio. Type your own primary and secondary voltage in whichever direction you need and the calculation is identical.
This tool is not for a voltage stabilizer (AVR), which holds one fixed output across a whole range of shifting input voltage using multiple switched taps rather than a single fixed ratio, that's a different design problem with its own calculator.
Design-estimation only: these are the standard formulas used for mains-frequency EI-lamination transformer design, but real-world results depend on winding technique, insulation, core assembly and the exact steel used. This is not a substitute for a qualified transformer winder or electrical engineer, especially above a few hundred VA or on anything mains-connected.
Frequently asked questions
What is SWG and why does the calculator use it instead of AWG or mm²?
SWG (Standard Wire Gauge) is the sizing system almost universally used in transformer rewinding shops. The calculator works out the exact wire diameter your current needs, then rounds up to the nearest standard SWG size, since going one size thicker is always safe and going thinner risks overheating.
Is this the same calculation for an inverter, a UPS and a simple voltage converter?
Yes. An inverter transformer (12V to 220V), a UPS charging transformer (220V to 12-15V) and a plain voltage converter (220V to 110V for a foreign appliance) are all the same underlying design problem, a transformer between two fixed voltages at a given power rating. Just enter your primary and secondary voltage in whichever direction you need.
Does this work for a voltage stabilizer (AVR)?
Not directly. A stabilizer holds one fixed output voltage across a whole range of input voltages by switching between several taps on one winding, it is not a single fixed turns ratio. This calculator is for a standard two-winding transformer with one primary voltage and one secondary voltage.
Why does my result change when I pick a different core steel quality?
Better steel (CRGO over ordinary hot-rolled laminations) carries more magnetic flux before saturating, so it needs both a smaller core area and fewer turns per volt for the same power rating, cheaper steel needs a bigger core and more turns to do the same job without overheating or buzzing.