Transformer kVA Sizing Calculator

About this calculator

A transformer's required rating in kVA is calculated from the load it must serve: for single-phase, kVA = (V × I) ÷ 1000, and for three-phase, kVA = (V × I × √3) ÷ 1000, where V is line voltage and I is load current. Adding a headroom margin (commonly 15–25%) for future load growth or inrush before rounding up gives a practical design figure rather than a bare-minimum one.

This calculator takes voltage, current, phase, and your chosen headroom, computes the required kVA, and rounds up to the next standard commercial transformer rating (such as 15, 25, 37.5, 50, 75, or 100 kVA) since transformers aren't custom-built to arbitrary sizes. Electrical engineers and electricians use it during panel or service upgrades to specify a transformer that won't be undersized for the connected load, or oversized enough to waste money and space.