AC Sizing Calculator

A rule of thumb is 20 BTU/h per sq ft, but accurate sizing depends on insulation, window area and orientation, occupancy, internal heat sources, and outdoor design temperature. Our calculator accounts for all these factors plus a built-in 15% safety margin to cover peak summer loads.

All three measure cooling capacity. 1 tonne of refrigeration = 12,000 BTU/h ≈ 3.517 kW. BTU/h is the US standard, kW is used across Europe and internationally, and tonnes are common amongst HVAC professionals. The calculator displays all three units simultaneously.

AC units remove both sensible heat (lowering air temperature) and latent heat (removing moisture). High humidity increases the latent cooling load, forcing the unit to work harder to dehumidify. We apply a ×1.15 multiplier for average humidity and ×1.30 for humid climates — set automatically per region.

No — oversizing is one of the most common and costly mistakes. An oversized unit short-cycles, which prevents proper dehumidification, causes temperature swings, increases wear, and wastes electricity. Our calculator includes a 15% safety margin — that is sufficient.

Select 'Poor' (U=1.8 W/m²K) for older properties without wall insulation. Choose 'Average' (U=1.2 W/m²K) for standard modern construction. Use 'Well Insulated' (U=0.6 W/m²K) for passive-house standard or recently retrofitted buildings. When unsure, 'Average' is a reliable default for most residential properties.

Significantly — south and west-facing windows in summer receive direct solar radiation adding 200–500 W per m². Shading (blinds or external overhangs) reduces solar gains by 50–70%. In Step 2 you can configure each window's orientation, glazing type, and shading individually for precise solar load calculations.