Cooling Load — Method
A first-pass cooling load sums the dominant heat gains in a room: the building fabric, glazing and infiltration (captured by an area gain in W/m2), plus the sensible heat from people (about 100 W each at rest) and from equipment and lighting. Q = area x area-gain + occupants x 100 + equipment. Divide watts by 3517 for refrigeration tons or multiply by 3.412 for BTU/hr. This rule-of-thumb is fine for picking a unit size, but a real design uses ASHRAE Manual J or CIBSE methods with orientation, glazing, U-values and ventilation.
Frequently Asked Questions
Add the heat gains: floor area x area gain (about 120-150 W per m2) + occupants x 100 W + equipment watts. Divide the total watts by 3517 for tons, or multiply by 3.412 for BTU/hr.
As a rough guide, 400-600 BTU/hr per m2 for typical rooms, more for kitchens, sunny rooms or high occupancy. This calculator builds it up from area, people and equipment instead of a flat figure.
One refrigeration ton equals 3.517 kW (12,000 BTU/hr). Divide the cooling load in watts by 3517 to get tons.
It is fine for selecting a unit size. For load that must be right (large glazing, high solar gain, tight efficiency targets) use a full Manual J or CIBSE calculation with fabric and solar data.
Avoid large oversizing - an oversized unit short-cycles, controls humidity poorly and wastes energy. Pick the next standard size above the calculated load, not several sizes up.
HVAC Cooling Load Explained
The cooling load is the rate of heat the air conditioner must remove to hold the room at setpoint. It comes from the building fabric and glazing, outside air leaking in, the people inside, and electrical equipment and lighting.
The quick estimate
Bundle fabric, glazing and infiltration into an area gain (W/m2), add sensible heat from people and equipment, and you have the sensible cooling load. Convert to tons or BTU/hr to match catalogue units.
When to go further
For accuracy, a Manual J or CIBSE calculation separates solar gain by orientation, glazing type and shading, fabric U-values and fresh-air ventilation, and adds latent (moisture) load. Pair with the HVAC duct sizing and heat exchanger tools.
Related: HVAC Duct Sizing, Heat Exchanger.