A load calculation adds conduction through the envelope, solar gain through glass, infiltration, ventilation air, people, lights and equipment, split into sensible and latent parts, at design weather. Residential loads are commonly calculated with Manual J and commercial loads with heat balance or radiant time series methods. Sizing equipment to the real calculated load, not square footage rules, avoids short cycling and humidity complaints.
Rule of thumb said five tons, the cooling load calc came out at 3.2, put in a three and a half and they'll be comfortable.
Rule-of-thumb sizing by square feet almost always oversizes, and oversized equipment short cycles and leaves the space humid.
Related terms
Design conditions
The outdoor weather and indoor targets a heating or cooling system is sized to handle.
Ton
A unit of cooling capacity equal to 12,000 Btu per hour, the heat needed to melt a ton of ice in a day.
Sensible heat
Heat that changes the air's temperature, which you can read on a thermometer.
Latent heat
Heat tied up in moisture, added or removed when water evaporates or condenses without changing air temperature.
Infiltration
Outdoor air that sneaks into a building through gaps in its shell, pushed by wind, stack effect and fans rather than by design.