Sizing a heating or cooling system starts with a load calculation: how much heat the room loses on the coldest design day and gains on the hottest. Guessing by square footage alone leads to oversized, inefficient equipment, so this tool walks through the major heat-transfer paths the way a simplified Manual J does.
How it works
Heat conducts through each part of the building envelope in proportion to its U-value, area, and the indoor-outdoor temperature difference:
Q_surface = U x Area x dT (BTU/h)The tool sums conduction through windows, walls, and the roof or ceiling, then adds the load from air leaking in and out:
infiltration_CFM = (room_volume_ft3 x air_changes_per_hour) / 60
Q_infiltration = 1.08 x CFM x dTFor cooling it also adds about 230 BTU/h of sensible heat per person. The cooling load divided by 12,000 gives the equipment size in tons.
Tips and notes
Use realistic U-values for your construction, and be honest about air-tightness: a leaky older home runs closer to 0.5 air changes per hour while a tight new build can reach 0.35 or lower. The result here is sensible-only and ignores duct losses, latent humidity, and solar orientation, so treat it as a planning figure. Critically, resist the urge to round up: an oversized air conditioner cools the air too fast, shuts off before it pulls out moisture, and leaves the room clammy. Have a contractor confirm with a full Manual J before purchasing.