What a cooling load actually is
The cooling load is the amount of heat a space gains each hour that the air conditioner has to remove, measured in BTU per hour. It is the foundation of right-sizing: a proper Manual J load calculation adds up heat conducted through the walls, roof, and floor, solar heat gain through windows, and internal gains from people, lights, and appliances. This tool uses a simplified additive method so you can estimate a single room, or add rooms up for a whole home.
What adds to the load
Floor area sets the baseline, but heat gain from windows, people, and appliances stacks on top. Here are the typical adders this calculator uses:
| Heat source | Typical load added |
|---|---|
| Floor area | 18 - 28 BTU per sq ft (by insulation & sun) |
| Each window | about 1,000 BTU |
| Each occupant beyond 2 | about 600 BTU |
| Kitchen | about 4,000 BTU |
Simplified planning adders. A full Manual J refines them with window orientation, glazing type, and local design temperatures.
Load first, size second. Once you have the load, convert it to equipment size with the AC BTU Calculator or AC Tonnage Calculator. Padding the number "to be safe" backfires - an oversized unit short-cycles and leaves the air humid.
Cooling Load Questions, Answered
What is a cooling load?
A cooling load is the amount of heat, measured in BTU per hour, that a space gains and the air conditioner has to remove to hold your target temperature. It combines heat conducted through walls, roof, and floor, solar heat gain through windows, and internal heat from people, lights, and appliances. Sizing equipment to the load is what a Manual J load calculation does.
How do I calculate the cooling load of a room?
A simplified method multiplies the room's square footage by 18 to 28 BTU depending on insulation and sun, then adds about 1,000 BTU per window, 600 BTU per occupant beyond the first two, and 4,000 BTU if it is a kitchen. The calculator above does that math. A full Manual J refines it with orientation, window type, and local design temperatures.
How many BTU do I need per square foot for a room?
Plan on roughly 18 BTU per square foot for a well-insulated, shaded room, about 22 for an average room, and up to 28 for a poorly insulated or sunny one. Then add for windows, people, and appliances. That is why two rooms of the same size can need very different capacity.
Do windows and kitchens increase cooling load?
Yes, significantly. Each window adds roughly 1,000 BTU of solar and conductive heat gain, and a kitchen adds about 4,000 BTU from cooking appliances. People add heat too, around 600 BTU each beyond the first two. A sunny, window-heavy kitchen can need far more cooling than its floor area alone suggests.
What is the difference between room load and whole-house load?
Room load is the cooling a single room needs, useful for sizing a ductless mini-split or balancing airflow. Whole-house load is the sum of all room loads and sets the size of a central system. Whole-house sizing also applies a diversity factor, since not every room peaks at the same time, so it is not simply every room added at maximum.
Why not just oversize the air conditioner?
An oversized AC cools the air fast but shuts off before it removes humidity, so the space feels clammy and temperatures swing. It also short-cycles, which wears out the compressor and wastes energy. Matching capacity to the actual load, rather than padding it, is what keeps a home comfortable and efficient.
Get an Exact Load Calculation.
We run a full Manual J - room by room - and size the system or mini-split to match, with a flat-rate written quote and no obligation.