Cooling Load (Manual J approx) Calculator
Sizing air conditioning starts with the cooling load: the rate of heat, in British thermal units per hour, that a system must remove to keep a space comfortable. A full Manual J load calculation accounts for insulation, climate, orientation and many other factors, but a quick approximation captures the largest drivers and is useful for a first estimate. This calculator combines three of them: the floor area, which sets the base load through a per-square-foot factor; the number of occupants, since each person adds body heat; and the number of windows, which admit solar gain. It multiplies the area by a typical 25 BTU per square foot, adds roughly 400 BTU for each occupant and 1,000 BTU for each window, and returns the estimated hourly load along with an approximate ton figure (one ton equals 12,000 BTU per hour). Enter your room's area, occupancy and window count to get a ballpark size. For equipment selection always commission a full Manual J load calculation; this quick estimate is a planning aid, not a substitute. Every figure here is computed deterministically from the formula shown below, with a worked example that reconciles exactly to the calculator defaults.
A quick cooling load is area x 25 + occupants x 400 + windows x 1,000 BTU/hr. A 1,000 sq ft room with 3 occupants and 5 windows needs about 31,200.00 BTU/hr, roughly 2.60 tons.
Cooling load approximation formula
Load (BTU/hr) = A x 25 + P x 400 + W x 1,000
A = floor area in square feet
P = number of occupants
W = number of windows
Tons = load / 12,000
This rule-of-thumb captures area, occupant heat and window gain. A full Manual J calculation adds insulation, climate, orientation and air leakage, and should be used for equipment selection.
Worked example
Suppose the room is 1,000 square feet with 3 occupants and 5 windows.
- Area load: 1,000 x 25 = 25,000.00 BTU/hr
- Occupant load: 3 x 400 = 1,200.00 BTU/hr
- Window load: 5 x 1,000 = 5,000.00 BTU/hr
- Total: 25,000 + 1,200 + 5,000 = 31,200.00 BTU/hr
- Tons: 31,200 / 12,000 = 2.60 tons
The estimated cooling load is 31,200.00 BTU per hour, about 2.60 tons. These are the calculator's default inputs, so the result matches the widget exactly.
Cooling Load (Manual J approx) Calculator: frequently asked questions
What is a cooling load?
A cooling load is the amount of heat, measured in BTU per hour, that an air conditioner must remove to hold a space at the desired temperature. It comes from many sources: heat conducted through walls and roof, sunlight through windows, people, lights and appliances. Sizing equipment to the load keeps a home comfortable and efficient.
Is this the same as a Manual J calculation?
No. Manual J is the detailed, industry-standard residential load calculation that accounts for insulation, climate zone, orientation, air leakage and internal gains. This tool is a simplified approximation of the largest factors for a quick estimate. Use a full Manual J before buying equipment.
What is a ton of cooling?
One ton of cooling equals 12,000 BTU per hour, a unit that dates from the heat needed to melt a ton of ice in a day. Residential systems are commonly sized in tons, so this calculator converts the BTU figure to an approximate ton value.
Why not just oversize the unit?
An oversized air conditioner cools quickly but short-cycles, which wastes energy, fails to remove humidity well and wears the equipment. The Department of Energy recommends right-sizing to the calculated load. Bigger is not better; matching the load gives comfort and efficiency.
What does this estimate leave out?
It does not directly account for insulation quality, ceiling height, climate, sun orientation, air leakage, or heat from lighting and appliances, all of which a full Manual J includes. Treat the result as a starting ballpark, then refine with a professional load calculation. The Department of Energy publishes guidance on efficient cooling.
Official sources
- Home cooling and energy efficiency guidance: US Department of Energy (DOE). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.