BTU & Heat Load Calculator
Get a quick cooling load for a space from its area, insulation/climate, occupants, and window area — the classic square-foot screening method, reported in BTU/hr and tons. Use it to ballpark equipment size in the first conversation, not to buy equipment: these factors are estimating rules of thumb, and an ACCA Manual J load calculation governs permit sets and final selection. Free, no login.
Space
Cooling Load (est.)
Square-foot estimating method (area × BTU/ft² climate factor, plus occupant and window adders) — a planning-grade estimate only, NOT a substitute for an ACCA Manual J load calculation.
How this calculator works
The square-foot method: a per-square-foot cooling factor for the envelope, plus flat adders for people and sunlit glazing, then a conversion to tons:
BTU/hr = area × factor + occupants × 400 + window ft² × 30 · tons = BTU/hr ÷ 12,000
Be clear about what this is: a screening estimate, not a Manual J. The three climate/insulation factors (20, 25, 35 BTU/hr·ft²) are coarse planning rules of thumb with no code citation behind them, and the 400 BTU/hr-per-person adder assumes seated, light activity — sensible plus latent combined. A real load calculation works wall by wall from orientation, glazing type, infiltration, duct location, and the local design temperatures, and it routinely lands well away from any square-foot number in both directions. Oversizing from a fat estimate is not conservative: an oversized air conditioner short-cycles and dehumidifies poorly. ACCA Manual J (or an equivalent load calculation) governs permit sets and equipment selection.
Worked example
A 1,000 ft² space of average construction with 2 occupants and 80 ft² of window (the calculator’s defaults):
- Base: 1,000 × 25 = 25,000 BTU/hr.
- People: 2 × 400 = 800 BTU/hr.
- Windows: 80 × 30 = 2,400 BTU/hr.
- Total: 25,000 + 800 + 2,400 = 28,200 BTU/hr, and 28,200 ÷ 12,000 = 2.35 tons — so you would be comparing 2- to 2.5-ton equipment, pending the Manual J.
The calculator opens with these inputs — the result panel shows the same 28,200 BTU/hr.
Reference: the estimating factors (labeled)
The factor table the engine carries — each row is a square-foot rule of thumb, exported with its basis and editable in spirit, never presented as fact:
| Construction / climate | Factor | Basis |
|---|---|---|
| Well-insulated / mild | 20 BTU/hr·ft² | Square-foot rule of thumb (mild) |
| Average | 25 BTU/hr·ft² | Square-foot rule of thumb (average) |
| Poorly insulated / hot | 35 BTU/hr·ft² | Square-foot rule of thumb (hot) |
Plus the two flat adders: 400 BTU/hr per occupant (seated, light activity — sensible + latent) and 30 BTU/hr per ft² of sunlit window. Gyms, kitchens, and shop floors run hotter per person than this tool assumes.
Frequently asked questions
How many BTU per square foot do I need for cooling?
As a planning rule of thumb, figure roughly 20 BTU/hr per square foot for a well-insulated space in a mild climate, 25 for average construction, and 35 for a poorly insulated space in a hot climate — those are the three factors this calculator uses. It then adds about 400 BTU/hr per occupant and 30 BTU/hr per square foot of window. These are estimating numbers, not design values: real loads swing widely with orientation, glazing type, infiltration, and duct location.
Is this calculator a substitute for a Manual J load calculation?
No. This is a square-foot estimate for budgeting and sanity-checking — an ACCA Manual J calculation, which accounts for the actual envelope, orientation, infiltration, internal gains, and local design temperatures room by room, is the authoritative basis for sizing equipment. Many jurisdictions and utility rebate programs require a Manual J before a permit or incentive. Use this tool to rough out a bid or check whether an existing unit is in the right ballpark, then run the real calc before you buy equipment.
What size air conditioner do I need for 1,000 square feet?
With the calculator defaults — 1,000 ft² of average construction, 2 occupants, 80 ft² of window — the estimate is 28,200 BTU/hr, about 2.35 tons, so you would look at 2- to 2.5-ton equipment. The same 1,000 ft² could land near 2 tons well-insulated in a mild climate or near 3 tons poorly insulated in a hot one, which is exactly why the insulation/climate factor matters. Treat any square-foot answer as a starting point and confirm with a Manual J.
How much cooling load does each person add?
This calculator adds 400 BTU/hr per occupant, a round number consistent with ASHRAE Fundamentals values for seated, light activity once you combine sensible and latent heat. People doing physical work give off substantially more, so for a gym, kitchen, or shop floor the per-person load should be higher than this tool assumes.
Why is oversizing an air conditioner a problem?
An oversized unit satisfies the thermostat quickly and short-cycles: it never runs long enough to wring moisture out of the air, so the space ends up cold and clammy, and the frequent starts wear the compressor and waste energy. Slightly undersized-to-right-sized equipment with long run times dehumidifies better and lasts longer. This is the main reason to resist rounding a 2.4-ton estimate up to 3 tons "to be safe" — and another reason the final number should come from a Manual J, not a rule of thumb.
Does this tool estimate heating load too?
No — the output is a cooling load. Heating load is driven by different factors, mainly the winter design temperature difference across the envelope and infiltration, and it doesn't get the people and solar window credits that show up here (those gains reduce heating load rather than add to it). For heating, run a Manual J heat-loss calculation against your local winter design temperature.
Method: square-foot cooling estimate (area × BTU/hr·ft² factor + 400 BTU/hr per occupant + 30 BTU/hr per ft² of window), tons at the definitional 12,000 BTU/hr per ton. The factors are uncited estimating heuristics — deliberately labeled as such — and the output is a cooling load only; heating load is a different calculation driven by winter design temperature difference and infiltration. ACCA Manual J (residential) or an ASHRAE-method load calculation (commercial) governs permit sets, and Manual S governs matching equipment to the load. Oversizing hurts dehumidification; never round a screening number up into a purchase.
Pair with the cooling load rules of thumb for sqft-per-ton ranges by building type, the BTU / watts / kW conversion chart for unit translation, the CFM per ton chart for the airflow the load implies, or the Cooling Tonnage Calculator to convert the result between BTU/hr, tons, and kW.