BTU Calculator

The rule of thumb, and what it is worth

The number every contractor carries in their head is 20 to 30 BTU per hour per square foot for cooling, which lands around 500 square feet per ton in a mixed climate. One ton is 12,000 BTU/hr — the rate at which melting a ton of ice over 24 hours absorbs heat, which is where the unit came from.

That range is wide for a reason. The bottom of it is a well-sealed, well-insulated house in a cool climate with modest glazing. The top of it is a leaky older house on the Gulf Coast with a lot of west-facing glass. Same square footage, fifty percent difference in load.

The formula this calculator uses

Envelope load = area × BTU per sq ft for your climate × (ceiling height ÷ 8) × insulation factor × sun factor

Window load = number of windows × 400 to 1,000 BTU/hr each, depending on glazing

Occupant load = (people − 2) × 600 BTU/hr

Kitchen load = 4,000 BTU/hr where cooking happens

Total ÷ 12,000 = tons

The ceiling-height term matters because you are conditioning a volume, not a floor. A 1,600 sq ft house with 10 ft ceilings has twenty-five percent more air in it than the same plan at 8 ft, and the rule-of-thumb figures all assume 8.

A worked example

A 1,600 sq ft house in a mixed climate, 8 ft ceilings, average insulation, 12 double-pane windows, normal sun, three occupants, kitchen included.

• Envelope: 1,600 × 25 × 1.0 × 1.0 × 1.0 = 40,000 BTU/hr
• Windows: 12 × 600 = 7,200 BTU/hr
• Occupants: (3 − 2) × 600 = 600 BTU/hr
• Kitchen: 4,000 BTU/hr
• Total: 51,800 BTU/hr = 4.3 tons

Nearest equipment size: 4.5 tons. And here is the important part — if a Manual J came back at 3.5 tons, the Manual J is right and this is wrong. What this calculation is good for is spotting the contractor who wants to sell you five tons for this house.

Why oversizing is the expensive mistake

The instinct is that bigger is safer. With air conditioning, it is the opposite, and it is the single most common failure in residential HVAC.

An oversized system cools the air to the thermostat setpoint very quickly and shuts off. That sounds efficient. It is not, for two reasons.

It never dehumidifies. Removing moisture is a slow process — humid air has to spend time passing over a cold coil so water condenses out and drains away. A unit that runs for six minutes and stops has barely started wringing out the air. You end up at 74 °F and 65% relative humidity, which feels clammy, cold and unpleasant, and which grows mould. A correctly sized unit runs long, steady cycles on a design day and pulls the humidity down with the temperature. In hot-humid climates the latent load is half the job.

It short-cycles. Compressor starts are where the wear and the inrush current live. A unit that starts and stops six times an hour instead of twice wears out its compressor and contactor years early, and uses more power doing it than a longer cycle would.

Undersizing is a real problem too — the system runs continuously on the hottest afternoons and never catches up — but it is far rarer, and its failure mode is discomfort on ten days a year rather than a house that is damp for the whole season.

Manual J is the real answer

ACCA Manual J is the industry-standard residential load calculation, and it is what your permit office is increasingly likely to ask for. It goes room by room and accounts for the things a per-square-foot number cannot see: which direction each window faces and what shades it, actual R-values in each assembly, measured or estimated infiltration, whether the ducts run through a 130 °F attic and how leaky they are, floor construction, and internal gains from appliances and lighting.

Properly done, Manual J routinely returns a smaller number than the rule of thumb — often twenty to forty percent smaller on a modern tight house — which is exactly why it is worth doing. Manual S then selects equipment to match that load, and Manual D sizes the ducts to carry the airflow. Ask any bidding contractor for the Manual J output; the ones who do it will hand it over, and the ones who do not will size by square feet and round up.

Heating is a different number

Do not assume the cooling tonnage sets the furnace or heat pump size. Heating load is driven by the temperature difference between inside and your design outdoor low, which in a northern climate is far larger than the summer difference. The same house can need three tons of cooling and 60,000 BTU/hr of heat — or, in Florida, five tons of cooling and almost no heat at all. Size each one to its own load.