How BTUs are calculated
The cooling estimate starts from a base for the floor area: 20 BTU/h per square foot, or the ENERGY STAR room air conditioner chart figure for that area if it is larger (it is for most single rooms under about 1,000 ft²). It then scales the base for ceiling height, insulation, sun and climate, and adds fixed amounts for extra people and for cooking.
floor area = length × width
height factor = ceiling height ÷ 8 ft
base = the larger of area × 20 and the ENERGY STAR chart value for the area
cooling = base × height factor × insulation × sun × climate
+ 600 × (people − 2, never below 0) + 4,000 if a kitchen
Cooling is then converted to equipment sizes. A window or room unit is the smallest standard size that covers the load (5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 15,000, 18,000, 24,000 BTU/h). Above 24,000 BTU/h there is no window unit, and the room or zone is central or mini-split territory.
tons = cooling ÷ 12,000
central size = tons rounded up to the next half ton, at least 1.5 tons over 12,000 BTU/h
Heating uses a per-square-foot figure for your climate, scaled by the same height and insulation factors:
heating = area × climate BTU per ft² × height factor × insulation
electric heat (kW) = heating ÷ 3,412
furnace input = heating ÷ 0.80 (an 80% AFUE furnace)
The 80% AFUE furnace is today's federal minimum for gas furnaces; furnaces made from December 18, 2028 must reach 95%, which needs less input for the same heat.
The results show cooling and heating rounded to the nearest 100 BTU/h. Equipment is sold in steps, so more precision than that would be false accuracy.
Worked example: 15 × 20 ft room
The calculator opens on a 15 × 20 ft room with an 8 ft ceiling, average insulation and sun, a moderate climate and 2 people.
- Floor area: 15 × 20 = 300 ft². Height factor: 8 ÷ 8 = 1.00.
- Cooling base: 300 × 20 = 6,000 BTU/h, less than the ENERGY STAR chart's 8,000 BTU/h for 300 ft², so the base is 8,000. Cooling: 8,000 × 1.0 × 1.0 × 1.0 × 1.0 = 8,000 BTU/h. With 2 people there is no extra people load, and it is not a kitchen, so the total is 8,000 BTU/h.
- Equipment: 8,000 ÷ 12,000 = 0.67 ton, so a 8,000 BTU/h window unit covers it.
- Heating (moderate climate): 300 × 40 × 1.0 × 1.0 = 12,000 BTU/h. That is 3.5 kW of electric heat, or a 15,000 BTU/h input furnace at 80% AFUE.
Adjustment factors
Every factor below is a rule of thumb. They stack by multiplication, so a sunny, poorly insulated room in a hot climate gets 1.584 times the base cooling load before people and cooking are added.
| Factor | Value | Applies to |
|---|---|---|
| Insulation: poor / average / good | 1.2 / 1.0 / 0.9 | Cooling and heating |
| Sun: shaded / average / sunny | 0.9 / 1.0 / 1.1 | Cooling |
| Climate: cool / moderate / hot | 0.9 / 1.0 / 1.2 | Cooling |
| Ceiling height | height ÷ 8 ft | Cooling and heating |
| Each person beyond two | +600 BTU/h | Cooling |
| Kitchen | +4,000 BTU/h | Cooling |
| Heating, mild / moderate / cold / very cold | 30 / 40 / 50 / 60 BTU/h per ft² | Heating |
Measuring and sizing tips
- Measure the floor wall to wall. For an L-shaped room, split it into rectangles and add the areas, then enter one rectangle with the same total area.
- Count the people who are usually in the room at the warmest time of day, not the most it will ever hold.
- Pick "very sunny" for big west- or south-facing windows with no shade, and "heavily shaded" for rooms under trees or on the north side.
- Window units below 5,000 BTU/h are uncommon. If the load is well under that, the smallest unit is still the right choice; do not step up.
- A unit that serves an open plan with a kitchen should be sized for the whole open area, with the kitchen allowance added once.
When to get a Manual J
Use this estimate for a single room, a window unit or a space heater. For a new central system, a heat pump replacement, an addition or a whole-house mini-split layout, have a contractor run ACCA Manual J (and Manual S for equipment selection). The International Residential Code (Section M1401.3) calls for heating and cooling equipment sized with Manual S from Manual J loads, and many building departments ask for the calculation with the mechanical permit. An oversized system costs more, short-cycles and leaves the house humid. If a contractor proposes simply matching the old unit's tonnage, ask for the load calculation.
Sources
- ENERGY STAR room air conditioner sizing chart (capacity by area, 8 ft ceiling) and adjustments: ±10% for sun or shade, +600 BTU/h per person beyond 2, +4,000 BTU/h for a kitchen
- Whole-area cooling rule of thumb: 20 BTU/h per ft² (about 600 ft² per ton), used where it exceeds the ENERGY STAR chart
- Federal furnace efficiency standard, 10 CFR 430.32(e)(1): 80% AFUE minimum for non-weatherized gas furnaces, rising to 95% for furnaces made from December 18, 2028
- Industry rules of thumb: ceiling-height factor (height ÷ 8 ft), insulation factors (poor 1.2, average 1.0, good 0.9), cooling-climate factors (hot 1.2, moderate 1.0, cool 0.9), heating 30–60 BTU/h per ft² by climate
- Unit conversions: 1 ton of refrigeration = 12,000 BTU/h; 1 kW = 3,412 BTU/h
- Air Conditioning Contractors of America (ACCA), Manual J: Residential Load Calculation
- International Code Council, 2021 International Residential Code, Section M1401.3 (equipment sizing per ACCA Manual S and Manual J)