A 12 × 15 ft bedroom (180 sq ft) with an 8 ft ceiling and average sun needs about 6,000 BTU/h of cooling, the ENERGY STAR figure for 150 to 250 sq ft. Heating the same room in climate zone 4 takes about 8,550 BTU/h. Both are rules of thumb for one space; a whole-house system is sized with an ACCA Manual J load calculation.
How the BTUs are calculated
Cooling follows the ENERGY STAR room air conditioner chart: find the floor area band, read the capacity, then apply ENERGY STAR's adjustments. A heavily shaded room takes 10% less, a very sunny one 10% more, every regular occupant beyond two adds 600 BTU/h and a kitchen adds 4,000 BTU/h. The chart assumes an 8 ft ceiling, so the calculator scales it by height ÷ 8 for taller rooms and applies the percentage before the fixed additions.
Cooling BTU/h = chart BTU × (ceiling ÷ 8 ft) × sun factor + 600 × (people − 2) + kitchen 4,000 + tool watts × 3.412
Heating multiplies the floor area by a BTU-per-square-foot figure for your climate zone, from 30 to 35 in zone 1 up to 50 to 60 in zone 5 (InspectAPedia's table built on the IECC zone map; Bryant gives the same 50 to 60 for cold northern states). New or well insulated buildings sit at the low end of the band, old or leaky ones at the top.
Heating BTU/h = floor area (sq ft) × BTU per sq ft for the zone × (ceiling ÷ 8 ft)
Worked example: a sunny kitchen
A 12 × 15 ft kitchen, 8 ft ceiling, very sunny, four people at dinner:
- Chart, 150 to 250 sq ft: 6,000 BTU/h
- Very sunny: 6,000 × 1.10 = 6,600 BTU/h
- Two people over the first two: 2 × 600 = 1,200 BTU/h
- Kitchen: 4,000 BTU/h
- Total: 11,800 BTU/h, so a 12,000 BTU window unit is the next chart size up.
ENERGY STAR room AC sizing chart
| Area to cool | Capacity needed |
|---|---|
| 100 to 150 sq ft | 5,000 BTU/h |
| 150 to 250 sq ft | 6,000 BTU/h |
| 250 to 300 sq ft | 7,000 BTU/h |
| 300 to 350 sq ft | 8,000 BTU/h |
| 350 to 400 sq ft | 9,000 BTU/h |
| 400 to 450 sq ft | 10,000 BTU/h |
| 450 to 550 sq ft | 12,000 BTU/h |
| 550 to 700 sq ft | 14,000 BTU/h |
| 700 to 1,000 sq ft | 18,000 BTU/h |
| 1,000 to 1,200 sq ft | 21,000 BTU/h |
| 1,200 to 1,400 sq ft | 23,000 BTU/h |
| 1,400 to 1,500 sq ft | 24,000 BTU/h |
| 1,500 to 2,000 sq ft | 30,000 BTU/h |
| 2,000 to 2,500 sq ft | 34,000 BTU/h |
ENERGY STAR warns that an oversized unit "costs more to purchase, wastes energy and does not provide better cooling." Buy the step that covers your number, not two steps above it.
What size window AC do I need?
Use the room's own floor area, not the apartment's. A 10 × 12 ft bedroom (120 sq ft) needs a 5,000 BTU unit; a 16 × 20 ft living room (320 sq ft) needs 8,000 BTU, or 8,800 BTU/h if it faces west with big windows, which rounds up to a 9,000 BTU unit. Past 34,000 BTU/h no single room unit on the chart fits; split the space or look at a mini split size calculator layout with one head per room.
BTUs for a garage and a garage heater
Garages are usually uninsulated and have taller ceilings, so the calculator starts the garage tab on heat, "uninsulated" (the top of the zone band) and a 9 ft ceiling. A 20 × 20 ft two-car garage in zone 4 needs 400 × 50 × 9 ÷ 8 = 22,500 BTU/h. That is:
- an electric garage heater of about 6,594 W, which is a 240 V appliance on its own circuit;
- a propane heater burning about 0.25 gallons an hour at full fire (22,500 ÷ 91,452 BTU per gallon, the EIA heat content);
- about 21.7 cubic feet of natural gas an hour (1,036 BTU per cubic foot).
Insulating the walls and door moves you to the low end of the band: the same garage at "Newer, good" needs 400 × 45 × 1.125 = 20,250 BTU/h. Check the insulation calculator for the R-value that gets you there.
BTU for a shop or workshop
Machines turn their power into heat: every watt adds 3.412 BTU/h (the EIA conversion, 3,412 BTU per kWh). A 24 × 30 ft shop with a 10 ft ceiling reads 18,000 BTU/h on the chart, 22,500 after the ceiling, and a 1,500 W saw running adds 5,118 BTU/h: 27,618 BTU/h, a 30,000 BTU unit.
Propane heater BTU
A heater's BTU rating is usually its input. To keep a space at the number above, pick a heater whose output (input × efficiency) reaches it; a vented unit at 80% needs 25,000 BTU/h of input to deliver 20,000. With an unvented heater, follow the maker's rules on room size and fresh air to the letter, and fit a carbon monoxide alarm.
Calculators for each part of the job
- Whole house: the AC size calculator turns square footage into tons, and the furnace size calculator turns it into a furnace input rating.
- Ductless: the mini split size calculator sizes one head per zone and the outdoor unit.
- Electric heat: the baseboard heater calculator gives watts, heater lengths and amps.
- Air: the CFM calculator converts tons and room volumes to airflow, the duct size calculator sizes one duct, and the ductwork calculator lays out a trunk and branches.
What a rule of thumb misses
Windows, orientation, air leakage, insulation levels and your local design temperatures all change the real load. Florida Solar Energy Center found that over a third of contractors oversize on purpose, and more than half of systems in one Florida study were over 120% of the Manual J load. Use these numbers to buy a room unit or a garage heater, and to check a contractor's quote; ask for a Manual J before buying central equipment.