What Is a BTU?
A BTU (British thermal unit) per hour is the standard measure of cooling capacity — how much heat a unit can move out of a room. Sizing is a Goldilocks problem: too few BTUs and the unit runs flat-out without ever catching up on a hot day; too many and it blasts the air cold in minutes, shuts off before dehumidifying, and leaves the room clammy. The chart method gets you the right window or portable unit without an engineering visit.
Enter the room's floor dimensions, then the conditions that move the number: ceiling height, how much sun the room takes, how many people regularly occupy it, and whether it's a kitchen. The result is the target BTU/hr rating to shop for — pick the closest available size, rounding up only slightly.
Need the floor area for an odd-shaped space first? Get it from the Square Footage Calculator.
BTU Chart: Room Size to Cooling Capacity
The published baseline — room floor area to cooling capacity (8′ ceilings, average conditions):
| Room area (sq ft) | Capacity (BTU/hr) | Room area (sq ft) | Capacity (BTU/hr) |
|---|---|---|---|
| 100–150 | 5,000 | 550–700 | 14,000 |
| 150–250 | 6,000 | 700–1,000 | 18,000 |
| 250–300 | 7,000 | 1,000–1,200 | 21,000 |
| 300–350 | 8,000 | 1,200–1,400 | 23,000 |
| 350–400 | 9,000 | 1,400–1,500 | 24,000 |
| 400–450 | 10,000 | 1,500–2,000 | 30,000 |
| 450–550 | 12,000 | 2,000–2,500 | 34,000 |
Note the curve isn't linear: small rooms need ~30–35 BTU per sq ft, big open areas closer to 15–20 — one reason the flat “20 BTU per square foot” rule misleads at both ends.
How to Calculate BTUs for Sun, Ceilings and Occupants
ENERGY STAR's published corrections, all applied automatically by this calculator:
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two people regularly in the room: add 600 BTU per extra person.
- Kitchen: add 4,000 BTU — cooking appliances are heat sources the chart doesn't otherwise see.
- Ceiling height: the chart assumes 8′; taller rooms hold proportionally more air to cool (a 10′ ceiling adds 25%).
Also from the guidance: place window units away from heat-generating appliances, and if the unit shares the room with a TV or electronics that run hot, err toward the next size up.
How Many BTUs for Heating?
Heating is a different calculation this tool deliberately doesn't fake. Cooling load is dominated by room size and sun; heating load is dominated by climate and insulation — the same room needs perhaps 30 BTU/sq ft of heat in a mild climate and 60 in a cold one with average insulation. A whole-home heating figure needs a Manual J load calculation (or at minimum a climate-zone factor), not a room chart.
Rough context only: a well-insulated bedroom in a moderate climate lands around 30–40 BTU/sq ft of heating; drafty rooms or severe winters push 50–60. For a real furnace, boiler, or heat-pump decision, get a load calculation from an HVAC contractor — oversized heating equipment short-cycles exactly like oversized cooling does.