BTU Calculator
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About the BTU Calculator
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
The ENERGY STAR Sizing Chart
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.
The Adjustments That Matter
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.
What About Heating BTUs?
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.
Frequently Asked Questions
How many BTUs do I need per square foot?
For cooling, it varies with room size — the ENERGY STAR chart works out to roughly 30–35 BTU/sq ft for small rooms and 15–20 for large open areas, which is why a flat per-foot rule misleads. Enter your dimensions and the calculator reads the chart plus adjustments for you.
How many BTU do I need to cool a 20x20 room?
400 sq ft lands at about 9,000–10,000 BTU/hr on the ENERGY STAR chart under average conditions — go 10% higher if it's very sunny, add 600 per person beyond two, and add 4,000 if it's a kitchen.
How many square feet does a 12,000 BTU AC cool?
Roughly 450–550 sq ft under average conditions per the ENERGY STAR chart — a large living room or a small open-plan area. Heavy sun, tall ceilings, or a crowd of people shrink that range; shade stretches it.
Is a bigger air conditioner better?
No. An oversized unit cools the air quickly and shuts off before it dehumidifies, leaving the room cold-but-clammy, and the constant start-stop cycling wastes energy and wears the compressor. Match the chart figure and pick the closest available size.
How big a room will a 15,000 BTU unit handle?
Around 700–850 sq ft under average conditions (it sits between the chart's 14,000 BTU / 550–700 sq ft and 18,000 BTU / 700–1,000 sq ft rows). Sunny exposure or a kitchen pulls that down; shade pushes it up.
What does BTU actually mean?
One BTU is the heat needed to raise a pound of water 1°F; an air conditioner's BTU/hr rating is how much heat it can remove per hour. 12,000 BTU/hr equals one “ton” of cooling — this calculator shows the tonnage equivalent too.
Sources & References
- [1]Room Air Conditioners — sizing guidance — ENERGY STAR (U.S. EPA / DOE)
Methodology. This calculator uses standard construction and material-estimation formulas. It is reviewed and maintained by the Vast Calculators editorial team.
Last updated · July 2026
Results are estimates for general use; verify critical figures independently.