What a BTU Is
A British thermal unit is the energy needed to raise one pound of water by one degree Fahrenheit — about 1,055 joules. In heating and cooling it is almost always quoted as BTU per hour, a rate of energy transfer rather than a quantity, even though the "per hour" is usually dropped in conversation.
| Unit | Equivalent |
|---|---|
| 1 BTU/hr | 0.293 watts |
| 1 kW | 3,412 BTU/hr |
| 1 ton of cooling | 12,000 BTU/hr |
| 1 horsepower | 2,545 BTU/hr |
The "ton" is a genuine historical unit: it is the cooling produced by melting one ton of ice over 24 hours, which is how buildings were cooled before mechanical refrigeration.
Sizing a Room
The starting point is roughly 20 BTU per square foot for cooling and 30–60 for heating, depending on climate. That baseline is then adjusted:
| Factor | Adjustment |
|---|---|
| Ceiling above 8 ft | Scale in proportion — you are conditioning volume, not floor |
| Very sunny room | +10% cooling |
| Heavily shaded room | −10% cooling |
| Poor insulation | +25% |
| Modern, well insulated | −15% |
| Each occupant beyond two | +600 BTU cooling |
| Kitchen | +4,000 BTU cooling |
Quick Reference by Room Size
| Area | Cooling BTU | Tons | Typical room |
|---|---|---|---|
| 100–150 sq ft | 5,000 | 0.42 | Small bedroom |
| 150–250 sq ft | 6,000 | 0.50 | Bedroom |
| 250–350 sq ft | 8,000 | 0.67 | Large bedroom, small living room |
| 350–450 sq ft | 10,000 | 0.83 | Living room |
| 450–550 sq ft | 12,000 | 1.00 | Large living room |
| 550–700 sq ft | 14,000 | 1.17 | Open-plan space |
| 700–1,000 sq ft | 18,000 | 1.50 | Small apartment |
| 1,000–1,400 sq ft | 24,000 | 2.00 | Apartment or small house |
Why Bigger Is Not Better
This is the most important thing on the page. An oversized air conditioner is worse than a correctly sized one, not better.
Cooling removes heat and moisture. Removing moisture takes time — the coil has to stay cold long enough for water to condense on it. An oversized unit drops the air temperature to the thermostat setpoint quickly, shuts off, and never runs long enough to dehumidify. The result is a room that is cold and clammy, with the compressor short-cycling, which wastes energy and shortens its life.
Undersizing has the opposite problem: the unit runs continuously and never reaches the setpoint on the hottest days. The correct size runs for long, steady cycles on a design-condition day.
Efficiency Ratings
| Rating | Measures | Good value |
|---|---|---|
| SEER2 | Seasonal cooling efficiency | 15–20+ |
| EER | Cooling efficiency at a fixed condition | 11–14 |
| HSPF2 | Seasonal heating efficiency of a heat pump | 8–10+ |
| AFUE | Furnace fuel efficiency | 90–98% |
Running cost falls roughly in proportion to SEER: moving from SEER 13 to SEER 20 cuts cooling electricity by about a third for the same capacity.
When to Use a Proper Load Calculation
The rule-of-thumb method here is fine for sizing a window unit or a single-room mini-split. For a whole-house system, HVAC professionals use Manual J, which accounts for wall construction, window type and orientation, air infiltration, duct losses and local design temperatures. It routinely produces a smaller answer than rules of thumb — oversizing is endemic in the trade — and a system sized to it will dehumidify properly and cost less to run.
Frequently Asked Questions
How many BTU do I need per square foot?
About 20 for cooling in a moderate climate, and 30–60 for heating depending on how cold the winters are. Both figures then need the adjustments in the table above.
What size air conditioner for 1,000 square feet?
Roughly 18,000–20,000 BTU, or about 1.5 tons, for an average room in a moderate climate. Poor insulation or a hot climate pushes that toward 24,000.
How do I convert BTU to kilowatts?
Divide by 3,412. A 12,000 BTU unit is about 3.5 kW of cooling capacity — note that this is the cooling output, not the electricity it consumes, which is far lower.
Does ceiling height really matter?
Yes. You are conditioning a volume of air. A 10 ft ceiling needs about 25% more capacity than an 8 ft ceiling over the same floor area.