Quick answer: A common starting point is about 20 BTU per hour per square foot of floor area, so a 300 sq ft room needs roughly 6,000 BTU. That is a rule of thumb - the real number changes with climate, ceiling height, sun exposure, insulation, windows, and how many people and appliances are in the room. For a proper answer, do a heat-load calculation with the HVAC heat load calculator rather than relying on square feet alone.
What a BTU actually is
A BTU (British Thermal Unit) is the amount of heat needed to raise one pound of water by one degree Fahrenheit. Air conditioners and heaters are rated in BTU per hour, which tells you how much heat they can move each hour - remove (cooling) or add (heating). Sizing is simply matching that BTU rating to how much heat the room gains in summer or loses in winter. Match it well and the unit keeps the room comfortable without short-cycling or running non-stop.
The rule of thumb: 20 BTU per square foot
For a quick estimate, use about 20 BTU/hr per square foot of floor area for cooling a typical room:
| Room size (sq ft) | Approx. cooling BTU/hr |
|---|---|
| 100 - 150 | 5,000 |
| 150 - 250 | 6,000 |
| 250 - 350 | 7,000 - 8,000 |
| 350 - 450 | 10,000 |
| 450 - 550 | 12,000 |
| 550 - 700 | 14,000 |
| 700 - 1,000 | 18,000 |
This is useful for picking a window or portable AC, but treat it as a check, not a final design. The 20 BTU figure assumes an average room with 8 ft ceilings, moderate sun, and decent insulation. Change any of those and the number moves.
What changes the BTU you need
The rule of thumb hides a lot. Here are the adjustments that matter, and roughly how much:
- Climate. A hot southern climate needs more cooling BTU than a mild one; a cold northern climate needs far more heating BTU. This is the single biggest variable.
- Sun exposure. A sunny, south- or west-facing room can need 10% more; a heavily shaded room about 10% less.
- Ceiling height. The rule assumes 8 ft. Taller ceilings mean more air volume - scale up by the ratio of your ceiling height to 8 ft, or add roughly 10-20% for 9-10 ft.
- Occupancy. Each person beyond the first two adds about 600 BTU of body heat.
- Kitchens. Add about 4,000 BTU for the appliances and cooking heat.
- Windows and insulation. Lots of glass or poor insulation raises the load; tight, well-insulated rooms lower it.
BTU and tons: the conversion
Whole-house and central systems are usually sized in tons, and the conversion is simple:
1 ton of cooling = 12,000 BTU/hr
So a 24,000 BTU system is 2 tons, and 36,000 BTU is 3 tons. (The word "ton" dates back to the cooling once provided by melting a ton of ice per day.) Room units and mini-splits are quoted in BTU/hr, while central air and heat pumps are quoted in tons. For the whole-house number you cross-check the tonnage against the load with the tonnage calculator.
Heating BTU: usually more than cooling
Heating starts near the same 20 BTU/sq ft in a mild climate, but cold climates need much more - often 30 to 45 BTU/sq ft or higher - because the temperature difference to the outdoors in winter is larger than in summer. Insulation, window area, and air leakage matter even more for heating. So a room that needs 6,000 BTU of cooling in a moderate climate might need 9,000-12,000 BTU of heating in a cold one. Always size heating for your coldest design day, not an average.
Why bigger is not better
It is tempting to "round up" and buy a larger unit, but oversizing hurts comfort, especially for cooling. An oversized AC cools the air fast and shuts off before it has run long enough to remove moisture, so the room feels cold and clammy. It also short-cycles - turning on and off repeatedly - which wastes energy, wears out the compressor, and gives uneven temperatures. A right-sized unit runs longer, steadier cycles that control both temperature and humidity. Undersizing is bad too (the unit runs constantly and still cannot keep up), so the goal is to match the load, not beat it.
Worked example
A 12 ft × 16 ft (192 sq ft) living room, sunny west-facing, 9 ft ceilings, three people often present, moderate climate:
- Base: 192 × 20 = 3,840 BTU.
- Sun (+10%): +384 → 4,224.
- Ceiling (9 ft, +12%): +460 → ~4,700.
- Extra person (+600): ~5,300 BTU.
You would choose a 6,000 BTU unit (the next standard size up), which lands just above the calculated load - right where you want it.
Whole-house AC size by square footage
For a whole house rather than a single room, the same idea scales up, and people usually think in tons. As a very rough starting point (moderate climate, average insulation, 8 ft ceilings), houses land near these sizes - but this is a sanity check, not a substitute for a load calculation:
| Home size (sq ft) | Approx. AC size |
|---|---|
| 600 - 1,000 | 1.5 ton (18,000 BTU) |
| 1,000 - 1,300 | 2 ton (24,000 BTU) |
| 1,300 - 1,600 | 2.5 ton (30,000 BTU) |
| 1,600 - 2,000 | 3 ton (36,000 BTU) |
| 2,000 - 2,600 | 4 ton (48,000 BTU) |
| 2,600 - 3,200 | 5 ton (60,000 BTU) |
A hot climate, lots of west-facing glass, or high ceilings push you up a size; a tight, shaded, well-insulated house pulls you down. Notice this works out to roughly 400-600 sq ft per ton, which is the classic contractor rule - useful for a gut check, dangerous as a final answer because it ignores your actual windows, insulation, and air leakage.
Heat pumps and the BTU question
A heat pump both heats and cools, so it is sized for whichever load is larger. In mild and moderate climates the cooling load usually governs, and the same BTU/ton logic applies. In cold climates the heating load can be much larger than cooling, and the heat pump's output falls as the outdoor temperature drops - so cold-climate heat pumps are sized carefully (often with backup heat or a cold-climate rated unit) rather than by the cooling number alone. This is exactly why you size heating for the coldest design day, not an average.
When to move beyond the rule of thumb
Per-square-foot numbers are fine for a single room unit. For a whole house, a new system, or a permit, you should do a full load calculation - ACCA Manual J for homes - which accounts for every wall, window, door, and the air infiltration, room by room. That is the difference between an estimate and a design, and it is what prevents the oversizing that plagues so many installed systems. See our detailed guide to the cooling load calculation and how it feeds equipment selection through the cooling coil calculator.
Standards and references
| Reference | What it covers |
|---|---|
| ACCA Manual J | Residential heating & cooling load calculation |
| ASHRAE Fundamentals | Heat gain/loss methods and design conditions |
| Key conversion | 1 ton = 12,000 BTU/hr; ~20 BTU/sq ft rule of thumb |
| ENERGY STAR | Room AC sizing recommendations |
The bottom line
Start at about 20 BTU/hr per square foot for cooling, then adjust for climate, sun, ceiling height, occupancy, kitchens, and insulation - and remember heating in a cold climate needs considerably more. Do not oversize: a right-sized unit controls temperature and humidity far better than a big one. Estimate a single room with the HVAC heat load calculator, and for a whole house use a full Manual J load calculation confirmed by a qualified HVAC professional.
Frequently asked questions
How many BTU do I need to cool a room?
A common starting point is about 20 BTU per hour per square foot of floor area, so a 300 square foot room needs roughly 6,000 BTU. That is a rule of thumb - the real figure changes with climate, ceiling height, sun exposure, insulation, windows, and how many people and appliances are in the room. For an accurate number, do a proper heat-load calculation rather than relying on square feet alone.
How many BTU per square foot for air conditioning?
Roughly 20 BTU per hour per square foot is the widely used cooling rule of thumb for a typical room, but it ranges from about 15 BTU in a cool, shaded, well-insulated space to 25 or more BTU in a hot, sunny, or poorly insulated one. Kitchens add around 4,000 BTU, and each regular occupant beyond two adds around 600 BTU. Treat per-square-foot numbers as a sizing check, not a final design.
What is a BTU and why does it matter for AC and heating?
A BTU, or British Thermal Unit, is the amount of heat needed to raise one pound of water by one degree Fahrenheit. Air conditioners and heaters are rated in BTU per hour, which tells you how much heat they can remove or add each hour. Matching the BTU rating to the room's heat gain or loss is what makes a unit keep the space comfortable without short-cycling or running non-stop.
What happens if my air conditioner is too big?
An oversized AC cools the air quickly but shuts off before it removes enough moisture, so the room feels cold and clammy, and the unit short-cycles - turning on and off frequently. That wastes energy, wears out the compressor, and gives poor humidity control and uneven temperatures. Bigger is not better; correct sizing from the actual load gives the best comfort and efficiency.
How do I convert BTU to tons of cooling?
One ton of cooling equals 12,000 BTU per hour. So a 24,000 BTU air conditioner is 2 tons, and a 36,000 BTU unit is 3 tons. The term ton comes from the cooling once provided by melting a ton of ice per day. For whole-house systems, capacity is usually quoted in tons, while room units and mini-splits are quoted in BTU per hour.
How many BTU to heat a room?
Heating also starts near 20 BTU per hour per square foot in a mild climate, but cold climates need far more - often 30 to 45 BTU per square foot or higher - because the temperature difference to the outdoors is larger. Insulation, windows, ceiling height, and air leakage matter even more for heating. A room heat-loss calculation gives a much better answer than square feet alone.
Does ceiling height change the BTU I need?
Yes. The rule-of-thumb per-square-foot numbers assume standard 8 foot ceilings. Taller ceilings mean more air volume and more surface area to lose or gain heat, so you scale the BTU up - a common adjustment is to increase the load by the ratio of the actual ceiling height to 8 feet, or simply add about 10 to 20 percent for 9 to 10 foot ceilings and more for higher spaces.
Is a BTU calculator accurate for sizing an AC or heater?
A BTU calculator that adjusts the per-square-foot baseline for climate, sun, ceiling height, insulation, occupancy and appliances gives a good sizing estimate for a room unit. For a whole house or a permit, a full load calculation (ACCA Manual J for homes) is the accurate method, accounting for every wall, window, and infiltration path, and should guide the final equipment selection.