A BTU — a British thermal unit — is the heat needed to raise one pound of water by one degree Fahrenheit, and an air conditioner rating in BTU per hour is the rate at which the unit removes heat from a room. Sizing is a heat-load question: how much heat enters the room, from the sun, the walls, the people, and the appliances, and how fast must it be removed to hold the temperature you want. The industry shortcut is 20 BTU per square foot of floor area for cooling at standard 8-foot ceilings, adjusted for what makes your room different from average. This guide works that rule, the adjustments, the conversion to tons, and the differences between window units, portable units, and mini splits, so the size you buy matches the room you have.

The 20 BTU per square foot rule

The rule is: cooling BTU per hour equals floor area in square feet times 20. It assumes 8-foot ceilings, average insulation, moderate sun exposure, and two occupants. It is a baseline, not a load calculation: a professional Manual J calculation measures the actual walls, windows, orientation, and insulation, and for a whole house that is the right tool. For a single room and a window unit or a mini split, the rule plus the adjustments below lands within one standard unit size of the correct answer, which is the precision the market offers — units are sold in steps, and the choice is which step to buy.

Work the rule on a 12 by 12 foot room. The area is 144 square feet. 144 times 20 is 2,880 BTU per hour. No manufacturer sells a 2,880 BTU unit, so the buy is the smallest standard size, 5,000 BTU. That gap is normal and harmless: the smallest unit covers the smallest rooms with margin, and the margin is absorbed by the adjustments. Work it on a 15 by 12 foot room: 180 square feet times 20 is 3,600 BTU, again a 5,000 BTU buy, or 6,000 BTU if the room is sunny. Work it on a 20 by 15 foot room: 300 square feet times 20 is 6,000 BTU, a 6,000 to 8,000 BTU buy depending on exposure. The rule is doing two jobs — estimating the load, and pointing at the shelf size — and the second job is where the rounding lives.

BTU by room size: the sizing table

Baseline before adjustments. Buy the standard size at or just above the adjusted figure.

Read the table as a starting point, then apply the adjustments. Two rooms of the same area can need different sizes: a shaded north-facing bedroom and a west-facing kitchen of the same size are different heat loads, and the table gives them the same baseline on purpose — the adjustments are where they diverge. If the adjusted figure falls between two standard sizes, the usual advice for cooling is to buy the closer size, not automatically the larger one, because of the short-cycling penalty described below; in a hot climate, one size up is the safer rounding.

Adjustments: sun, ceilings, people, kitchens

Ceiling height: the rule assumes 8-foot ceilings. For taller ceilings, scale by height divided by 8: a 10-foot ceiling adds 25%, a 12-foot ceiling adds 50%, because there is that much more air to cool. Sun exposure: add about 10% for a room with strong afternoon or west-facing sun, and 10% to 15% where large west-facing windows dominate; subtract about 10% for a heavily shaded room. Insulation: add 10% to 20% for poor insulation, subtract up to 10% for a well-insulated newer build. People: add about 600 BTU per hour for each person beyond two regularly in the room, because each person is a heat source of roughly that output at rest. Kitchen: add 4,000 BTU per hour, because the range, oven, refrigerator, and other appliances dump heat all day.

Worked example with adjustments: a 300-square-foot kitchen-dining room with 10-foot ceilings, west-facing windows, and four regular occupants. Baseline: 300 times 20 is 6,000 BTU. Ceiling: times 10 divided by 8 is 7,500 BTU. Sun: add 10% to get 8,250 BTU. People: two extra people add 1,200 BTU, giving 9,450 BTU. Kitchen: add 4,000 BTU, giving 13,450 BTU. The buy is a 14,000 BTU unit, or a 12,000 BTU (1 ton) mini split in a mild climate with the understanding that it will run hard on the hottest days. The same 300 square feet as a shaded bedroom with 8-foot ceilings and two occupants is 6,000 BTU and a 6,000 BTU unit. Same area, double the load: the adjustments are the sizing.

BTU to tons, and what to actually buy

One ton of cooling equals 12,000 BTU per hour. The term comes from the heat needed to melt one ton of ice in 24 hours, and it survives because mini splits and central systems are sold in tons: 9,000 BTU is three quarters of a ton, 12,000 BTU is 1 ton, 18,000 BTU is 1.5 tons, 24,000 BTU is 2 tons, 36,000 BTU is 3 tons. Window units are usually sold in BTU, mini splits in both, and central systems in tons. To convert, divide BTU per hour by 12,000. A 14,000 BTU requirement is 1.17 tons, so the buy is a 1.5-ton system unless a 12,000 BTU unit with a small margin is acceptable in a mild climate.

Buy in the steps the market sells. Window units commonly come in 5,000, 6,000, 8,000, 10,000, 12,000, and 14,000 BTU sizes, with larger units beyond. Mini splits commonly come in 9,000, 12,000, 18,000, and 24,000 BTU sizes per indoor head. If the adjusted requirement is 7,200 BTU, the window buy is 8,000 BTU and the mini-split buy is 9,000 BTU. If it is 13,000 BTU, the window buy is 14,000 BTU and the mini-split buy is 12,000 or 18,000 BTU depending on climate and on whether the head will serve one room or an open plan. The hub calculator returns the adjusted requirement and the tonnage; the shelf step is the last decision, and it is the one this guide exists to make easy.

Why oversizing hurts

An oversized air conditioner cools the air fast, reaches the thermostat setting, and shuts off — before it has run long enough to remove humidity. The room ends up cold and clammy, the compressor cycles on and off constantly, which wears it and wastes electricity on restarts, and comfort is worse than with a correctly sized unit. An undersized unit, by contrast, runs continuously and may never catch up on the hottest day, but it dehumidifies while it runs and it does not short-cycle. That asymmetry is why the advice is to size to the calculated load, or one standard size up only when between sizes in a hot climate, rather than buying two sizes up to be safe. Being safe by oversizing buys the one failure mode — humidity — that a correctly sized unit avoids.

There is one exception worth naming: humid climates reward a slightly larger unit, not a smaller one, because a unit that never gets ahead of the load never dehumidifies either, and in sustained humidity the load includes moisture that the baseline does not fully capture. Even there, the oversize is one step, not two, and a variable-speed inverter mini split changes the calculus: it can run at part load continuously, dehumidifying without short-cycling, which is why mini splits tolerate sizing error better than fixed-speed window units. If the choice is between a fixed-speed unit one size up and an inverter unit at the calculated size, the inverter at the calculated size is usually the better comfort buy.

Mini splits: sizing by zone

A mini split cools the zone its indoor head serves, not the whole house, so each head is sized by its own room using the same rule and adjustments. A 1,000-square-foot open plan served by one head needs about 20,000 BTU baseline — a 24,000 BTU (2 ton) head, or two 12,000 BTU heads, which is often the better layout because two heads distribute air and allow different setpoints. A 1,200-square-foot home with three bedrooms needs a head per bedroom plus one for the living area, each sized to its room, and the outdoor unit sized to the sum of the heads it serves, with the multi-zone derating the manufacturer specifies. The kept evidence asks repeatedly how many BTU a mini split needs for 1,000 and 1,200 square feet: the answer is 20,000 and 24,000 BTU baseline respectively, before adjustments, and the zone breakdown is what turns that total into a purchase.

Mini-split sizing has one trap that window units do not: the smallest head is 9,000 BTU. A 100-square-foot bedroom needs 2,000 BTU baseline and gets a 9,000 BTU head, which is more than four times the load. With an inverter compressor that is fine — the head modulates down — but with a fixed-speed head it would short-cycle badly. Confirm the head is inverter-driven, which nearly all current mini splits are, and confirm the minimum capacity in the specifications, not just the nominal capacity. The nominal figure is the maximum; the minimum is what decides whether a small room stays comfortable.

Window units vs portable units

Window units sit in the window, exhaust heat outside, and deliver their rated BTU to the room with relatively little loss. Portable units sit inside the room, exhaust through a hose in the window, and lose efficiency two ways: the hose radiates heat back into the room, and single-hose units pull conditioned air out of the room to cool the condenser, drawing warm air in from elsewhere. A portable unit rated 12,000 BTU therefore cools less effectively than a window unit rated 12,000 BTU, and buyers commonly report needing a larger portable rating for the same room. If the room allows a window unit, it is the more efficient buy at the same rating; if only a portable unit is possible, size one step up and prefer a dual-hose model, which draws condenser air from outside rather than from the room it is cooling.

Portable and window units also differ in what they do to the room: a window unit blocks part of a window and needs a suitable sash, a portable unit needs floor space and a window within hose reach, and both need the condensate dealt with — most window units evaporate it, portable units collect it in a tank or a drain. None of that changes the BTU arithmetic, but all of it changes which unit is actually installable, and an installable unit at the right size beats a theoretically better unit that cannot be fitted.

Heating BTU: a different rate

Heating uses a different rate because winter heat loss depends on climate, not just area. Common rules of thumb are about 30 BTU per square foot in hot climates, 40 in moderate climates, and 55 in cold climates, adjusted for insulation in the same way as cooling. A 1,000-square-foot home in a moderate climate therefore needs about 40,000 BTU of heating, and in a cold climate about 55,000 BTU. Furnace sizing by zip code, which the kept evidence asks for, is a climate-zone lookup: the same house needs a larger furnace where design temperatures are lower. A heat pump mini split sized for cooling may not cover the heating load in a cold climate, and the heating requirement should be checked separately rather than assumed from the cooling size. For a whole house, a Manual J load calculation is the right tool for both heating and cooling; the per-foot rules in this guide are for single rooms and for sanity-checking a contractor quote, not for replacing the calculation on a new system.

Frequently asked questions

How many BTUs do I need for a 12×12 room?

About 2,880 BTU per hour by the baseline rule, 144 square feet times 20 BTU per square foot. The usual buy is a 5,000 BTU window unit, the smallest standard size, which covers the room with margin. Add 10% for strong sun or 10-foot ceilings, and round to the next standard size if the adjusted figure passes 5,000 BTU.

How many BTUs per square foot do I need?

For cooling, 20 BTU per square foot at 8-foot ceilings, adjusted for ceiling height, sun, shade, insulation, occupants, and kitchens. For heating, the rule depends on climate: about 30 BTU per square foot in hot climates, 40 in moderate climates, and 55 in cold climates. Both rules are baselines; a Manual J calculation is the precise method for a whole house.

How many BTUs is a ton of air conditioning?

12,000 BTU per hour. An 18,000 BTU system is 1.5 tons, 24,000 BTU is 2 tons, and 36,000 BTU is 3 tons. Window units are usually sold in BTU and mini splits and central systems in tons, so divide the BTU requirement by 12,000 to convert.

Is it better to oversize or undersize an air conditioner?

Neither, but a small undersize is less harmful than an oversize. An oversized unit short-cycles, leaves the room humid, and wears the compressor, while an undersized unit runs continuously and dehumidifies but may not catch up on the hottest day. Size to the calculated load, or one standard size up when between sizes in a hot climate. Inverter mini splits tolerate sizing error better than fixed-speed units.

How many BTUs do I need for a mini split for 1,000 square feet?

About 20,000 BTU per hour baseline, 1,000 square feet times 20 BTU per square foot, before adjustments for sun, ceilings, and occupants. That is usually a 24,000 BTU head or two 12,000 BTU heads, one per zone. Each zone should be sized to its own room rather than dividing the total evenly.

Do I add BTUs for a kitchen?

Yes, add 4,000 BTU per hour for a kitchen, because the range, oven, refrigerator, and other appliances add heat all day. Also add about 600 BTU per hour for each person beyond two regularly in the room, and scale for ceiling height above 8 feet by multiplying by height divided by 8.