BTU Calculator

Buy too small and your AC runs all day without ever quite cooling the room. Buy too big and it cools the room in fifteen minutes flat, then shuts off — leaving you cold, clammy, and paying for it twice: once at the store and again on every electric bill. This calculator sizes your air conditioner the way the pros do: room area times 20 BTU per square foot, then adjusted for the things that actually change the answer — afternoon sun, extra people, a kitchen in the space, tall ceilings, and insulation. Enter your room below and you get a cooling load plus the standard AC size to buy, in BTU, tons, or kilowatts.

How to use this calculator

Measure the room length and width at the longest walls, and multiply to get the floor area. If the room is L-shaped, measure it as two rectangles and add them — our room area calculator does the fiddly version for you.
Measure the ceiling height. The baseline rule assumes 8 ft (2.44 m); taller ceilings mean more air to cool, and the calculator scales for it.
Judge the sun exposure honestly. Big west- or south-facing windows that bake in the afternoon count as sunny; a room tucked behind trees or facing away from the sun counts as shaded.
Count the regular occupants. The baseline covers two people — every extra person adds about 600 BTU per hour of body heat.
Say whether the space includes a kitchen or is open-plan to one. An oven throws around 4,000 BTU of heat, which dwarfs almost every other adjustment.
Pick the insulation quality. Drafty single-pane windows and uninsulated walls leak cool air fast; a modern well-sealed room holds it.
Read the recommended size and buy that standard AC size — not the next one up "to be safe." Overshooting by more than about 15% is how you end up cold and clammy.

Formula

The industry rule of thumb: start with room area × 20 BTU per square foot (about 210 watts per square metre). Then adjust: +10% for a sunny room, −10% for a heavily shaded one, +600 BTU per hour for each occupant beyond two, +4,000 BTU if the space includes a kitchen, multiply by (ceiling height ÷ 8 ft) for tall ceilings, and +15% for poor insulation or −10% for good insulation. Finally, round up to the nearest standard AC size (5,000 / 6,000 / 8,000 / 10,000 / 12,000 / 14,000 / 18,000 / 21,000 / 24,000 BTU and up) — but if that size overshoots your load by more than 15%, step back down one size, since slightly under beats badly over.

Worked example

Take a bedroom: 12 × 15 ft = 180 sq ft. It faces west and gets hammered by afternoon sun. Two people sleep there. Standard 8 ft ceilings, average insulation, no kitchen. Step one — base load: 180 × 20 = 3,600 BTU per hour. Step two — sunny room adds 10%: 3,600 × 1.10 = 3,960 BTU. Step three — two occupants is the baseline, so no change; no kitchen, standard ceiling, average insulation, so nothing else moves. The real cooling load is about 3,960 BTU/hr. Step four — round to a standard size: the next size up is 5,000 BTU. That overshoots the load by about 26%, which is more than the 15% guideline — but there is no standard size between 3,960 and 5,000, so 5,000 BTU is the buy. In tons that is 5,000 ÷ 12,000 = 0.42 tons; in kilowatts, 5,000 × 0.293 ÷ 1,000 ≈ 1.5 kW of cooling.

Measurement tips

Measure the room yourself before trusting the listing. Floor plans round, agents exaggerate, and a "12 ft" wall that is really 11 ft 4 in changes the answer on a small room.
Afternoon sun matters more than morning sun. A room that is shady at 9am but bakes from 2pm on is a sunny room — mark it that way.
If two rooms connect with no door between them, they are one space. Add both areas together instead of sizing for one room and hoping.
Check the AC label for the BTU rating, not the marketing name. "12,000 BTU" and "1 ton" are the same thing; some brands round their model numbers optimistically.
For portable ACs, read the fine print: many list an inflated "ASHRAE" rating next to the real "SACC" cooling capacity. Size from the SACC number.
A slightly undersized unit that runs steadily dehumidifies better than an oversized one that short-cycles. If you are between sizes, the smaller one is usually the kinder choice.

Common mistakes

Buying bigger "just to be safe." An oversized AC cools the air in 15–20 minutes and shuts off before it has wrung the moisture out — so you get a cold, damp, clammy room, higher bills from constant compressor restarts, and a unit that dies years early.
Sizing from memory of the old unit. The old one might have been wrong too — that is possibly why you are replacing it.
Forgetting the kitchen in open-plan spaces. That 4,000 BTU is not optional; skip it and the "right" size will never keep up at dinner time.
Ignoring ceiling height. A 12 ft ceiling holds 50% more air than an 8 ft one. The flat ×20 rule quietly under-sizes every tall room.
Counting a shaded morning as a shaded room. What matters is the hottest part of the day — west and south windows in the afternoon.
Using a room-AC rule for a whole house. Whole-home systems need a proper Manual J load calculation done by a professional; this shortcut is for single rooms only.

Factors that affect accuracy

Sun exposure. Direct afternoon sun through big windows is the single biggest adjustment most rooms need — it can add 10% or more to the load.
Occupants. Each person is roughly a 600-BTU heater. A home office with three people in it needs noticeably more cooling than the same room as a spare bedroom.
Kitchens and heat sources. Ovens, and in home offices lots of electronics, dump heat into the room. An open-plan kitchen-living space is really one big hot room.
Ceiling height. Taller ceilings mean more cubic feet of air per square foot of floor. Lofts and double-height spaces need the height factor applied.
Insulation and windows. Old single-pane windows and drafty walls leak cooled air as fast as the AC makes it; good insulation and modern glazing hold it in.
Climate. This calculator sizes the room; it does not know your climate zone. In very hot regions, treat the result as a minimum and consider a professional load calculation.

Sources & assumptions

Base load = room area × 20 BTU per square foot, the rule of thumb behind ENERGY STAR's sizing chart.
Adjustments: +10% for a sunny room / −10% for a shaded one, +600 BTU per occupant beyond two, +4,000 BTU for a kitchen, ceiling-height scaling, +15% / −10% for insulation.
Room-size to BTU bands follow ENERGY STAR's room air conditioner sizing chart.
1 ton of cooling = 12,000 BTU/hr; 1 BTU/hr ≈ 0.293 watts.

Please note: material estimates are approximate. Always confirm product coverage with the manufacturer and buy a little extra for cuts, waste and future repairs.

What this calculator doesn't do

This is a rule-of-thumb, not a Manual J load calculation — pros size whole-house systems room by room.
Duct losses in central systems and extreme humidity aren't modeled.
Very leaky old windows or uninsulated additions can push the real number higher.

Frequently asked questions

How many BTU do I need per square foot?

The standard rule is 20 BTU per square foot of room area, assuming 8 ft ceilings and average conditions. So a 250 sq ft room starts at 5,000 BTU. Then adjust: +10% for strong sun, −10% for heavy shade, +600 BTU per extra person beyond two, +4,000 BTU for a kitchen, scale up for tall ceilings, and ±10–15% for insulation quality.

What happens if my AC is too big for the room?

It cools the air in 15–20 minutes and shuts off — before it has removed the humidity. You end up with a room that is cold but damp and clammy (above 60% humidity invites mold and dust mites), higher electric bills from constant compressor restarts, and a unit that wears out years early. Bigger is not safer; it is more expensive in every direction.

Is a 12,000 BTU air conditioner the same as 1 ton?

Yes. One "ton" of cooling equals exactly 12,000 BTU per hour — the term comes from the cooling power of a ton of melting ice. So a 2-ton system is 24,000 BTU/hr, and a 1.5-ton system is 18,000 BTU/hr.

How do I convert BTU to kW or watts?

Multiply BTU per hour by 0.293 to get watts. A 12,000 BTU unit is about 3,520 watts, or 3.5 kW of cooling. Going the other way, 1 watt ≈ 3.412 BTU/hr.

Should I round up or down to the nearest AC size?

Round up to the nearest standard size, but do not overshoot your calculated load by more than about 15%. If the next size up is way bigger than you need, take the smaller one — a slightly undersized unit that runs steadily beats an oversized one that short-cycles.

Does ceiling height really change the BTU I need?

Yes. The ×20 rule assumes 8 ft (2.44 m) ceilings. A 10 ft ceiling means 25% more air volume to cool, so multiply your result by (ceiling height ÷ 8). For rooms with very tall or vaulted ceilings, measure the average height.

Can I use this for a whole house or apartment?

No — this calculator is for single rooms. Whole-home cooling needs a Manual J load calculation, which accounts for the building envelope, ductwork, orientation, and climate zone. That is a job for an HVAC professional, not a rule of thumb.

Why does my room still feel humid with the AC running?

Almost always oversizing. An AC dehumidifies only while the compressor runs; an oversized unit satisfies the thermostat in minutes and shuts off before it has pulled meaningful moisture from the air. Right-sizing — or in stubborn cases a standalone dehumidifier — fixes it.

Next step

Take the recommended BTU size to the store and check it against the AC's yellow EnergyGuide label — then measure the room once more before you buy, since a wrong area number is the most common sizing error.

Save this estimate: use the Copy link button in the results box above to share these exact inputs, or Print results for a paper copy to take to the store.