Air Conditioner BTU Calculator
Cooling capacity for a room, in BTU and in tons. The important thing this will tell you is that bigger is not better — an oversized unit leaves a room cold and clammy.
Cooling needed
10,000BTU/hr
- Floor area
- 500.0 sq ft
- Tons
- 0.83
- Window unit
- 10,000 BTU
- Central system
- 1.0 tons
- Kilowatts of cooling
- 2.93
- Bigger is not better. An oversized unit cools the air fast, shuts off, and never runs long enough to pull the humidity out — so the room ends up cold and clammy, and the compressor wears out short-cycling.
- This is a sizing estimate from the published per-square-foot rule. A Manual J calculation is the real method and accounts for insulation, air leakage, window type and which way the house faces.
About the BTU calculator
A BTU is a unit of heat, and an air conditioner is rated by how many of them it can move out of a room in an hour. The published sizing rule is about twenty BTU per square foot in a moderate climate, and every window-unit sizing table you have seen comes from that one number: five hundred square feet needs ten thousand BTU.
Four things move it. Ceiling height scales the whole load, because what is really being cooled is a volume of air and the tables all assume eight feet. Sun exposure moves it ten per cent either way. Each person past two adds six hundred BTU. A kitchen adds four thousand, because the oven and the fridge are both heat sources in the room they are cooling.
The counterintuitive part is that oversizing is worse than being slightly under. An air conditioner dehumidifies by running long enough for water to condense on the coil and drain away. A unit that is too large drops the air temperature quickly, satisfies the thermostat and shuts off before that happens — so the room reads the right temperature and feels cold and clammy, and the compressor wears out from short-cycling.
This is a rule of thumb, and an honest description of it is that it is what a salesperson uses to get in the right range. The real method is a Manual J load calculation, which accounts for insulation levels, air leakage, window type and area, and which way the house faces. For a whole-house system it is worth paying for.
What it works out
- Cooling capacity in BTU, tons and kilowatts
- The nearest window unit and central system size actually sold
- Ceiling height, sun exposure, occupancy and kitchens
- Four climate bands, from cool to hot
The formula
BTU = Area × Climate factor × (Ceiling ÷ 8) ± Sun 10% + 600 per extra person + 4,000 for a kitchen
The base is floor area times a per-square-foot figure. Twenty BTU a square foot is the moderate-climate number behind every published sizing table, and it moves to eighteen where summers are mild and thirty where they are long and very hot.
Ceiling height scales that, because the load is really about the volume of air in the room and the tables were written for eight feet. A twelve-foot ceiling is half as much air again, so a five hundred square foot room goes from ten thousand BTU to fifteen thousand.
Then the adjustments, which are added rather than scaled. Ten per cent off a heavily shaded room and ten per cent on a very sunny one. Six hundred BTU for each person past two, since two is already assumed. Four thousand for a kitchen.
Finally it rounds to something you can buy. Window and portable units come in thousand-BTU steps; central systems come in half-ton steps, where a ton is twelve thousand BTU an hour. It rounds up, because a system slightly under the load never catches up on the hottest day of the year.
- Area
- Floor area of the space being cooled.
- Climate factor
- BTU per square foot: 18 cool, 20 moderate, 25 warm, 30 hot.
- Ceiling
- Ceiling height in feet. Everything scales by how far it is from eight.
- Ton
- 12,000 BTU an hour — how central systems are sized.
A worked example
A twenty-five by twenty foot living room with eight-foot ceilings, in a moderate climate, normally lit, with two people in it.
That works out to 10,000 BTU/hr.
- Floor area
- 500.0 sq ft
- Tons
- 0.83
- Window unit
- 10,000 BTU
- Central system
- 1.0 tons
- Kilowatts of cooling
- 2.93
Questions
What size air conditioner do I need for 500 square feet?
10,000 BTU in a moderate climate with eight-foot ceilings — that is a large window unit, or about one ton of central cooling.
How many BTU do I need for a 1,500 square foot house?
About 30,000 BTU, or 2.5 tons, in a moderate climate. In a hot climate the same house works out closer to 45,000. For a whole house it is worth having a Manual J calculation done rather than relying on the square-foot rule.
Is a bigger air conditioner better?
No, and this is the single most useful thing on this page. An oversized unit cools the air fast, hits the thermostat setting and shuts off before it has run long enough to condense any humidity out. The room ends up cold and clammy, and the compressor wears out from constantly starting and stopping.
How many BTU is a ton of cooling?
12,000 BTU an hour. The name is literal: it is roughly the rate of cooling you would get from a ton of ice melting over a day, which is how the unit was defined before mechanical refrigeration.
Does ceiling height matter?
Yes, and the published tables ignore it. They assume eight feet. A room with a twelve-foot or vaulted ceiling holds half as much air again and needs the load scaled to match, which this does.
Why does a kitchen need more?
Because the oven, hob, dishwasher and fridge are all putting heat into the room the air conditioner is trying to cool. Four thousand BTU is the standard allowance for it.
What is a Manual J calculation?
The industry method for working out a heating and cooling load properly. It accounts for insulation levels, air leakage, window type and area, orientation and local design temperatures, room by room. The square-foot rule here gets you in the right range; Manual J gets you the right answer.
Does this size heating as well?
No. Heating load depends on how cold it gets and how well the house holds heat, which is a different calculation with different numbers behind it. A heat pump has to satisfy both, and which one governs depends on the climate.
What about a room with a lot of glass?
Use the very sunny setting, which adds ten per cent. If the room is largely glazed on a south or west face, even that is likely to be light — glass is the single largest variable a square-foot rule cannot see, and it is one of the main reasons Manual J exists.