BTU Calculator for a Room
A room's cooling load comes from its glass and its gadgets, not its floor area. Add up the real heat sources and size the unit to match.
Room BTU calculator
Where a room's heat comes from
| Source | BTU/hr | Share | Basis |
|---|
Same room, different window wall
| Windows face | Cooling needed | Unit size |
|---|
Identical room, identical glass area — only the direction changes. This is why a west bedroom needs a bigger unit than a north bedroom of the same size.
BTU per hour, not BTU
A BTU is a quantity of heat: the energy that raises one pound of water by one degree Fahrenheit. An air conditioner does not store BTUs, it moves them at a rate, which is why the honest unit is BTU per hour and why the number on the box is really a rate wearing a shortened label. One ton of cooling is 12,000 BTU per hour, so a 12,000 BTU window unit is nominally a one-ton machine — nominally, because the rating point is not your window.
The conversion worth memorising is electrical: 1 watt of equipment load becomes 3.412 BTU per hour of heat inside the room. Almost all of a computer's or a television's input power ends up as heat in the space it sits in, which is why a gaming PC at 400 watts adds more load than a third occupant.
Why the 20 BTU per square foot rule fails in one room
Whole-house rules average across a building. A room does not average. Two rooms of identical floor area can differ by a factor of three depending on how much glass they have, which way it faces, and how many of their walls are exterior. A 12 by 12 bedroom on the north side of a shaded house may need 2,800 BTU per hour. A 12 by 12 room with a west wall of glass and a home office in it can pass 6,000. The square footage is the same; the load is not.
Interior rooms invert the rule entirely. A windowless interior office with two people and a laptop has no conduction gain worth counting through its walls, because both sides are conditioned. Its load is people plus plug loads plus whatever air leaks in from the ceiling plenum. Sizing it at 20 BTU per square foot would specify a unit more than twice the size needed.
The four inputs that actually move the number
- Glass area and orientation. West glass is the worst case in almost every US climate, because low afternoon sun hits it nearly perpendicular while the outdoor air is at its hottest. Peak solar gain through clear double pane runs near 58 BTU per hour per square foot of glass; north glass at the same moment is closer to 16. A 30 square foot west window is worth about 1,700 BTU per hour; the same window facing north is worth under 500.
- Exterior wall count. Each exterior wall adds conduction area. A corner room with two exposures is the normal case; a sunroom with four is a different building.
- What is above the ceiling. This model assumes attic above the room, which is the worst realistic case. A room under another conditioned floor has almost no ceiling gain, and a basement room has none at all — in which case the calculated number overstates the load, often by 15 to 25 percent.
- Equipment watts. The cheapest input to change. Replacing a 400 watt desktop with a 65 watt laptop removes 1,140 BTU per hour from the room, which is roughly a tenth of a ton, for free.
Why the box rating is not the delivered capacity
Window units are rated under AHRI 310/240 at 95°F outdoor dry bulb and 80°F indoor dry bulb. Capacity falls as the outdoor temperature rises above the rating point — a few percent by 105°F for a standard compressor, less for an inverter unit that can ramp rather than cycle. If your design afternoon is 104°F, treat the nameplate as a ceiling, not a target, and size one step up rather than one step down.
Portable units have a bigger problem, and it is mechanical rather than meteorological. A single-hose portable pulls air from the room, passes it over the condenser and blows it out the window. That air has to come from somewhere, and it comes back in through every gap in the envelope — hot outdoor air, at exactly the hour you are trying to keep out. Laboratory testing and manufacturer documentation put the effective loss at 20 to 30 percent of rated capacity, which is why a 12,000 BTU single-hose portable often performs like an 8,000 BTU window unit. Dual-hose portables pull condenser air from outside and return it outside, which removes the penalty but not the price.
Reading the result
The calculator returns the total load and the nearest standard window-unit size at or above it. The standard sizes are 5,000, 6,000, 8,000, 10,000, 12,000, 15,000, 18,000, 24,000 and 30,000 BTU per hour. If you land within about 10 percent of a size, take the smaller one unless the room is a kitchen or has an unusual internal gain: an oversized unit reaches the setpoint and shuts down before it has pulled much moisture out of the air, and the room ends up cold and damp rather than comfortable.
Bedrooms typically land at 5,000 to 8,000 BTU. Living rooms and open-plan spaces at 10,000 to 15,000. Anything above 18,000 is usually better served by a ductless mini-split than by a window unit, because window units at that size are loud, heavy, and draw more current than a standard 15 amp circuit comfortably supplies.
What this cannot see
Whether the door stays open. A room that is open to a hallway is not a closed box, and a window unit fighting an open doorway to a 78°F house will run continuously. It also cannot see shade from a tree or a neighbouring wall, skylights, a west-facing wall with no windows at all (which still conducts), or the air imbalance created by a central system whose return is undersized. Those need a site visit; the arithmetic here gets you to the right shelf at the store, not to a stamped load calculation.
Frequently asked questions
How many BTU do I need for a 12x12 room?
Usually 5,000 to 6,000 BTU per hour, which is the smallest window unit sold. Floor area alone is a poor guide though: the same 12 by 12 room with a large west window and a desktop computer can reach 7,000 to 8,000, while a shaded north-facing bedroom may need under 3,000.
Is a bigger window unit always better?
No. An oversized unit cools the air quickly and shuts off before the coil has run long enough to condense moisture, so the room gets cold and clammy. If your calculation lands close to a size, choose the smaller one, and consider a unit with a dehumidify or low-fan mode.
Why does my portable AC not cool as well as its rating?
If it has a single exhaust hose, it is pulling conditioned room air out of the window and drawing hot outdoor air back in through the envelope. That penalty is typically 20 to 30 percent of rated capacity. A dual-hose unit avoids it.
Do people really add that much load?
Each resting adult adds roughly 230 BTU per hour of sensible heat plus about 200 BTU per hour latent, so four people are worth around 1,700 BTU per hour — a meaningful share of a small room's load, and the reason a crowded room feels stuffy even when the AC is keeping up with the thermostat.
Does the ceiling height matter?
It changes the air volume, which changes infiltration, and it changes wall area, which changes conduction. A 10 foot ceiling in the same footprint adds roughly 10 percent to the load. It does not change the floor area, which is why square-footage sizing misses it.
Should I size a mini-split the same way?
The load number is the same, but the equipment sizing is not: mini-splits are sold in quarter-ton steps and are usually selected to the calculated load or one step above, not rounded up to the next window-unit size. Use this result as the load and let the installer match the head to it.
Can I subtract the AC from an adjacent room?
Only if the door is open and the adjacent system has spare capacity. In practice, a room with the door closed is its own zone, and borrowing capacity from a central system rarely works because the thermostat is somewhere else.
Disclaimer: This tool estimates a single room's cooling load for planning purposes. It is not a Manual J room-by-room calculation and does not account for ductless system line-set limits, room-to-room air transfer, or the effect of an open doorway to the rest of the house.
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