Calculator
Room AC BTU Calculator
Room AC BTU sizing starts from a baseline of roughly 20 BTU per square foot, but that baseline alone is a rough starting point, not the finished answer — ceiling height, sun exposure, and occupant count each shift the real number meaningfully, and skipping those adjustments is exactly what leads to an AC unit that's oversized (short-cycles, doesn't dehumidify well) or undersized (never quite cools the room on hot days).
This calculator starts from the 20 BTU/sq ft baseline and layers in the real adjustment factors most quick BTU calculators skip entirely. See choosing the right AC size: BTU explained for why oversizing is a genuine mistake, not just a safe-margin choice.
Calculate your quantity
AC size needed
8,000 BTU
- Area
- 400 sq ft
Shopping summary
- • 8000 BTU air conditioner
This is an estimate — confirm structural work with a professional.
How this calculation works
Baseline BTU = room area (sq ft) x 20. Then adjust: standard 8-foot ceilings need no adjustment, but taller ceilings increase the actual air volume being cooled beyond what floor area alone captures, so a room with 10 or 12-foot ceilings needs a higher BTU figure than the same floor-area room with an 8-foot ceiling. Heavy sun exposure (south- or west-facing rooms with a lot of window area) adds heat load beyond the baseline; a shaded, north-facing room needs slightly less than the baseline suggests.
Occupant count matters too — each person in a room adds a real, if modest, heat load (roughly 400-600 BTU per additional person beyond the first one or two), which is why a home office or bedroom sized for one person needs adjusting up if the room regularly holds several people, like a family room.
The formula
btu = area×20; if height>8: btu×=1+((height−8)/8)×0.1; if sun=2: btu×=1.1; if sun=0: btu×=0.9; if occupants>2: btu+=(occupants−2)×600; round to nearest 100
- 1+((height−8)/8)×0.1
- The ceiling-height multiplier — 10% for every 2 ft of ceiling above 8, so a 10 ft ceiling adds 10% and a 12 ft ceiling adds 20%. It is capped at 50%, because past roughly 12 ft the rule of thumb this comes from stops applying and the room needs a real load calculation rather than a larger multiplier.
- sun=2 / sun=0 multipliers
- A flat ±10% adjustment applied AFTER the ceiling-height multiplier, so it compounds on top of whatever the ceiling term already produced rather than being calculated from the unadjusted baseline.
- (occupants−2)×600
- A flat BTU amount ADDED, not multiplied, and only once occupancy exceeds 2 — this is applied last, after both the ceiling and sun multipliers, so it is the one factor whose effect doesn't scale with room size at all.
- round to nearest 100
- The final figure is rounded to the nearest 100 BTU, not up — a result of 7,965 rounds DOWN to 8,000 only because 65 happens to round up past the midpoint; a result of 7,940 would round to 7,900, slightly under the unrounded figure.
Where these numbers come from
- 20 BTU per sq ft baseline
- Trade convention widely used as a first-pass room-AC sizing rule; it is a starting point this calculator layers real adjustments onto, not a substitute for a full Manual J load calculation.
- ceiling-height adjustment of 10% per 2 ft above 8 ft
- Trade convention: the widely published room-air-conditioner guidance is to add 10% for a ceiling above 8 ft, and this lands exactly on that figure at the common 10 ft case before scaling beyond it. Cooling load is not purely a function of air volume — envelope area and solar gain matter more — which is why the adjustment is proportional to height rather than to volume, and why it is capped rather than extrapolated indefinitely.
- roughly 400-600 BTU per occupant beyond two
- Trade convention for body heat load in a cooled room; applied here as a flat 600 per person over the baseline two, added rather than scaled by room size, so the same per-person addition applies whether the room is small or large.
Worked examples
A 220 sq ft shaded home office, one occupant
| Room area | 220 sq ft |
|---|---|
| Ceiling height | 8 ft |
| Sun exposure | Shaded |
| Occupants | 1 |
| AC size needed | 4000 BTU |
|---|---|
| Area | 220 sq ft |
220 sq ft at the 20 BTU/sq ft baseline is 4,400 BTU. A standard 8 ft ceiling triggers no height adjustment at all, one occupant doesn't cross the 2-person threshold either, so the only adjustment here is the shaded-room 10% reduction, landing at 3,960 and rounding to 4,000.
This is close to the cleanest possible reading of the baseline formula: with height and occupancy both neutral, shade alone pulled the figure down by exactly 10% before rounding — a useful reference point for how much a single factor moves the result on its own.
A 480 sq ft sunny sunroom, tall ceiling, six occupants
| Room area | 480 sq ft |
|---|---|
| Ceiling height | 12 ft |
| Sun exposure | Very sunny |
| Occupants | 6 |
| AC size needed | 15100 BTU |
|---|---|
| Area | 480 sq ft |
480 sq ft baselines to 9,600 BTU. The 12 ft ceiling adds 20% (to 11,520), heavy sun adds another 10% on top of that (to 12,672), and six occupants add a flat 2,400 BTU — four people beyond the two-person baseline at 600 each — for 15,072, rounding to 15,100.
Worth noticing which factor did the work. Occupancy contributed a flat 2,400 BTU regardless of room size, and would have contributed the same in a cupboard. The ceiling contributed 1,920 and the sun 1,152 — so on this room the height mattered more than the exposure, which is the reverse of the usual assumption and only visible because the two are applied as separate multipliers rather than bundled into one comfort fudge.
Common mistakes
- Assuming 'bigger is always safer' when choosing a BTU rating — an oversized unit cools the air quickly but shuts off before it's run long enough to properly dehumidify, leaving the room feeling cool but clammy.
- Ignoring sun exposure — a west-facing room with large windows can need meaningfully more cooling capacity than the same-size room on the shaded side of the house.
- Not adjusting for ceiling height, which increases the actual volume of air being cooled beyond what floor-area math alone accounts for.
- Sizing for the room's normal occupancy and ignoring occasional higher-occupancy use (like a home office that becomes a guest room), which can leave the unit undersized when it matters most.
- Not accounting for heat-generating equipment in the room (a kitchen, a room with several computers or a server rack), which adds real load beyond the standard baseline and occupant adjustment.
- Placing the unit poorly (blocked by furniture, in a corner with poor air circulation) which undermines even a correctly sized unit's ability to cool the room evenly.
Shopping summary
Choose a unit rated at or near the adjusted BTU figure (baseline plus ceiling-height, sun-exposure, and occupancy adjustments) rather than rounding up 'to be safe' — an oversized unit is a real comfort and efficiency problem, not just extra capacity in reserve.
Worked example: a 300 sq ft room at the 20 BTU/sq ft baseline needs 6,000 BTU. With a 10-foot ceiling (25% more air volume than standard 8-foot), heavy west-facing sun exposure (add roughly 10%), and four regular occupants (add about 1,200-1,800 BTU for the two beyond the baseline assumption), the adjusted target lands closer to 8,000-8,500 BTU — meaningfully above the unadjusted 6,000 BTU baseline for the identical floor area.
FAQ
Is it better to size an AC unit slightly bigger than calculated?
Not usually — an oversized unit cools the air fast and shuts off (short-cycles) before it's dehumidified the room properly, which often feels less comfortable than a correctly sized unit running a longer, steadier cycle, even though the air temperature drops just as fast.
How much does sun exposure really change BTU needs?
A heavily sun-exposed room (especially with large west-facing windows in the afternoon) can need a noticeably higher BTU rating than the floor-area baseline alone suggests, since direct sun through glass adds a real, sustained heat load beyond ambient room temperature.
Does the 20 BTU/sq ft baseline apply to a whole house, or just one room?
This calculator and the 20 BTU/sq ft figure are specifically for sizing a single-room unit (window or portable AC); whole-house central air sizing uses a more detailed load calculation (a Manual J calculation) performed by an HVAC professional, which accounts for the entire building envelope rather than one room's specific conditions.
What BTU ratings do window and portable AC units commonly come in?
Residential units are typically available in steps around 5,000, 6,000, 8,000, 10,000, 12,000, and higher BTU ratings, so after calculating your adjusted target, round up to the nearest commonly available rating rather than assuming an exact custom BTU figure will be sold.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.