The 20 BTU per square foot starting point
The standard residential rule of thumb is about 20 BTU of cooling per square foot of floor area, for rooms with normal 8–9 ft ceilings. A 150 sq ft bedroom comes to about 3,000 BTU/h; a 300 sq ft living room to about 6,000.
That baseline is only the opening bid. The adjustments are what actually decide which unit you buy, and they routinely move the answer by a full model size.
| Room size | Recommended BTU | Typical room |
|---|---|---|
| 100–150 sq ft | 5,000 BTU | Small bedroom, home office |
| 150–250 sq ft | 6,000 BTU | Bedroom |
| 250–300 sq ft | 7,000 BTU | Large bedroom |
| 300–350 sq ft | 8,000 BTU | Small living room |
| 350–400 sq ft | 9,000 BTU | Living room |
| 400–450 sq ft | 10,000 BTU | Large living room |
| 450–550 sq ft | 12,000 BTU | Open living space |
| 550–700 sq ft | 14,000 BTU | Studio apartment |
| 700–1,000 sq ft | 18,000 BTU | Large open area |
| 1,000–1,400 sq ft | 24,000 BTU | Multi-room, 230V |
The adjustments that matter more than the floor area
ENERGY STAR's sizing guidance and the standard trade practice agree on these, and this calculator applies them:
- Direct afternoon sun: add about 10%. South- and west-facing glass is the usual cause.
- Heavily shaded: subtract about 10%. A big tree outside the window genuinely counts.
- Kitchen: add a flat 4,000 BTU for appliance and cooking heat. A 300 sq ft kitchen and a 300 sq ft bedroom are not the same load.
- Extra occupants: add about 600 BTU per person beyond two. People are heaters.
- Ceilings above 8–9 ft: the volume being cooled grows with height, which the per-square-foot rule does not see.
- Poor insulation or leaky single-pane windows: push the whole result up.
So the full calculation is BTU = area × 20 × sun factor + kitchen + extra people, then round up to the next real model size.
Three worked examples
- 144 sq ft bedroom (12×12), average sun, two people. 144 × 20 = 2,880 BTU/h → the next standard size is 5,000 BTU. This is why small bedrooms almost always end up on the smallest unit available rather than on the raw number.
- 300 sq ft room with direct afternoon sun, two people. 300 × 20 = 6,000, +10% for sun = 6,600 BTU/h → a 7,000 BTU unit.
- 350 sq ft kitchen-diner, sunny, four people. 7,000 baseline, +10% sun = 7,700, +4,000 kitchen = 11,700, +1,200 for two extra people = 12,900 BTU/h → a 14,000 BTU unit. The kitchen alone accounts for nearly a third of the total.
Notice that in all three cases the rounded model size is well above the raw number. That gap is the safety margin, and it is deliberate — which is also why you should not then add your own extra on top.
Why bigger is not better
An oversized window AC cools the air quickly, satisfies the thermostat and shuts off before it has run long enough to remove humidity. The room ends up cold and clammy. Short cycling also wears the compressor.
Undersizing fails the other way: the unit runs continuously, never quite reaches setpoint on the hottest days, and uses more electricity over the season than a correctly sized unit that cycles.
Right-sizing is the single biggest decision in the purchase. Industry guidance commonly puts the benefit at 20–30% lower running cost and materially longer service life versus an oversized unit.
When a window unit is the wrong answer
A window AC stops being the right tool when: the space is over about 700 sq ft; the layout is long, open or split into several areas; you need quiet all-day cooling in a main living space; or the capacity you need pushes you into 230V equipment that will not run on a standard outlet (generally above about 12,000–15,000 BTU). At that point a mini-split is usually the better buy despite the higher installed cost.
Two practical checks before you buy either: measure the window opening — most units fit standard double-hung windows 23–36 in wide, and large units may need a sliding or casement kit — and confirm the circuit.
Common mistakes
- Buying from floor area alone. Sun, kitchen heat and occupancy regularly add a full size.
- Adding a "safety margin" on top of an already rounded-up size. The rounding is the margin.
- Ignoring ceiling height. The 20 BTU/sq ft rule assumes 8–9 ft.
- Using a bedroom rule for a kitchen. Kitchens carry a flat +4,000 BTU for a reason.
- Not checking the outlet. Larger units move to 230V and will not run on a normal receptacle.
- Skipping the window measurement. A correctly sized unit that does not fit the opening is not a solution.
- Forgetting the tilt. Most manufacturers require a slight downward tilt so condensation drains outside instead of into the frame.
Getting a correctly sized unit into the window
Most window AC problems after purchase are not capacity problems — they are fit, seal and drainage problems. This is the short list that prevents them.
- Window AC support bracket — Many manufacturers require one, and most windows will not carry the weight safely without it. Check the unit weight and bracket rating before you buy.
- Window AC weather seal foam — Seals the gap around the unit. Without it you are cooling the outdoors, and the extra load can push a right-sized unit into running flat out.
- Window AC cover — For the off-season, or for a unit left in place. Stops cold draughts and keeps dust out of the coil.
- Plug-in energy monitor — The honest way to see what the unit costs to run, and to compare whether a bigger or smaller size would have been cheaper over the season.
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Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on the equipment manufacturer, the page says so instead of guessing one.
- ENERGY STAR — room air conditioner sizing guidance: kitchen allowance, occupancy allowance and sun exposure adjustments.
- ACCA — Manual J residential cooling load calculation standards.
- U.S. Department of Energy — air conditioning sizing, efficiency ratings and home cooling guidance.
Figures, formulas and reference tables last verified: 21 September 2026.