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AC Sizing: BTU Math and Why Oversized Units Underperform
Home & Garden · 6 min read · Last updated September 2026
The sizing formula
Tons = BTU ÷ 12,000
Worked example: 150 sq ft bedroom, hot climate, decent insulation: 150 × 80 = 12,000 BTU → 1.0 ton. The same room in a temperate climate with good insulation might need only 150 × 20 = 3,000 BTU — climate and insulation swing the answer 4×, which is why "one rule" sizing misleads.
Adjustments that matter
| Factor | Adjustment |
|---|---|
| Sunny room | +10% |
| Shaded room | −10% |
| Kitchen | +4,000 BTU (appliance heat) |
| Each regular occupant beyond 2 | +600 BTU |
| High ceiling / open plan | Size by volume, not floor area |
Why oversizing fails
An oversized unit drops temperature fast, satisfies the thermostat in minutes, and shuts off — before it has dehumidified the air. Result: cold-but-clammy rooms, more on/off cycling (wear + higher peaks), and worse comfort than the "smaller" correct unit. The AC/BTU Size Calculator applies all adjustments; the Electricity Bill Calculator estimates what your chosen tonnage costs to run.
Try the calculators
Worked example: AC/BTU Size Calculator at its default inputs
Take the default setup on AC/BTU Size Calculator: no fields changed, nothing tuned to a nice round number. Run in the site's own engine, those values produce the answer below.
| Input | Default |
|---|---|
| Room Length (ft) | 14 |
| Room Width (ft) | 12 |
| Ceiling Height (ft) | 8 |
| People in Room | 2 |
Result: 5000 BTU (min. 0.4 ton)
Room Length (ft): half, as given, and half again
Room Length (ft) is the first input worth testing, so here it is at three sizes.
| Room Length (ft) | Result |
|---|---|
| 7 | 5000 BTU (min. 0.2 ton) |
| 14 | 5000 BTU (min. 0.4 ton) |
| 21 | 7000 BTU (min. 0.6 ton) |
The middle row is the default. The outer two show how far the answer drifts when room length (ft) is wrong by half or by a half in the other direction.
Related guides
Tonnage is capacity, not comfort
One ton of cooling is about 3,517 watts, or 12,000 BTU per hour. The number describes how much heat the unit can remove, so it has to be matched to the room's heat load rather than to its floor area alone.
A room can be small and still need a large unit: a west-facing wall, a glass façade, an uninsulated roof or a kitchen all add load that square footage cannot express. That is why two rooms of the same size can need different tonnage.
What adds heat load
Sun on glazing, especially west and south, is usually the largest single factor after area. Insulation, ceiling height, the number of occupants (roughly 400-600 BTU per person) and appliances — a computer, a television, an oven — all add to the total.
Ventilation matters too: an open doorway to a hot corridor can undo a correctly sized unit. Fix the envelope before buying a bigger machine, because cooling a leaky room is a permanent running cost.
Oversizing is a real failure mode
A unit that is too large cools the air quickly, reaches the setpoint and switches off, leaving the room cold and clammy because it never ran long enough to remove humidity. Short cycling also wears the compressor and uses more energy than a steady run.
Undersizing fails differently: the unit runs continuously, never reaches the setpoint on the hottest days, and consumes maximum power while doing it. Both errors are expensive; the correct size is the one that runs steadily at design conditions.
Inverter units change the arithmetic
An inverter air conditioner varies compressor speed instead of switching on and off, so it holds a temperature rather than chasing it. That makes slight oversizing less harmful than with fixed-speed units, and it makes part-load efficiency the number to compare.
Check the seasonal efficiency rating rather than the headline tonnage when comparing two units. Two 1.5-ton inverters from different years can differ by a third in electricity use over a summer.
Servicing matters as much as the rating. A clogged filter and a fouled coil can add close to a fifth to consumption within one season, which is a larger effect than the gap between two closely matched models.
Frequently asked questions
How many BTU do I need per square foot?
Start at 20 BTU/sq ft for temperate, well-insulated rooms; hot climates with average insulation run 60–80 BTU/sq ft. Adjust ±10% for sun exposure and add for kitchens and extra occupants.
What size AC for a 150 sq ft room?
About 12,000 BTU (1 ton) in a hot climate: 150 × 80 = 12,000. Cooler climates with good insulation may need far less — run the full formula with your conditions.
Is a bigger AC better?
No. Oversized units short-cycle: they cool quickly, shut off before dehumidifying, and deliver cold-but-clammy air with more wear. Correct sizing beats bigger.
How do I convert BTU to tons?
Divide BTU by 12,000. 12,000 BTU = 1 ton, 18,000 = 1.5 tons, 24,000 = 2 tons.
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