AC Size
Room facts in, cooling load out — BTUs and the tonnage equivalent, sized to cycle right instead of short-cycling clammy.
Cooling load
0
BTU / hr · 0.7 ton system- Base load
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- With adjustments
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- Shop range
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Whole-house loads need a Manual J — this sizes one room or zone.
How AC sizing works
The industry shorthand is 20 BTU per hour for every square foot of floor, which lands most American rooms within reach of the shelf labels. The real load also scales with volume (taller ceilings hold more air), gains through sunny glass, leaks through thin walls, and the raw heat of your summer — each nudge above moves the number a fixed, stated amount.
Round to the nearest unit actually sold: window units come in 5,000 / 6,000 / 8,000 / 10,000 / 12,000 / 15,000 BTU steps, and mini-splits in 9k / 12k / 18k / 24k. Undershooting runs the compressor flat-out on August afternoons; overshooting chills without dehumidifying.
Kitchens are different
Cooking equipment adds serious heat. If the room is a kitchen, bump the result another 4,000 BTU before you shop — the checkboxes don't know about your stove.
The other half of the year
This sizes cooling. The same room facts size heat, in different units: the electric heat calculator turns them into watts, feet of baseboard and the breaker the circuit needs. If the heat is coming from a heat pump, the cooling number above is the one the installer will start from anyway.
On the counter
The assumptions this calculator ships with — every one is adjustable in the form.
- Base rate
- 20 BTU/hr per sq ft — the standard sizing rule of thumb
- Ceiling factor
- Scales with height above/below an 8 ft ceiling
- Adjustments
- Sun +10%, shade −10%, poor insulation +15%, hot climate +10%
Questions from the aisle
How many BTUs does a 400 sq ft room need?
At the standard 20 BTU/hr per square foot, about 8,000 BTU. With tall ceilings, strong sun, or a hot climate it climbs toward 10,000 — which is exactly what the adjustment boxes above account for.
What size window AC for 300 square feet?
Around 6,000–8,000 BTU/hr depending on sun and insulation. Most stores label units by this range, and 8,000 BTU covers the shaded-to-typical spread safely.
Is bigger always better?
No. An oversized unit cools fast then shuts off before it dehumidifies, leaving rooms cold and clammy. Sizing close to the real load keeps the compressor cycling properly.
What is a ton of air conditioning?
12,000 BTU per hour. The tag on central systems reads in tons; window units read in BTU. This calculator shows both so you can shop either shelf.
The reasoning behind this one
Sizing an air conditioner
Where the assumptions above come from, what changes them, and the mistakes that cost a second trip. Free to read, no account needed.