BTU & AC Sizing Calculator
Work out the BTU load of a room and which air conditioner size to buy — with sun, insulation, ceiling height, occupancy and kitchen adjustments.
Enter the room length and width — the BTU load updates as you type.
- Room area
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- Cooling load
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- Air conditioner to buy
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- Same size in tons / kW
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- Heating load (rule of thumb)
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ENERGY STAR room air conditioner sizing table
| Room area (sq ft) | Capacity (BTU/h) |
|---|---|
| 100 – 150 | 5,000 |
| 150 – 250 | 6,000 |
| 250 – 300 | 7,000 |
| 300 – 350 | 8,000 |
| 350 – 400 | 9,000 |
| 400 – 450 | 10,000 |
| 450 – 550 | 12,000 |
| 550 – 700 | 14,000 |
| 700 – 1,000 | 18,000 |
| 1,000 – 1,200 | 21,000 |
| 1,200 – 1,400 | 23,000 |
| 1,400 – 1,500 | 24,000 |
| 1,500 – 2,000 | 30,000 |
| 2,000 – 2,500 | 34,000 |
This is a rule-of-thumb estimate for picking a room air conditioner, mini-split or portable unit. A whole-house system, a heat pump or anything you are paying an installer for should be sized with a proper ACCA Manual J load calculation, which accounts for window glazing, orientation, air leakage and local design temperatures.
How it works
The calculator uses the ENERGY STAR / AHAM room air conditioner rule: 20 BTU per hour per square foot of floor space with a standard ceiling (8 ft / 2.4 m). It then applies the adjustments from the same guidance — reduce the capacity by 10% for a heavily shaded room, increase it by 10% for a very sunny one, add 600 BTU for each regular occupant beyond two, and add 4,000 BTU if the unit is cooling a kitchen. Two extra factors are included because they matter in practice: taller ceilings scale the load by height ÷ standard height (you are cooling volume, not floor), and insulation quality adds 15% for a leaky older building or takes off 10% for a well-sealed one.
The result is rounded to the nearest 50 BTU and matched to the next standard unit size on the shelf (5,000, 6,000, 8,000, 9,000, 10,000, 12,000, 14,000, 18,000, 24,000 BTU and up), then also shown in tons (1 ton = 12,000 BTU/h) and kilowatts (1 kW = 3,412.14 BTU/h). The heating figure is the usual furnace rule of thumb: 30–60 BTU per square foot depending on how cold your winters are, with the same insulation and ceiling adjustments. Oversizing is a real mistake — an air conditioner that is too big short-cycles, never runs long enough to pull humidity out of the air and leaves the room cold and clammy. Everything is worked out in your browser with plain JavaScript: nothing is uploaded, there are no sign-ups and no ads.
Frequently asked questions
How many BTU do I need for a room?
Start from the ENERGY STAR / AHAM rule of 20 BTU per hour per square foot of floor area with a standard 8 ft (2.4 m) ceiling, then adjust. A 20 × 25 ft room is 500 sq ft, so 500 × 20 = 10,000 BTU/h. Take 10% off if the room is heavily shaded, add 10% if it is very sunny, add 600 BTU for every regular occupant beyond two, and add 4,000 BTU if the unit is cooling a kitchen. Scale by ceiling height ÷ 8 ft, because you are cooling volume rather than floor area, and allow roughly 15% more for a leaky older building.
What size air conditioner is 12,000 BTU, and how many tons is that?
One ton of cooling is 12,000 BTU per hour — the term comes from the heat needed to melt a ton of ice in 24 hours and has nothing to do with the weight of the unit. So 12,000 BTU/h is exactly 1 ton, 24,000 BTU/h is 2 tons and 9,000 BTU/h is 0.75 tons. In kilowatts, divide by 3,412.14: a 12,000 BTU/h unit is about 3.52 kW of cooling. This calculator shows all three figures side by side, plus the nearest off-the-shelf unit size.
Is it bad to buy an air conditioner that is too big?
Yes, and it is the most common sizing mistake. An oversized unit reaches the thermostat setpoint quickly and shuts off, so it short-cycles: it never runs long enough to pull moisture out of the air, and the room ends up cold and clammy instead of cool and dry. Short-cycling also wears the compressor and wastes energy. Aim for the smallest unit that covers the calculated load rather than the biggest one you can afford — and for a whole-house system, or anything an installer is quoting on, insist on a proper ACCA Manual J load calculation instead of a rule of thumb.