This AC BTU calculator focuses on air conditioner capacity estimates from the cooling related fields on the page.
For a general BTU planning page, open the BTU calculator. Measure the room with the square footage calculator first.
How the AC BTU calculator turns three inputs into a size
BTU/h = room area (sq ft) × climate factor × (ceiling height ÷ 8)
| Climate setting | Factor (BTU per sq ft) | 400 sq ft, 8 ft ceiling |
|---|---|---|
| Mild | 20 | 8,000 |
| Average | 25 | 10,000 |
| Hot | 30 | 12,000 |
The average factor lines up with the ENERGY STAR room chart at 400 sq ft (10,000 BTU). The mild and hot settings are this tool’s own ±20% planning bands, not ENERGY STAR figures.
Choosing a climate setting in the AC BTU calculator
- Mild: summers where most days are warm rather than hot, and nights cool down.
- Average: regular hot spells, but not weeks of extreme heat.
- Hot: long stretches of very hot days, warm nights, or a top-floor room under a sun-baked roof.
If you’re between two settings, run both and treat the results as a range.
Why ceiling height scales the result
Standard sizing charts assume 8-foot ceilings. A 10-foot ceiling holds 25% more air than an 8-foot one over the same floor, so the AC BTU calculator multiplies by 10 ÷ 8 = 1.25. Vaulted ceilings vary in height, so use the average height across the room, not the peak.
AC BTU calculator worked example: hot climate, 9 ft ceiling
- Room: 15 × 22 ft = 330 sq ft. You can check this with the square footage calculator.
- Hot climate: 330 × 30 = 9,900 BTU.
- Ceiling: 9,900 × (9 ÷ 8) = 9,900 × 1.125 = 11,137.5 BTU.
- Shopping size: common room units come in steps such as 10,000, 12,000 and 14,000 BTU. 12,000 BTU is the nearest size that covers the estimate.
For comparison, the same room on the average setting with an 8-foot ceiling comes to 330 × 25 = 8,250 BTU. Climate and ceiling together add about 35%.
Don’t oversize “to be safe”
ENERGY STAR’s room air conditioner guidance warns that an oversized unit cools the room before it can remove humidity, leaving it damp and clammy, and wastes energy by cycling on and off. If the estimate is just over a step, check the room first. Shading, insulation and an installation sealed with the supplied foam can reduce the load.
Turning BTUs into running cost
BTU/h measures cooling output, not power use. Electricity use depends on the unit’s efficiency rating. Divide BTU/h by the rated efficiency (BTU per watt-hour) to estimate watts, then use the electricity calculator for the cost per day or season.
Rooms the AC BTU calculator can’t see
The AC BTU calculator knows area, climate and height only. Bump your estimate toward the next step for a kitchen, a room that regularly holds more than two people, west-facing glass or a top-floor room; drop toward the lower step for a heavily shaded room. For those adjustments in exact figures, use the BTU calculator, which applies ENERGY STAR’s percentages and per-person additions.
Last reviewed: October 2026