Room BTU Calculator
Get a quick, rough estimate of the cooling capacity (in BTU/hr) a room air conditioner needs, based on room size, ceiling height and climate. This is a planning estimate only — see the limitations below.
Result
| Base estimate (moderate climate) | |
| Climate selected |
Units & conversions used
Imperial inputs are square feet and feet; the result is BTU per hour (BTU/hr), the standard rating unit for room air conditioners. Metric inputs are converted first: 1 m² = 10.7639 ft², 1 m = 3.28084 ft.
Formula & source
adjusted BTU/hr = base × climate factor (mild 0.85 / moderate 1.0 / hot 1.2)
Source: 20 BTU/hr per ft² at a standard 8-ft ceiling is an industry rule of thumb for room cooling, scaled by ceiling height and adjusted by climate. As a sanity reference, the ENERGY STAR room air conditioner sizing chart lists roughly 7,000 BTU/hr for a 250–300 ft² room — this calculator gives 7,200 BTU/hr for 300 ft² in a hot climate, in the same ballpark. This is a rough planning estimate, not a design basis.
Worked example (you can verify by hand)
Input: 300 ft² room, 8 ft ceiling, hot climate (factor 1.2).
- Base: 300 × 20 × (8 ÷ 8) = 6,000 BTU/hr
- Adjusted: 6,000 × 1.2 = 7,200 BTU/hr
Expected output: 7,200 BTU/hr. For reference, the ENERGY STAR room-AC chart suggests about 7,000 BTU/hr for a 250–300 ft² room.
How to choose the right size HVAC system
For a single-room air conditioner, the table below gives the commonly cited ENERGY STAR room-AC sizing ranges. Find your room area and read across — then adjust: go toward the high end for sunny rooms, kitchens, or hot climates, and toward the low end for shaded rooms or mild climates.
| Room area (ft²) | Recommended capacity (BTU/hr) |
|---|---|
| 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 |
| 1,000–1,500 | 21,000–24,000 |
Important: this table applies to room air conditioners only. Whole-house central AC, ductwork and heat pumps must be sized with a Manual J load calculation by an HVAC professional — guessing there is expensive in both directions (see limitations below).
Limitations — what this cannot do
⚠ This is an estimate only. It cannot replace a Manual J load calculation or an HVAC professional. Real cooling load depends on insulation, window size and orientation, sun exposure, air leakage, occupancy and appliances — none of which this calculator measures.
- Do not use this to size central air conditioning, ductwork or heat pumps — those require a proper Manual J calculation by a qualified professional.
- Oversized units short-cycle: they cool quickly but shut off before dehumidifying, leaving the room cold and clammy. Undersized units run constantly and never catch up.
- Only suitable for a rough starting point when shopping for a single-room air conditioner; confirm with the unit's sizing guidance and, for anything beyond a simple room, a professional.
FAQ
What is a BTU?
A British Thermal Unit is a unit of heat energy. For air conditioners, BTU/hr measures cooling power — how much heat the unit can remove per hour. Higher BTU/hr means a more powerful unit.
Can I use this to size my central AC?
No. Central systems must be sized with a Manual J load calculation that accounts for insulation, windows, orientation and more. This calculator is only a rough starting point for single-room units.
Why does ceiling height matter?
Taller ceilings mean more air volume to cool. The formula scales the 20 BTU/hr-per-ft² rule (based on a standard 8-ft ceiling) proportionally — a 10-ft ceiling needs about 25% more capacity.
Why does climate change the result?
The same room needs more cooling power where summers are hotter and longer. The factors (mild 0.85, moderate 1.0, hot 1.2) are rough multipliers — your actual climate zone and sun exposure matter more than any single number.
What happens if I buy an oversized AC?
An oversized unit cools the room fast but shuts off before removing humidity, leaving the air cold and damp. It also cycles on and off frequently, which wastes energy and wears the unit. Slightly under the estimate is usually safer than far over it.
How do I calculate the cooling load for my home?
For a single room, use the calculator at the top of this page or the sizing table above — room area, ceiling height and climate give a reasonable planning estimate. For a whole home, there is no shortcut: a proper cooling load requires a Manual J calculation by an HVAC professional, accounting for insulation, window size and orientation, sun exposure, air leakage, occupancy and appliances. Rule-of-thumb numbers are fine for shopping for a window unit; they are not a basis for sizing central equipment.