BTU Calculator
Estimate the cooling BTU and tons an air conditioner needs for a room from its size, ceiling height, sun exposure, occupants, and whether it’s a kitchen. Calculates in your browser.
Cooling capacity needed
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Show the math
Look up the room's area in the ENERGY STAR room air conditioner sizing chart (for example 300 up to 350 sq ft → 8,000 BTU/hr), then adjust:
btu = chart size × sun factor (shaded 0.9, very sunny 1.1)
+ 600 per regular occupant over 2, + 4,000 for a kitchen
The chart assumes an 8-foot ceiling and covers 100 to 2,500 sq ft. It sizes a room air conditioner; it is not a full Manual J load calculation. One ton of cooling equals 12,000 BTU per hour. All math runs locally in your browser.
What this does
A BTU calculator estimates the cooling BTU and tons an air conditioner needs for a room from its size, ceiling height, sun exposure, occupants, and use.
How to use it
- Enter the room size.
- Set the ceiling height and sun exposure.
- Add the occupants.
- Read the BTU and tons.
How it works
It looks up the room’s area in the ENERGY STAR room air conditioner sizing chart (100 to 2,500 sq ft, 8-foot ceilings), then applies ENERGY STAR’s adjustments: −10% if heavily shaded, +10% if very sunny, +600 BTU per regular occupant over two, and +4,000 BTU for a kitchen. It is not a full Manual J.
1 ton = 12,000 BTU/hour
Understanding your result
This sizes a room air conditioner from the ENERGY STAR chart, not a full Manual J load calculation, and bigger is not better: an oversized air conditioner short-cycles, cools the air without removing humidity, and leaves the room clammy. Use the figure to find the right size band, then lean toward correct sizing rather than rounding up to the next large unit.
Example
A 300 sq ft room falls in the chart’s 300–350 sq ft band: 8,000 BTU/hr, about two-thirds of a ton, or 8,800 BTU/hr if the room is very sunny.
Last updated .
Frequently asked questions
How many BTU do I need per square foot?
It is not a fixed number per square foot: the ENERGY STAR chart runs from about 33 BTU per sq ft for small rooms (5,000 BTU for 150 sq ft) down to about 14 for large ones (34,000 BTU for 2,500 sq ft), before adjusting for sun, occupants, and kitchens.
How many BTU are in a ton?
One ton of cooling equals 12,000 BTU per hour. The tool reports both so you can match a unit’s rating.
Is this a Manual J load calculation?
No, it is a room air conditioner sizing chart with simple adjustments. A full Manual J accounts for insulation, windows, and climate in detail.
Do you store my numbers?
No. Everything is calculated locally in your browser.
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