Complete guide
BTU calculator for room air-conditioner sizing
Enter room dimensions, ceiling height, people, insulation, sunlight, climate and room type. Calculator24 estimates BTU per hour, cooling tons and kilowatts and shows every adjustment.
What this BTU estimate covers
The result targets a single room air conditioner. It starts with an area-based capacity table and exposes every multiplier or addition used.
Capacity and conversion formulas
The area table provides a base at an eight-foot ceiling. Multipliers adjust the base; occupants beyond two add 600 BTU/h each and a kitchen adds 4,000 BTU/h.
adjusted base=table BTU/h×ceiling factor×condition factors1 cooling ton=12,000 BTU/h1 W≈3.412 BTU/hWorked 300-square-foot example
A 20 by 15 foot room has 300 square feet and begins near 7,000 BTU/h. Average conditions preserve the base before occupant and kitchen additions.
Why oversizing is a problem
DOE guidance warns that an oversized room air conditioner can cost more, use more energy and remove humidity poorly because it cycles too quickly.
Envelope and climate limitations
Window area and orientation, air leakage, wall and roof insulation, ventilation, shading and outdoor design conditions affect the real load. Broad dropdowns cannot capture those details.
When to obtain a load calculation
For central systems, additions, unusual glazing, code compliance or equipment purchasing decisions, obtain a room-by-room professional load calculation such as ACCA Manual J and follow manufacturer sizing data.
Area-only cooling-capacity starting points
| Area (ft²) | Base 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 |
| 1,000–1,400 | 23,000 |
| 1,400–2,000 | 30,000 |
| 2,000–2,500 | 34,000 |
Frequently asked questions
What does BTU/h measure?
It is a rate of heat removal or delivery, not a one-time energy total.
Is a bigger air conditioner always better?
No. Oversizing can shorten cycles and reduce humidity removal.
How are metric rooms handled?
Meters are converted to feet so the same base table and ceiling adjustment apply.
How much does an extra person add?
This rough method adds 600 BTU/h for each regular occupant above two.
Does this replace Manual J?
No. It is a preliminary single-room estimate.
How many BTU/h are in one cooling ton?
One cooling ton equals 12,000 BTU/h.
What to keep in mind
This is a rough room-air-conditioner estimate, not an ACCA Manual J load calculation. Building envelope, windows, orientation, ventilation, ducts and local design weather can materially change required capacity.
Results display up to 8 decimal places; exported numbers preserve calculation precision. This is a calculation summary, not an official certificate.