BTU Calculator
A rough range for the heating or cooling a room needs, in BTU per hour, tons and kilowatts, from its floor area and your own BTU-per-square-foot figures.
Result
12,000 BTU/hEstimate
A rough range from your own factors. It is not HVAC sizing; a Manual J calculation is what to act on.
- Estimate, high
- 18,000 BTU/h (5 kW)Estimate
- This is not a Manual J calculation
- A rough estimate, not HVAC sizing. Equipment is sized by a Manual J load calculation, which accounts for construction, windows, orientation, air leakage and your climate. An oversized air conditioner short-cycles and dehumidifies poorly, so bigger is not safer.
- In tons of cooling, low
- 1 ton (3.52 kW)Estimate
- In tons of cooling, high
- 1.5 ton (5.28 kW)Estimate
https://calcgallery.com/home-systems/btu-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the BTU Calculator
- Enter the room's floor area and ceiling height.
- Enter a low and a high figure for BTU per hour per square foot, from a source you trust for your climate. The page supplies none, because the right figure depends on climate and construction.
- Add the people regularly in the room and whether it is a kitchen, then read the range. Treat it as a rough guide, not a sizing.
What this is, and is not
This page multiplies a floor area by a factor. That is the rough rule of thumb behind many sizing charts, and it is useful for a first idea of scale. It is not how heating and cooling equipment should be sized. The industry method, ACCA Manual J, works out the heat a house actually gains and loses through its walls, roof, windows and air leakage, for your local design temperatures. Two rooms of the same area can need very different capacities.
BTU/h = floor area × factor × (ceiling height ÷ 8 ft) + 4,000 for a kitchen + 600 per regular occupant beyond twotons = BTU/h ÷ 12,000The result is a range, from your low and high factors, because a single number would suggest more precision than the method has. Ask an HVAC contractor for a Manual J calculation before buying equipment.
With your numbers
- BTU/h = area × factor × ceiling ÷ 8 ft, + 4,000 for a kitchen, + 600 per person beyond two
Quick reference
A rough range from your own factors. It is not HVAC sizing; a Manual J calculation is what to act on.
| Job | Estimate, low |
|---|---|
| 600 ft² at 20 BTU, 8 ft | 12,000 BTU/h |
| 600 ft², 9 ft ceiling | 13,500 BTU/h |
| 600 ft² kitchen | 16,000 BTU/h |
| 600 ft², 4 people | 13,200 BTU/h |
| 1,200 ft² at 30 | 36,000 BTU/h |
| 55 m² at 20 to 30 | 11,654 BTU/h |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
3 sources
ENERGY STAR, Room Air Conditioners, section 3 "Adjust as necessary": "If the unit is used in a kitchen, increase capacity by 4,000 BTUs." https://www.energystar.gov/products/room_air_conditioners
Used for
- Cooling added for a kitchen in the area-based estimate: 4,000 BTU/h.
ENERGY STAR, Room Air Conditioners, section 3 "Adjust as necessary": "If more than two people regularly occupy the room, add 600 BTUs for each additional person." https://www.energystar.gov/products/room_air_conditioners
Used for
- Cooling added per regular occupant beyond two in the area-based estimate: 600 BTU/h.
ENERGY STAR, Room Air Conditioners, section 2: "The capacities are calculated based on an 8-foot ceiling." https://www.energystar.gov/products/room_air_conditioners
Used for
- The ceiling height ENERGY STAR's capacity chart assumes: 8 feet. Scaling taller rooms up in proportion is this site's method, not ENERGY STAR's.
Common questions about the BTU Calculator
- How many BTU do I need per square foot?
- There is no single figure. It depends on your climate, insulation, windows, sun and air leakage, which is why this page takes a low and a high figure from you rather than assuming one. Published homeowner guides give ranges by climate; a professional Manual J calculation replaces the rule of thumb with the house’s actual heat gains and losses.
- What is a ton of air conditioning?
- A cooling rate of 12,000 BTU per hour, by definition. It comes from the heat needed to melt a ton of ice in a day. A 3-ton unit removes 36,000 BTU of heat an hour.
- Is a bigger air conditioner better?
- No. An oversized unit cools the air quickly and switches off before it has removed much moisture, so the room feels cold and clammy, and the short cycles wear the equipment. Sizing to the actual load, which is what Manual J does, is better than sizing up for safety.
- Why do kitchens and people add to the figure?
- Both add heat. The adjustments here follow ENERGY STAR’s room air conditioner guidance: about 4,000 BTU per hour more for a kitchen, and 600 more for each person who regularly uses the room beyond two.