BTU Calculator
BTU/h = the ENERGY STAR chart value for the room's area: a 15 × 20 ft room (300 ft²) needs 8,000 BTU/h. Add 10 % for a very sunny room, 600 BTU/h per person above two and 4,000 BTU/h for a kitchen.
Cooling capacity needed
8,000 BTU/h
2.34 kW · 0.67 tons
- Room area
- 300 ft²
- ENERGY STAR chart base
- 8,000 BTU/h
Chart values assume 8 ft ceilings. For higher ceilings or poor insulation, choose the next size up or ask an HVAC pro.
How it works
Area = length × width
Base capacity = ENERGY STAR chart value for that area (e.g. 450–550 ft² → 12,000 BTU/h)
Heavily shaded: −10 % · very sunny: +10 %
+600 BTU/h for each regular occupant above two · kitchen: +4,000 BTU/h
Assumptions
- Chart rows run "from X up to Y" square feet: an area exactly on a boundary uses the higher row.
- ENERGY STAR calculates the chart capacities for an 8 ft ceiling. The chart stops at 2,500 ft², so the calculator refuses larger areas.
- 1 ton of cooling = 12,000 BTU/h; 1 BTU/h = 0.2930711 W.
Worked examples
A 450 ft² living room, very sunny, 4 people
- Chart: 450 ft² falls in 450–550 → 12,000 BTU/h
- Very sunny: 12,000 × 1.10 = 13,200
- Two people above two: + 2 × 600 = 14,400 BTU/h
Result: 14,400 BTU/h.
Checked by the automated test “BTU — 450 ft², very sunny, 4 people = 14,400 BTU/h”.
A 12 × 15 ft kitchen
- 12 × 15 = 180 ft² → 6,000 BTU/h
- Kitchen: + 4,000 = 10,000 BTU/h
Result: 10,000 BTU/h.
Checked by the automated test “BTU — 180 ft² kitchen = 10,000 BTU/h”.
Picking a unit size
- Choose the closest size on the shelf. Room units come in steps (8,000, 10,000, 12,000 BTU/h…). Pick the one nearest the result rather than jumping to the next size up.
- Ceilings above 8 ft: the chart assumes 8 ft, and ENERGY STAR notes that higher ceilings may call for a more powerful unit.
- Check the plug and circuit. Large units can need a dedicated circuit or a different outlet: read the electrical requirements on the box before buying.
Common mistakes
- Measuring the whole floor. Count only the space the unit will cool. An open kitchen-living area is one room; a closed hallway is not.
- Ignoring the sun. Rooms under the roof or with large west-facing windows often behave like the “very sunny” case; use the sun setting rather than adding a size.
- Sizing central air with a room chart. The chart stops at 2,500 ft² and is meant for one room, not a ducted system.
Frequently asked questions
How many BTU per square foot do I need?
There is no fixed rate in the ENERGY STAR method: it uses a chart. A 300 ft² room gets 8,000 BTU/h (about 27 per ft²), a 1,000 ft² room 21,000 BTU/h (21 per ft²). Sun, people and a kitchen then change the figure.
How many tons of air conditioning do I need?
Divide the BTU/h by 12,000, the size of one ton of cooling (NIST). The 14,400 BTU/h room above needs 1.2 tons. For a whole house, ENERGY STAR says the contractor should calculate your home's heating and cooling loads with ACCA Manual J or an equivalent method.
What room size does a 10,000 BTU air conditioner cool?
Read the ENERGY STAR chart backwards: 8,000 BTU/h is listed for 300 to 350 ft², 10,000 BTU/h for 400 to 450 ft² and 12,000 BTU/h for 450 to 550 ft², before any adjustment for sun, people or a kitchen. Bigger is not better: ENERGY STAR warns that a unit that is too large cools the room before it has a chance to remove the humidity, so the room feels damp and clammy.
Can I use this for heating?
No. The ENERGY STAR chart sizes room air conditioners for cooling. To size a heating system, ENERGY STAR asks the contractor to calculate your home's heating load with ACCA Manual J or an equivalent method.
Sources
Formula, examples and sources checked on by the Yardstick Math team. How we check our formulas.
- Room Air Conditioners – how to choose the right size (cooling capacity chart and adjustments) — ENERGY STAR (US EPA)
- Room Air Conditioner fact sheet — ENERGY STAR (US EPA)
- Heating & Air Conditioning Installation Bid Comparison Checklist (load calculation with ACCA Manual J or equivalent) — ENERGY STAR (US EPA and US DOE)
- NIST Guide to the SI, Appendix B.8 – Factors for units listed alphabetically (ton of refrigeration = 12,000 BTU/h) — NIST