What does a BTU calculator say a room needs? Use ENERGY STAR’s chart: 5,000 Btu per hour for 100 to 150 square feet, rising to 12,000 for 450 to 550 and 34,000 for 2,000 to 2,500. Then adjust for sun, shade, occupants and kitchens. The chart is for single room units, not central air.
Key Takeaways
- ENERGY STAR’s chart has 14 bands, from 5,000 Btu/h (100 up to 150 sq ft) to 34,000 Btu/h (2,000 up to 2,500 sq ft), assuming an 8-foot ceiling (ENERGY STAR).
- Adjustments: minus 10% if heavily shaded, plus 10% if very sunny, plus 600 Btu per regular occupant above two, plus 4,000 Btu for a kitchen, then round down to the nearest thousand (ENERGY STAR, February 2024).
- One ton of cooling equals 12,000 Btu per hour (DOE/NREL, April 2013).
- For central systems, DOE Energy Saver points to ACCA Manual J and Manual S, not a room chart (archived copy).
- Portables are rated in seasonally adjusted cooling capacity, weighting a 95°F test at 0.2 and an 83°F test at 0.8 (10 CFR 430, Appendix CC).
What does a BTU calculator for a room air conditioner do?
A BTU calculator turns one room’s floor area into the cooling capacity, in Btu per hour, that a window or through-the-wall unit should deliver. The most defensible version is a lookup table: the sizing chart on ENERGY STAR’s room air conditioner page.
Federal rules define a room air conditioner as a window-mounted or through-the-wall encased assembly running on single-phase power (10 CFR 430.2). If you are still choosing equipment, start with the main ways homes are air conditioned. Oversizing is the usual mistake: ENERGY STAR warns a unit that is too large cools the room before removing humidity, leaving it damp and clammy, the same problem behind a house that feels cold but humid.
What is the ENERGY STAR room air conditioner sizing chart?
It is a 14-row table pairing a square-footage band with one recommended capacity. ENERGY STAR states the capacities are calculated for an 8-foot ceiling.
| Area to be cooled (sq ft) | Capacity needed (Btu/h) |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
| 1,400 up to 1,500 | 24,000 |
| 1,500 up to 2,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 |
Source: ENERGY STAR, Room Air Conditioners, accessed September 2026.
The chart is not a flat per-square-foot rule. The top of the smallest band works out to about 33 Btu per square foot (5,000 divided by 150); the top of the largest, 13.6 (34,000 divided by 2,500). An archived DOE Energy Saver page instead gives 20 Btu for each square foot (archived copy).
How do you use a BTU calculator step by step?
Measure, multiply, look up the band, adjust, then round down. ENERGY STAR sets out that sequence in its Ask the Experts article.
- Measure length and width in feet (9 inches is 0.75 ft, 6 inches 0.5 ft, 3 inches 0.25 ft).
- Multiply length by width. For a triangle, divide by two; split complex shapes and add them.
- Find the square footage in the chart.
- Apply the adjustments that fit the room.
- Round down to the nearest thousand and shop for that capacity.
ENERGY STAR’s example: a room 14 ft 9 in by 10 ft is 147.5 square feet and needs 5,000 Btu. Pick your band below.
Choose your room size (square feet):
5,000 Btu/h
Sunny: 5,500, buy 5,000. Shaded: 4,500, buy 4,000. Kitchen: 9,000.
6,000 Btu/h
Sunny: 6,600, buy 6,000. Shaded: 5,400, buy 5,000. Kitchen: 10,000.
7,000 Btu/h
Sunny: 7,700, buy 7,000. Shaded: 6,300, buy 6,000. Kitchen: 11,000.
8,000 Btu/h
Sunny: 8,800, buy 8,000. Shaded: 7,200, buy 7,000. Kitchen: 12,000.
9,000 Btu/h
Sunny: 9,900, buy 9,000. Shaded: 8,100, buy 8,000. Kitchen: 13,000.
10,000 Btu/h
Sunny: 11,000, buy 11,000. Shaded: 9,000, buy 9,000. Kitchen: 14,000.
12,000 Btu/h
Sunny: 13,200, buy 13,000. Shaded: 10,800, buy 10,000. Kitchen: 16,000.
14,000 Btu/h
Sunny: 15,400, buy 15,000. Shaded: 12,600, buy 12,000. Kitchen: 18,000.
18,000 Btu/h
Sunny: 19,800, buy 19,000. Shaded: 16,200, buy 16,000. Kitchen: 22,000.
21,000 Btu/h
Sunny: 23,100, buy 23,000. Shaded: 18,900, buy 18,000. Kitchen: 25,000.
23,000 Btu/h
Sunny: 25,300, buy 25,000. Shaded: 20,700, buy 20,000. Kitchen: 27,000.
24,000 Btu/h
Sunny: 26,400, buy 26,000. Shaded: 21,600, buy 21,000. Kitchen: 28,000.
30,000 Btu/h
Sunny: 33,000, buy 33,000. Shaded: 27,000, buy 27,000. Kitchen: 34,000.
34,000 Btu/h
Sunny: 37,400, buy 37,000. Shaded: 30,600, buy 30,000. Kitchen: 38,000.
HVAC Brief arithmetic on ENERGY STAR figures, one adjustment at a time; ENERGY STAR states no order for combining them. “Buy” is the total rounded down to the nearest thousand.
Which adjustments change the BTU calculator result?
ENERGY STAR publishes four numeric adjustments, for shade, sun, occupants and kitchens, worded identically on its Ask the Experts article and product page.
| Room condition | ENERGY STAR adjustment | On a 10,000 Btu/h base |
|---|---|---|
| Heavily shaded | Reduce 10 percent | 9,000 |
| Very sunny | Increase 10 percent | 11,000 |
| More than two regular occupants | Add 600 Btu per extra person | 10,600 with three people |
| Kitchen | Add 4,000 Btu | 14,000 |
| Ceiling above 8 feet | No numeric factor published | Not calculable |
Rounding down has a side effect. Every chart value is a whole thousand, so one extra occupant (600 Btu) never changes the shopping number alone, and on bases of 9,000 Btu/h or less the 10 percent sun increase disappears too.
ENERGY STAR’s old flyer disagrees with its chart
A June 2003 ENERGY STAR flyer still on energystar.gov says a 525 square foot room needs 14,000 Btu/hr, and prints overlapping bands of “450 to 550” and “500 to 700” (2003 flyer). The current chart puts 525 square feet at 12,000. Use the current chart.
How many BTUs are in a ton of cooling?
One ton of cooling is 12,000 Btu per hour. A DOE Uniform Methods Project protocol defines a ton as 12,000 Btu/hr, the power required to melt 1 ton of ice in 24 hours (DOE/NREL, 2013).
ENERGY STAR uses the same conversion, noting a 3-ton system equals 36,000 Btuh (ENERGY STAR guide). On the room chart, one ton serves 450 up to 550 square feet.
Why a room BTU calculator does not size central air
ENERGY STAR’s chart is guidance for a single room unit, and it should not be scaled up to a house. For ducted systems, DOE Energy Saver tells homeowners to have contractors size equipment with an ACCA Manual J load calculation and a Manual S equipment selection protocol (archived copy).
That is why our guide to sizing a whole-house HVAC system publishes no Btu-per-square-foot table, and the two positions are consistent. A room unit faces one room’s sun, occupants and appliances, which the four adjustments roughly capture; a ducted central system also carries duct losses and air leakage across every room.
Whole-house loads also move with the envelope, which is why insulating before replacing equipment comes first.
Do portable and wall units use the same BTU numbers?
Through-the-wall units do, because federal rules class them as room air conditioners alongside window units. Portables do not: DOE rates them in seasonally adjusted cooling capacity (SACC) under a separate test procedure (Appendix CC).
It subtracts duct and infiltration heat transfer from measured capacity, then weights the 95°F condition at 0.2 and 83°F at 0.8. ENERGY STAR’s chart covers room units, so treat a portable’s SACC as a separate measure. Our explainer on why single-hose portables underperform covers the airflow reason, and hoseless portables get their own page.
Capacity also sets the federal efficiency floor. A cooling-only louvered unit of 8,000 to 13,900 Btu/h manufactured from May 26, 2026 must reach a combined energy efficiency ratio of 16.0 (10 CFR 430.32(b)); see the 2026 CEER minimums for every class, or compare through-the-wall units.
Check the circuit too. DOE Energy Saver says units under 7.5 amps can use a 15- or 20-amp, 120-volt circuit not shared with major appliances; larger units need a dedicated circuit, the largest 240 volts. Local code varies, so confirm with an electrician. Running cost depends on local rates, as our look at air conditioning running costs explains.
How we researched this
Researched September 17, 2026: 13 sources consulted, 10 cited. The chart was copied from the live ENERGY STAR product page and checked against the Ask the Experts article, whose text cites “the table below” but no longer shows it. Federal text is from the current eCFR; DOE Energy Saver pages now return 404, so archived copies are cited.
Excluded: retailer calculators, contractor blogs and online ceiling or insulation multipliers, none traceable to a federal source. A Reddit search surfaced no usable threads.
Frequently asked questions
What size AC do I need for a 12 by 12 room?
A 12 by 12 foot room is 144 square feet, inside ENERGY STAR’s 100 up to 150 square foot band, which calls for 5,000 Btu per hour. Reduce 10 percent if heavily shaded or add 10 percent if very sunny, then round down to the nearest thousand.
What size window air conditioner do I need for 500 square feet?
ENERGY STAR’s current chart puts 500 square feet in the 450 up to 550 band, which calls for 12,000 Btu per hour, or one ton of cooling. A 2003 ENERGY STAR flyer printed overlapping bands here, so use the current chart. Add 4,000 Btu for a kitchen.
How many BTUs does ENERGY STAR add per person?
ENERGY STAR adds 600 Btu for each additional person when more than two people regularly occupy the room. Four regular occupants therefore add 1,200 Btu to the chart value. Because chart values are whole thousands, a single extra person disappears when you round down.
How many BTUs is a 1-ton air conditioner?
A 1-ton air conditioner delivers 12,000 Btu per hour. A Department of Energy Uniform Methods Project protocol defines a ton as 12,000 Btu/hr, the power needed to melt 1 ton of ice in 24 hours, and ENERGY STAR notes a 3-ton system equals 36,000 Btuh.
Can I use a BTU calculator to size central air conditioning?
No. ENERGY STAR’s chart is written for room air conditioners serving one space. For central systems, DOE Energy Saver advises contractor sizing with an ACCA Manual J load calculation and Manual S equipment selection, which account for insulation, air leakage and ducts.
Does ceiling height change the BTU calculation?
ENERGY STAR says its chart capacities are calculated for an 8-foot ceiling and that higher ceilings may call for a unit with a higher CADR. It publishes no percentage or Btu figure for that adjustment, so fixed multipliers seen online do not come from ENERGY STAR.
Sources
- ENERGY STAR. “Room Air Conditioners.” https://www.energystar.gov/products/room_air_conditioners Accessed September 2026.
- ENERGY STAR. “How To Choose the Right Sized Window AC (Ask the Experts, February 2, 2024).” https://www.energystar.gov/products/ask-the-experts/how-choose-right-sized-window-ac Accessed September 2026.
- ENERGY STAR. “ENERGY STAR Qualified Room Air Conditioners (flyer, June 2003).” https://www.energystar.gov/ia/partners/manuf_res/marketing_res/ROOM_AIRCONDITIONER.pdf Accessed September 2026.
- U.S. Department of Energy / NREL. “Uniform Methods Project, Chapter 4: Unitary and Split System HVAC Cooling Equipment (April 2013).” https://www.energy.gov/sites/prod/files/2013/05/f0/53827-4.pdf Accessed September 2026.
- ENERGY STAR. “Using the CEE/ARI Online Database.” https://www.energystar.gov/sites/default/files/asset/document/CEE-ARI_Guide.pdf Accessed September 2026.
- U.S. Department of Energy, Energy Saver. “Central Air Conditioning (archived copy).” https://web.archive.org/web/2025id_/https://www.energy.gov/energysaver/central-air-conditioning Accessed September 2026.
- U.S. Department of Energy, Energy Saver. “Room Air Conditioners (archived copy).” https://web.archive.org/web/2025id_/https://www.energy.gov/energysaver/room-air-conditioners Accessed September 2026.
- eCFR. “10 CFR 430.2, Definitions.” https://www.ecfr.gov/current/title-10/chapter-II/subchapter-D/part-430/subpart-A/section-430.2 Accessed September 2026.
- eCFR. “10 CFR 430.32, conservation standards.” https://www.ecfr.gov/current/title-10/chapter-II/subchapter-D/part-430/subpart-C/section-430.32 Accessed September 2026.
- eCFR. “10 CFR 430, Subpart B, Appendix CC (portable air conditioners).” https://www.ecfr.gov/current/title-10/chapter-II/subchapter-D/part-430/subpart-B/appendix-Appendix%20CC%20to%20Subpart%20B%20of%20Part%20430 Accessed September 2026.
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