BTU Calculator for Cooling and Heating a Room
A 12 × 15 ft bedroom (180 ft²) needs 6,000 BTU/h of cooling on the ENERGY STAR sizing chart, about 33 BTU per square foot. Big rooms need less per square foot. Heating depends on how cold your winter gets: the same room needs about 3,670 BTU/h in Chicago with average insulation.
How this BTU calculator works
Cooling and heating are sized from different data, so the tool runs two calculations on one set of inputs.
Cooling: the ENERGY STAR room chart plus four adjustments
The starting number comes from the room air conditioner sizing chart published by ENERGY STAR (EPA). The chart assumes an 8-foot ceiling. We then apply the chart's own corrections and one of ours:
+ 600 × (people − 2) + 4,000 if kitchen
The ceiling factor is our assumption: ENERGY STAR only says the chart is based on an 8 ft ceiling, so a 10 ft ceiling gets 25% more air volume and 25% more BTU. Everything else is quoted from the chart page. The steps of your own result are listed under the readout, so you can check each one.
Heating: design temperature × how leaky the house is
The outdoor design temperature is the 99% heating value from ASHRAE climate data: the winter temperature your area stays above 99% of the hours in a year. UA is heat loss per square foot of floor per degree. It runs from 0.12 for a tight new house to 0.60 for an old uninsulated one. These four levels are our simplified version of a room-by-room Manual J load. The methodology page shows how we got each one from typical wall, attic, window and air-leak values.
ENERGY STAR BTU chart by square footage
The chart below is the ENERGY STAR table, plus two columns we added: BTU per square foot at the middle of each range and the same capacity in tons. Notice how BTU per square foot falls as rooms grow, which is why a flat "20 BTU per sq ft" rule undersizes bedrooms and oversizes open-plan rooms.
| Area (ft²) | BTU/h needed | BTU per ft² (mid) | Tons |
|---|---|---|---|
| 100 – 150 | 5,000 | 40 | 0.42 |
| 150 – 250 | 6,000 | 30 | 0.50 |
| 250 – 300 | 7,000 | 25 | 0.58 |
| 300 – 350 | 8,000 | 25 | 0.67 |
| 350 – 400 | 9,000 | 24 | 0.75 |
| 400 – 450 | 10,000 | 24 | 0.83 |
| 450 – 550 | 12,000 | 24 | 1.00 |
| 550 – 700 | 14,000 | 22 | 1.17 |
| 700 – 1,000 | 18,000 | 21 | 1.50 |
| 1,000 – 1,200 | 21,000 | 19 | 1.75 |
| 1,200 – 1,400 | 23,000 | 18 | 1.92 |
| 1,400 – 1,500 | 24,000 | 17 | 2.00 |
| 1,500 – 2,000 | 30,000 | 17 | 2.50 |
| 2,000 – 2,500 | 34,000 | 15 | 2.83 |
What size room does a 5,000 to 24,000 BTU unit cool?
Read the chart the other way round when you already have a unit in mind. The watts column assumes a typical room AC efficiency (CEER) of 11 BTU per watt-hour. Your unit's label shows its own CEER.
| Unit size | Room it fits | Tons | Power draw (CEER 11) |
|---|---|---|---|
| 5,000 BTU/h | 100 – 150 ft² | 0.42 | 455 W |
| 6,000 BTU/h | 150 – 250 ft² | 0.50 | 545 W |
| 8,000 BTU/h | 300 – 350 ft² | 0.67 | 727 W |
| 10,000 BTU/h | 400 – 450 ft² | 0.83 | 909 W |
| 12,000 BTU/h | 450 – 550 ft² | 1.00 | 1,091 W |
| 14,000 BTU/h | 550 – 700 ft² | 1.17 | 1,273 W |
| 18,000 BTU/h | 700 – 1,000 ft² | 1.50 | 1,636 W |
| 24,000 BTU/h | 1,400 – 1,500 ft² | 2.00 | 2,182 W |
Heating BTU per square foot by climate zone
This table is not in the usual BTU charts. It shows why one national "BTU per square foot for heat" number cannot work: the answer changes about six times from Miami to Fairbanks. Values are BTU/h per ft² with 8 ft ceilings at the 99% design temperature of one representative city per zone.
| Zone | City | 99% design | ΔT | Poor | Average | Good | Tight |
|---|---|---|---|---|---|---|---|
| 1 | Miami, FL | 52 °F | 18 °F | 10.8 | 5.4 | 3.6 | 2.2 |
| 2 | Houston, TX | 33 °F | 37 °F | 22.2 | 11.1 | 7.4 | 4.4 |
| 3 | Atlanta, GA | 26 °F | 44 °F | 26.4 | 13.2 | 8.8 | 5.3 |
| 4 | Baltimore, MD | 17 °F | 53 °F | 31.8 | 15.9 | 10.6 | 6.4 |
| 5 | Chicago, IL | 2 °F | 68 °F | 40.8 | 20.4 | 13.6 | 8.2 |
| 6 | Minneapolis, MN | -8 °F | 78 °F | 46.8 | 23.4 | 15.6 | 9.4 |
| 7 | Duluth, MN | -14 °F | 84 °F | 50.4 | 25.2 | 16.8 | 10.1 |
| 8 | Fairbanks, AK | -38 °F | 108 °F | 64.8 | 32.4 | 21.6 | 13.0 |
The 35–60 BTU per square foot sometimes quoted for northern homes comes from older, leaky houses. It also includes a generous safety margin, which is how many houses ended up with furnaces twice the size they need. For a furnace, the result here is the minimum output. ACCA Manual S allows furnace output up to about 140% of the load, so a 40,000 BTU/h load can take a furnace rated for 40,000 to 56,000 BTU/h of output.
BTU, watts, kWh and tons
- 1 watt = 3.412 BTU/h, so a 1,500 W space heater gives about 5,118 BTU/h.
- 1 kWh = 3,412 BTU of heat. BTU/h is a rate of energy; BTU and kWh are amounts of energy.
- 1 ton = 12,000 BTU/h. A 2-ton central AC is 24,000 BTU/h.
- Power an AC draws = BTU/h ÷ efficiency. A 10,000 BTU/h window unit with CEER 11 uses about 909 W, or 0.91 kWh per hour of running.
When to pick the next size up or down
- Go up for a west-facing room with big windows, a top-floor room under a dark roof, or a garage or workshop with no insulation. Use the "Very sunny" option and the "Poor" envelope.
- Stay down in humid climates (zones 1–3). An oversized unit cools the air before it has time to pull out moisture, so the room ends up cold and clammy, and the compressor wears from short cycling.
- Open-plan spaces: add up only the area the unit can actually blow into. A window unit in a living room will not cool the hallway behind it.
- Two rooms, one unit: usually a mistake. Two small units or a two-head mini-split beat one big unit pushing air through a doorway.
For a whole house with ducts, the room chart stops being the right tool. Use the AC size calculator, which sizes central systems in tons, and the duct size calculator for the airflow that follows.
Sources
- ENERGY STAR, Room Air Conditioners: sizing chart and adjustments (energystar.gov/products/room_air_conditioners), checked November 14, 2026.
- ASHRAE Handbook of Fundamentals 2009, climatic design conditions: 99% heating dry-bulb for MIA, IAH, ATL, BWI, ORD, MSP, DLH, FAI (via ashrae-meteo.info).
- ACCA Manual J (residential load calculation) and Manual S (equipment selection), used for the furnace 100–140% window.
Questions people ask
How many BTU per square foot do I need?
For cooling a single room, the ENERGY STAR chart works out to roughly 15 to 40 BTU/h per square foot: about 33 for a 180 ft² bedroom and 21 for a 1,000 ft² space. For heating, it ranges from about 5 BTU/h per square foot in Miami to 23 in Minneapolis for an average house.
How many square feet will a 12,000 BTU air conditioner cool?
About 450 to 550 square feet with 8 ft ceilings and average sun, according to the ENERGY STAR chart. Subtract roughly 10% of that area for a very sunny room, or add about 10% for a shaded one.
How many BTU do I need to heat a 10x10 room?
A 100 ft² room with average insulation needs about 2,000 BTU/h at a 70 °F temperature difference (Chicago design weather). A 1,500 W heater gives about 5,100 BTU/h, so it is more than enough. In a mild climate, a 750 W setting is often enough.
Is 18,000 BTU enough for 1,000 sq ft?
Just about. The ENERGY STAR chart gives 18,000 BTU/h for 700 up to 1,000 ft² and 21,000 BTU/h from 1,000 ft². For a sunny 1,000 ft² space, or one with more than two regular occupants, choose about 21,000 to 24,000 BTU/h.
Why does the calculator give a different answer for heating and cooling?
Cooling is driven by sun, people and appliances, while heating is driven by how cold it gets outside and how fast the house leaks heat. For a whole house in Miami the cooling load is several times the heating load; in Minneapolis or Duluth the heating load matches or passes the cooling load.
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