What Is a BTU?
A British Thermal Unit is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. In HVAC, the rating is really BTU per hour: an 8,000 BTU window AC removes 8,000 BTUs of heat from your room every hour it runs. Getting this number right matters more than most people think. An undersized unit runs constantly and never quite catches up on the hottest day of the year, while an oversized unit blasts cold air for a few minutes, shuts off, and leaves the room cold but clammy because it never ran long enough to pull moisture from the air.
How to Calculate BTUs for a Room
Contractors size rooms with a transparent rule of thumb, and our calculator follows the same approach:
BTU = floor area x climate factor x sun factor x insulation factor x ceiling factor + occupant and window additions
Start with length times width for the area. Multiply by a climate-zone factor (20 to 28 BTU per square foot for cooling, 20 to 56 for heating). Adjust for sun exposure (no direct sun 1.0, partial sun 1.1, full sun 1.2), insulation (good 0.9, average 1.0, poor 1.25), and ceiling height above 8 feet. Then add 600 BTU for each occupant beyond the first two, 1,000 BTU per large window or glass door, and 4,000 BTU if it is a kitchen with a stove and oven. Divide the total by 12,000 to get tons, or by 3,412 to get kilowatts.
BTU Per Square Foot by Climate Zone
Climate is the biggest lever in the whole calculation. These factors assume average insulation and standard 8-foot ceilings:
| Climate zone | Cooling BTU/sq ft | Heating BTU/sq ft |
| Zone 1, very hot (Miami, Honolulu) | 24 | 28 |
| Zone 2, hot (Houston, Phoenix) | 23 | 32 |
| Zone 3, warm (Atlanta, Dallas, LA) | 22 | 36 |
| Zone 4, mixed (NYC, DC, Seattle) | 20 | 40 |
| Zone 5, cool (Chicago, Boston, Denver) | 19 | 44 |
| Zone 6, cold (Minneapolis, Burlington) | 18 | 50 |
| Zone 7, very cold (Duluth, Fairbanks) | 17 | 56 |
Notice how cooling and heating move in opposite directions: the hotter the zone, the more cooling you need per square foot and the less heating. If you are comparing an old unit's wattage to a new one's capacity, convert between the two with our unit converter.
Heating vs Cooling: Why the Numbers Differ
Heating loads are almost always bigger than cooling loads for the same room, because the temperature gap a furnace must close is wider. Cooling a room from 95 to 75 degrees is a 20-degree lift; heating it from 10 to 70 is a 60-degree lift. That is why a 400 square foot room might need 8,000 BTU of cooling but 16,000 BTU of heating in a cold climate. Heat pumps complicate the picture in a good way: they move heat instead of making it, so a cold-climate heat pump rated for the heating load often covers cooling easily. Look for the HSPF2 rating on heating efficiency and SEER2 on cooling efficiency when you shop; both are Energy Star benchmarks worth taking seriously.
What Size AC or Heater Do I Need? Equipment Guide
| Room size | Cooling BTUs | Heating BTUs | Typical equipment |
| 150 sq ft | 5,000 | 5,000 to 7,000 | Window AC / space heater |
| 300 sq ft | 7,000 | 9,000 to 15,000 | Window or portable AC |
| 500 sq ft | 10,000 | 15,000 to 25,000 | Mini split |
| 1,000 sq ft | 20,000 | 30,000 to 50,000 | Central AC / furnace |
| 2,000 sq ft | 40,000 | 60,000 to 100,000 | Heat pump / furnace |
These are starting points for average conditions. If your result lands between standard equipment sizes, size up for heating in cold climates and size down (or hold) for cooling, since oversizing an AC hurts dehumidification. For a whole house, a contractor should still run an ACCA Manual J load calculation, the industry standard that accounts for windows, orientation, ductwork, and air leakage room by room. Wondering what the efficiency difference is worth in dollars? Work out the savings as a percentage of your bill with our percentage calculator.
Enter your room above and you will get an exact BTU target for heating or cooling, the equivalent tonnage and kilowatts, the equipment class that fits, and an estimated monthly running cost at your electricity rate. It takes less than a minute, and it is the difference between a unit that sips energy and one that fights your climate all summer.
Frequently Asked Questions
How many BTUs do I need per square foot?
For cooling, plan on 20 to 25 BTUs per square foot in most US climates (up to 28 in very hot zones like South Florida and Arizona). For heating, plan on 30 to 60 BTUs per square foot, with colder climates at the high end. Adjust for insulation, ceiling height, sun, and occupants.
How do I convert BTU to tons?
Divide BTUs by 12,000. One ton of cooling equals 12,000 BTU per hour. A 24,000 BTU mini split is a 2-ton unit, and a 36,000 BTU central AC is a 3-ton unit.
Is it bad to oversize an air conditioner?
Yes. An oversized AC cools the room so fast that it shuts off before removing humidity, leaving the air cold and clammy. It also short-cycles, which wastes energy and wears out parts. Size within about 10 to 15 percent of your calculated load.
What is the difference between SEER and BTU?
BTU measures capacity: how much heat the unit moves per hour. SEER (Seasonal Energy Efficiency Ratio) measures efficiency: how much cooling you get per watt of electricity. Two 12,000 BTU units cool the same room, but the higher-SEER one costs less to run.
How many BTUs do I need for a 400 square foot room?
For cooling, a 400 sq ft room typically needs 8,000 to 10,000 BTU with average insulation and 8-foot ceilings. For heating, the same room needs 12,000 to 20,000 BTU depending on climate. Add BTUs for poor insulation, high ceilings, or heavy sun.
Last updated: 2026 | Climate factors follow common ACCA-style rules of thumb. For whole-house sizing, ask a contractor for a Manual J calculation. This calculator is for estimation only.