How Many BTU Do You Need? Quick Room and House Estimate

Type a room size and you get cooling BTU, tons and airflow in one line. For example, a 350 ft² living room needs about 9,000 BTU/h, and a 1,800 ft² house in a mixed climate needs about 3.5-ton central AC moving 1,400 CFM through a 18″ trunk.

Cooling for one room

9,000BTU/h

Tons

0.75

Airflow

300 CFM

Room mode uses the ENERGY STAR chart, so climate zone does not change it. Switch to whole house to size a central system.

One house, four numbers

Heating and cooling questions come as a chain. Each answer is the input to the next one, and a mistake early on carries all the way through. This is the chain for a 1,800 ft² house in zone 4 with average insulation:

1,800 ft²→39,600 BTU/h→3.5 tons→1,400 CFM→18″ trunk

Insulation sits before all of it. Better attic and wall R-values shrink the BTU load, which shrinks the equipment, which shrinks the ducts. That is why the R-value chart is part of this site and not a separate topic.

Calculators

BTU calculatorCooling and heating BTU for one room: ENERGY STAR chart, sun, people, kitchen, and your zone's winter design temperature. AC size calculatorCentral AC tonnage for a house, with every nominal size that fits the ACCA 90–115% window and the airflow it needs. Duct size calculatorRound and rectangular duct from CFM or tons, at the friction rate you choose, with a velocity and noise check. R-value chartR per inch for 9 materials, 2021 IECC minimums by zone, inches for R-30 to R-60, and the real R-value of a stud wall.

Find your climate zone

Three of the tools ask for a climate zone, the DOE / IECC map that splits the US into zones 1 (hottest) to 8 (coldest). Rough guide: 1 South Florida and Hawaii; 2 Gulf Coast, Houston, Phoenix; 3 Atlanta, Dallas, Los Angeles, Las Vegas; 4 Baltimore, St. Louis, Seattle; 5 Chicago, Denver, Boston; 6 Minneapolis, northern Wisconsin, Burlington VT; 7 Duluth and northern Minnesota; 8 interior Alaska. The county list in the IECC gives the exact zone for your county.

What these tools are, and are not

Every number on this site comes from a published table or a formula that is written out on the page: ENERGY STAR, ASHRAE climate data, the 2021 IECC, and the ACCA manuals as reference. The math is a single script, and the same script runs the automated tests. The tools are built to help you plan, compare quotes and catch a unit that is a size off. A contractor's Manual J and Manual D for your house remain the right basis for buying equipment. How each number is made is on the methodology page.