BTU Calculator
Estimate residential cooling capacity, AC tonnage, and furnace size from the home and climate.
This is a rule-of-thumb estimate
Correct equipment sizing requires an ACCA Manual J load calculation. Oversizing causes short cycling and poor dehumidification. Ask a qualified contractor to run Manual J before selecting equipment.
Home Inputs
Colorado and similar areas, cold winters
Nearest standard AC sizes: 3 ton and 3.5 ton
Required input at 95% AFUE: 94,737 BTU/hr. Nearest common input sizes: 80k and 100k
Furnace ratings are input BTU. Output BTU equals input multiplied by AFUE. Compare delivered output, not input alone.
Windows, orientation, infiltration, duct location, local design temperatures, and actual construction can materially change the load.
How BTU Sizing Works
BTU per hour measures heating or cooling capacity. This BTU calculator starts with square footage and a climate-zone rate, then adjusts for ceiling height, insulation, sun exposure, and cooling occupancy. It can serve as a quick furnace size calculator and AC tonnage calculator, but it cannot model the actual heat gain and heat loss of a home.
Why Bigger Is Not Better
Oversized cooling equipment runs short cycles, removes less moisture, creates temperature swings, and can wear components faster. Oversized furnaces can also cycle rapidly and create comfort problems. Manual J gives the design load needed for a defensible equipment selection.
Climate Zone Factors Used
| Zone | Description | Cooling BTU/sq. ft. | Heating BTU/sq. ft. |
|---|---|---|---|
| 1 | South Florida and Hawaii, very hot | 30 | 30 |
| 2 | Gulf Coast and desert Southwest, hot | 29 | 35 |
| 3 | Southeast and southern California, warm | 27 | 40 |
| 4 | Mid-South and mixed climates | 25 | 45 |
| 5 | Colorado and similar areas, cold winters | 23 | 50 |
| 6 | Upper Midwest and northern New England, colder | 21 | 55 |
| 7 | Northern Minnesota and Alaska, very cold | 20 | 60 |
AC Tonnage Conversion
BTU Sizing Questions
Turn the final equipment choice into a clear estimate
Plenum Estimates helps contractors present approved equipment options and scope after the load calculation is complete.
See Estimates