Heating BTU Calculator

The rule of thumb varies by US climate zone: 40 BTU/sq ft in cold zones (Minneapolis, Boston, Chicago), 30 BTU/sq ft in mixed zones (Denver, Washington DC, Seattle), 22 BTU/sq ft in warm zones (Atlanta, Dallas, Los Angeles), and 18 BTU/sq ft in hot zones (Miami, Phoenix, Houston). Multiply your room size by the right factor to get the baseline before adjusting for ceiling height, insulation, and room type.

A lot. A poorly insulated room (pre-1980 home, single-pane windows) needs 1.3x the baseline BTUs, while an excellent envelope (new construction with triple-pane windows) drops the load to 0.7x. That is an 86 percent swing between the two extremes. A 12,000 BTU baseline can become 15,600 BTU with poor insulation or 8,400 BTU with excellent insulation, which is the difference between a small and a midsize heater.

One ton equals 12,000 BTU/hr. The unit comes from the cooling capacity of melting one ton of ice over 24 hours, which is exactly 12,000 BTU/hr. Heat pumps and central HVAC equipment are often spec'd in tons, so converting your heating BTU need to tons gives you a quick way to compare against equipment ratings: a 30,000 BTU/hr heating load is roughly 2.5 tons of equipment capacity.

One watt of electric resistance heat is about 3.412 BTU/hr. A 9,000 BTU/hr estimate converts to about 2,638 W, which exceeds the continuous capacity of a standard 120 V, 15 A branch circuit. Do not combine heaters on one circuit to reach that load; have the circuit and heating equipment sized for the installation.

No. This is a rough planning estimate. A Manual J calculation uses design temperatures, envelope assemblies, windows, infiltration, orientation, and internal gains. Neither method has a universal error percentage, and results depend on accurate inputs and field conditions. Use a qualified load calculation before selecting equipment.