HVAC Load & BTU Sizing Calculator

The estimate starts from a per-square-foot rule of thumb — about 20 BTU/hr per ft² for cooling and 30 BTU/hr per ft² for heating — then adjusts that baseline for your ASHRAE climate zone, insulation quality, and (for cooling only) sun exposure. On the cooling side it also adds a fixed amount for each window and for occupants beyond two. Tonnage is the cooling figure divided by 12,000 BTU/hr, rounded up to the nearest half ton, and the recommended size maps that tonnage back to a conventional unit label.

cooling:
  base_cool = 20 × area_ft²
  cool_btu  = base_cool × zone_cool × insulation × sun
              + windows × 1,000
              + max(occupants − 2, 0) × 100
  tons      = ceilToHalf(cool_btu / 12,000)      // round UP to nearest 0.5
  recommended = tons × 12,000 BTU/hr

heating (no sun, no window/occupant adds):
  base_heat = 30 × area_ft²
  heat_btu  = base_heat × zone_heat × insulation

zone multipliers (cooling | heating):
  z1 1.30|0.70  z2 1.20|0.80  z3 1.10|0.90  z4 1.00|1.00
  z5 0.90|1.15  z6 0.80|1.30  z7 0.70|1.45
insulation: poor 1.15 · average 1.00 · good 0.90 · excellent 0.85
sun (cooling only): shaded 0.90 · partial 1.00 · full 1.10
metric echo: kW = BTU/hr ÷ 3,412

Sun, windows, and occupants raise the cooling demand only — solar gain and internal heat add to what your AC has to remove, not to what your furnace has to supply. Heating is reported in BTU/hr only; tonnage is a cooling concept.

Two identical 1,600 ft² homes can need anywhere from 2.5 to 4 tons depending on climate zone, insulation, orientation, and air-sealing. Treat this single number as a starting point, not a precise spec. The climate zone is a manual ASHRAE selection — there is no ZIP code or weather lookup, by design. Baselines from miniwebtool; window/occupant adds and the 12,000 BTU/ton constant from Omni Calculator and HVAC.com.

Formula: cool = 20·area · zone · insulation · sun + windows·1,000 + max(occ−2,0)·100 ; tons = ceilToHalf(cool / 12,000)

See Sources below ↓