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.

Sizing is the first half of a replacement conversation and the price is the second. Once the tonnage band is settled, the HVAC job pricing calculator builds the quote from equipment cost, labor hours, parts markup and overhead, and a published menu of common job prices keeps the routine work off the back of an envelope.

The install itself is dispatched on the HVAC work order template, and the equipment mix a growing shop plans to sell is part of what the HVAC business plan template asks you to forecast. The unit the tonnage lands on still has to be fed, and the wire size and voltage drop calculator is the quick read on whether the run out to it holds its voltage before an electrician prices the circuit.

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

See Sources below ↓