HVAC Load Calculator (Square-Foot Method)

Sizing a furnace or air conditioner correctly is one of the few residential decisions where bigger is actually worse. An oversized unit short-cycles, burns efficiency, and fails to dehumidify during cooling. The professional standard for residential load calculation is ACCA Manual J, which goes room by room with infiltration rates, window U-values, duct losses, and solar orientation. This tool is not Manual J. This is the square-foot rule-of-thumb method published widely by HVAC manufacturers and consumer guides for ballparking equipment size before a contractor visit.

Enter the conditioned square footage (floor area inside the thermal envelope, excluding unheated garages and detached structures), pick a climate zone that roughly matches your region, and pick an insulation tier based on when the home was built and what envelope work has been done. The tool returns a cooling load in BTU per hour and in AC tons (1 ton = 12,000 BTU/h), plus a heating load in BTU per hour and in MBH for furnace sizing (1 MBH = 1,000 BTU/h).

Use the result to sanity-check a contractor bid, not to specify equipment. For an actual equipment order see the methodology page and request a full Manual J load calculation, which a qualified contractor should perform before placing an order or pulling a permit.

Cooling BTU = sqft × climate_cooling_base × insulation_modifier. Heating BTU uses the same structure with a heating base. Climate cooling base values: mild 22, moderate 28, hot 35, cold 22 BTU/sqft. Climate heating base values: mild 32, moderate 40, hot 30, cold 52 BTU/sqft. Insulation modifier: poor build 1.15, average 1.00, good 0.90.

These per-square-foot ranges reflect the consumer rule-of-thumb values published by HVAC manufacturers and energy-efficiency guides for residential sizing checks. They assume typical single-family residential construction, standard ceiling heights, and average window area. They do NOT account for specific window U-values, air infiltration rates, duct location, solar orientation, or internal heat loads. For any of those, a full Manual J is required.

Use this to ballpark an equipment size before you get a contractor bid, to sanity-check that a bid is not wildly oversized (a common problem), or to compare rough sizing across two different build conditions (before vs after an insulation retrofit). Do not use this number for the actual equipment order. A full Manual J is required for any permit-set mechanical plan and for any warranty claim later. Pair with the paint coverage calculator when scoping an interior remodel, since HVAC and finish work usually sequence together.