HVAC Load Calculator

Manual J is the ACCA (Air Conditioning Contractors of America) standard method for calculating the heating and cooling loads of a residential building. It accounts for insulation, windows, climate, infiltration, internal gains, and duct losses to determine the minimum BTU capacity needed from your HVAC equipment. This calculator uses a simplified version of those inputs.

No. Permit-level designs require a full ACCA Manual J calculation using actual construction documents, window schedules, infiltration testing data, and software certified for that purpose. This tool is designed for preliminary sizing, customer conversations, and rough equipment selection — not permit submittals.

BTU/h stands for British Thermal Units per hour. It is the standard unit for measuring HVAC heating and cooling capacity. One ton of cooling equals 12,000 BTU/h. A typical 2,000 sq ft home in a mixed climate requires roughly 24,000–36,000 BTU/h of cooling (2–3 tons) depending on insulation and window quality.

ASHRAE climate zones in the US are determined by county. Zone 1–2 covers the deep south and southwest. Zone 4A covers the mid-Atlantic states. Zones 5–7 cover the northern US. If you are unsure, search "[your city] ASHRAE climate zone" or use the DOE climate zone map.

Ducts in an unconditioned attic are subject to extreme temperatures — 130°F+ in summer and near-freezing in winter. This creates significant conductive losses even through insulated duct wrap. The multiplier in this calculator reflects the ACCA Manual J duct loss correction factors for attic vs conditioned space installations.

The right size depends on your climate, insulation, and windows — not just square footage. A 2,000 sq ft home in Miami with average insulation might need 4 tons of cooling. The same house in Chicago might need 2.5 tons. This is why load calculations exist and why oversizing or undersizing HVAC equipment is such a common and costly mistake.