HVAC Load Calculator: Free Heating & Cooling Load Calculation

An HVAC load calculation measures how much heat your home gains on the hottest design day (cooling load) and loses on the coldest design day (heating load), expressed in BTU per hour. It accounts for walls, windows, ceilings, floors, air leakage, sun exposure, and the people and appliances inside. The result is what determines the correct furnace, AC, or heat pump size - not square footage alone.

Measure your conditioned square footage, note your insulation level, window type and amount, air tightness, foundation, and duct location, then compute each component's heat flow: area × U-value × design temperature difference, plus air leakage (1.08 × CFM × ΔT), window solar gain, and internal gains from people and appliances. The calculator above runs all of these steps and shows you the worksheet line by line, so you can see exactly where the BTUs come from.

For cooling, most US homes land between 12 and 28 BTU per square foot of load depending on climate and construction - around 15-20 for a typical home in a mixed climate, 20-28 for an older home in Texas or Florida. Heating loads run about 18-28 BTU per square foot in moderate climates and 25-45 in the cold North. Note these are calculated loads - installed furnaces are usually rated in input BTU and sized above the load, which is why furnace nameplate numbers look roughly twice as big.

It's a Manual-J-style block load: the same physics (component U-values, design temperatures, infiltration, internal and solar gains) applied at the whole-house level rather than room by room. For typical homes it lands within about 10-15% of full ACCA Manual J software. For permit paperwork, new construction, or room-by-room duct design, you still need a full Manual J - see our Manual J guide for how those work and what they cost.

Hiring an energy auditor or mechanical designer to run a full ACCA Manual J typically costs $99-$400 for a single-family home, depending on whether it's a remote calculation from your plans or an on-site visit with measurements. Some HVAC contractors include it free with a replacement quote - but many skip it and size by rule of thumb, which is exactly when you should sanity-check them with a calculator like this one.

A block load treats the whole house as one zone and answers 'what size system do I need?' - that's what this calculator does. A room-by-room load calculates each room separately and answers 'how much air does each room need?', which is required for designing ductwork (ACCA Manual D). Equipment replacement usually only needs a block load; new duct systems need room-by-room.

In a mixed US climate with average 1990s-2000s construction, a 2,000 sq ft house typically calculates to about 28,000-34,000 BTU of cooling load (a 2.5-3 ton AC) and 35,000-45,000 BTU of heating load - which maps to roughly a 60,000-80,000 BTU input furnace once efficiency (AFUE) and sizing allowances are applied. Hot climates push cooling toward 3.5-4 tons; a tight new build can drop to 2 tons. Insulation and windows swing the answer more than square footage does.

Commercial loads are dominated by people, lighting, equipment, and code-required fresh air (ASHRAE 62.1) rather than the envelope, so commercial work uses ACCA Manual N or full ASHRAE procedures instead of Manual J. Rules of thumb also differ - offices often run one ton per 300-400 sq ft versus one ton per 500-600 sq ft for homes. This calculator is residential; use it for houses, townhomes, and apartments only.

Metal buildings and garages behave differently from houses: they're usually uninsulated or lightly insulated, very leaky, and have big door openings, so their loads per square foot run 2-3x higher than a home's. Set insulation to 'Poor' and air tightness to 'Drafty' in the calculator for a rough number, and consider that an uninsulated metal shop in the South can need 40-60 BTU per square foot of cooling. Insulating first is almost always cheaper than upsizing equipment.

Include a finished, heated basement in your conditioned square footage and select 'Heated basement' as the foundation - below-grade walls lose far less heat than above-grade walls because the ground stays around 50-60°F. Leave an unfinished, unconditioned basement out of the square footage and select 'Unheated basement'; it acts as a buffer that reduces floor losses.

In most US jurisdictions, yes - the IRC (International Residential Code) requires equipment to be sized per ACCA Manual J and Manual S, and many permit offices ask for the paperwork on new construction and full system replacements. Enforcement on like-for-like replacements varies widely by area. Even where nobody checks, an oversized system short-cycles, dehumidifies poorly, and costs more - so the calculation is worth doing regardless.

The ACCA-approved packages are Wrightsoft Right-J, Elite RHVAC, and Cool Calc (a free web option that generates permit-ready Manual J reports but requires an account and more detailed inputs). Free simplified tools like this one trade some precision for speed - useful for homeowners checking quotes, while the approved software is what pros use for stamped reports.