HVAC Load Calculator

A room BTU calculation sizes one space in isolation — useful for a window AC or mini-split. A whole-home load calculation accounts for the entire building envelope and is what a contractor uses to size a central air system. The whole-home number is not the sum of your rooms: shared interior walls, hallways, and conditioned attic space reduce the total load compared to adding every room separately.

ACCA Manual J is the industry standard for residential load calculations, required for permitted HVAC installations in most jurisdictions. A full Manual J uses your local 99th-percentile design temperatures, measured wall U-values, duct loss factors, and a room-by-room breakdown. This tool applies the same logic with rule-of-thumb multipliers — accurate enough to plan, not enough to pull a permit.

Each decade of tighter energy codes has meaningfully reduced baseline loads. Pre-1970 homes often have R-7 or less in walls, no vapor barriers, and single-pane windows with visible daylight around the frames. A 2010+ home built to IECC 2009 or 2012 standards might have R-20 walls, R-49 attics, and triple-pane windows — effectively a different physics problem. The construction year input captures this without requiring you to look up your actual wall assemblies.

Glass is the thermal weak point of any building envelope. Even good double-pane glass (U-0.3) conducts heat about six times faster than a well-insulated wall (U-0.05). A home with large picture windows on the south and west faces can see its cooling load increase 10–15% compared to the same home with fewer windows. The window-to-wall ratio captures this without requiring a window-by-window count.

A south-facing home (long axis running east–west, main windows facing south) receives the most direct solar gain — roughly 5% more cooling load than a neutral orientation. A north-facing home receives the least. Orientation matters most in hot climates and in homes with high window-to-wall ratios.

In hot climates (Florida, Texas, Arizona), the cooling load almost always dominates — size your system for cooling and heating is covered. In cold climates (Minnesota, New England, mountain states), the heating load dominates. Moderate climates can go either way; this calculator shows both loads so you can size for the larger.

Ducts routed through unconditioned attic space can lose 10–30% of their conditioned air before it reaches the living space. This calculator assumes no duct loss. If your ducts run through an unconditioned attic — common in 1970–2000 construction — add roughly 15% to the result when shopping for equipment.

The cooling load is your AC system's target capacity. The heating load is your furnace or heat pump heating target. The tonnage range gives you shopping flexibility — any unit within that band is a reasonable fit. If you're installing a heat pump that handles both, size it for whichever load is larger. For a gas furnace paired with AC, size each independently.