Room BTU Load Calculator

How the math works

The simplified Manual J shortcut starts from a base cooling load per square foot keyed to your climate zone, then layers on per-component loads. We multiply the base by three envelope modifiers (insulation, ceiling height, exterior-wall count), then add discrete loads for windows, doors, people, and appliances.

The DOE-derived climate zone numbers are deliberately conservative. Zone 1 (Miami) needs about 35 BTU/sqft of cooling; zone 7 (Duluth) needs 16. Heating reverses: Miami needs nearly zero, Duluth needs 65 BTU/sqft. Insulation tier modulates both directions — a Passive-House-quality envelope cuts loads by 35% vs. a typical 2000s build.

Why room-by-room beats square-foot rules of thumb

The 20-BTU/sqft rule you hear from box-store sales staff is the right ballpark for the average US room in zone 4 with average everything. The moment you have any of: more glass than average, an exposed exterior corner, vaulted ceilings, an attic above, or a west-facing wall — the rule of thumb undersizes by 25-50%.

The oversizing trap

Going one size up "just in case" is the most common HVAC mistake. An oversized AC reaches the thermostat setpoint quickly, shuts off, and never runs long enough to remove humidity. The room ends up cold and clammy. Modern variable-speed equipment partially mitigates this but doesn't eliminate it.

Size to the load. If your peak day in the year is 95°F and the load is 8,200 BTU/hr, buy an 8,500-BTU unit, not a 12,000-BTU unit.

When this calculator is the wrong tool

Use a full Manual J procedure for: whole-home system sizing, equipment selection for a heat pump (which has unique heating capacity curves), and any commercial application. This tool is for sizing a window AC, a single mini-split head, a portable unit, or evaluating whether an existing room's HVAC is reasonably sized.