BTU Calculator

The classic rule of thumb is 20 BTU per square foot for cooling and 30 to 40 BTU per square foot for heating in Texas Climate Zone 3. That is enough to get you within about 20 percent of the correct size on a typical residential space with 8 foot ceilings and average insulation.

For serious sizing on a new install or a full replacement, the industry uses ACCA Manual J. Manual J accounts for wall U-values, window SHGC, infiltration rates, orientation, internal loads, and duct losses. It is the difference between a system that runs efficiently for 15 years and one that short-cycles for 3 years and dies.

This calculator uses an adjusted rule of thumb suitable for room-by-room mini-split sizing and quick load estimates. For full-house replacement, always run Manual J.

A cooling system that is too large for the space cools the air quickly but does not run long enough to remove humidity. In humid climates like the Texas Gulf Coast and East Texas, an oversized AC leaves the house at 72 degrees with 65 percent humidity, which feels clammy and encourages mold.

Oversized systems also short-cycle, which shortens compressor life and wastes efficiency. Every start pulls inrush current and provides less cooling per kilowatt-hour than a longer run at steady state.

The correct size is one that runs for extended periods on hot afternoons. If your AC never runs longer than 15 minutes at a time on a 100 degree day, it is oversized.

Ceiling height: multiply the BTU load proportionally. A 10 foot ceiling means 25 percent more volume than an 8 foot ceiling, and about 15 to 20 percent more load.

Sun exposure: west and southwest-facing rooms in Texas get 10 to 20 percent higher afternoon cooling load. North-facing and heavily shaded rooms get 5 to 10 percent less.

Insulation: R-13 wall insulation with single-pane windows can add 30 to 50 percent to the load compared to R-19 walls with double-pane low-E windows. Spray-foam encapsulated attics reduce cooling load by 10 to 25 percent.

Internal loads: each person contributes about 400 to 600 BTU per hour. A kitchen with a range and dishwasher can add 3,000 to 5,000 BTU during meal prep. An office with two people and multiple computers adds 2,000 to 3,000 BTU.

1,500 sq ft single-story tract home, average insulation, DFW: about 2.5 to 3 tons of cooling (30,000 to 36,000 BTU).

2,500 sq ft two-story, spray foam attic, Austin: about 3.5 to 4 tons (42,000 to 48,000 BTU).

3,500 sq ft custom home, high ceilings, west-facing, San Antonio: about 5 tons plus a supplemental zone (60,000+ BTU total).

A single-room mini-split for a 400 sq ft master bedroom in North Texas: 9,000 to 12,000 BTU (0.75 to 1 ton).

Sizing calls come from homeowners planning renovations, additions, and system replacements. Peak Local Leads builds and ranks local HVAC sites for one contractor per Texas market. Free week of calls, flat monthly after.