Enter your room.
Where the number comes from.
Every legitimate residential cooling-load formula starts the same way: square footage times a base BTU/sq ft factor for the climate, then adjusted for ceiling height, insulation, sun, and occupants.
The base rate
We use 25 BTU per square foot because Birmingham sits in the humid subtropical Cfa climate zone. Dry-climate calculators (Denver, Phoenix) use 18–20. Wet-climate calculators (Houston, Miami) use 28–32. Twenty-five is the right anchor for Alabama.
Ceiling adjustment
The 25 BTU/sq ft baseline assumes an 8-foot ceiling. A 10-foot ceiling means 25% more air volume to cool. The calculator multiplies by (ceiling / 8) automatically.
Insulation modifier
A 1965 brick ranch in Hoover with original single-pane windows and R-11 attic insulation needs 15% more cooling than a 2018 build with R-49 attic and double-pane low-E glass. The modifier handles the gap.
Sun exposure
A west-facing master bedroom over a Pelham driveway gets baked from 2 PM until sundown in July. We add 15% over baseline. North-facing rooms with tree cover get 10% off.
Occupant adder
Every adult occupant beyond the first two adds about 600 BTU/hr of sensible and latent heat. A bedroom with two people is the base case; a home office with three people and three monitors is a different problem.
Source standards
The full residential cooling-load standard is ACCA Manual J 8th Edition and the ASHRAE Handbook. Every After Hours HVACR install begins with a measured Manual J, not this rule of thumb.
Three services that fix sizing problems.
AC Installation
Every install starts with a measured Manual J. Right-size the equipment, pull the permit, verify startup.
READ MORE →Duct Cleaning & Sealing
Right-sized AC paired with leaky ductwork loses 20-30% of capacity. Test, seal, recover the BTU.
READ MORE →AC Repair
If sized right and still not cooling, the issue is mechanical — capacitors, refrigerant, coils. We diagnose, you decide.
READ MORE →BTU questions, answered straight.
How accurate is this BTU calculator?+
This tool uses the ACCA Manual J residential rule of thumb — about 25 BTU per square foot for Alabama's humid subtropical climate — adjusted for ceiling height, insulation, sun exposure, and occupants. Accurate within 10-15% for a typical Birmingham home. For a real installed system, every install gets a measured Manual J on paper.
How many BTU does a Birmingham room need per sq ft?+
Twenty-five BTU per square foot is the residential baseline for Birmingham — humid Köppen Cfa climate zone. A 200 sq ft bedroom needs about 5,000 BTU/hr; a 14×16 living room about 5,600; a 600 sq ft master suite with western sun about 17,000.
Why does oversized AC cool worse than right-sized AC?+
Oversized AC short-cycles — it cools the thermostat fast, shuts off, and never runs long enough to pull humidity out of the air. The result is a cold, clammy house and an early compressor failure. Birmingham summers run 95°F+ with 70°F+ dewpoint — sizing right matters more here than in dry climates.
What if my room is L-shaped or has vaulted ceilings?+
L-shaped rooms: treat each rectangle separately and add the BTU numbers together. Vaulted ceilings: the calculator scales proportionally — a 10-foot ceiling needs 25% more cooling than 8-foot for the same floor area. Older Birmingham bungalows with high ceilings are routinely undersized for this reason.
Does this work for whole-house cooling?+
No — this is per-room. For whole-house cooling, add up the BTU/hr for every conditioned room, then subtract about 10% for shared interior walls. Or call us — a whole-house Manual J is something we do before quoting any install. Call (205) 994-6402.
How much should I add for sun exposure in Alabama?+
West and south-facing rooms with no shade get baked all afternoon — add 15% over baseline. North-facing or heavily shaded rooms: 10% reduction. Sunrooms and skylights need a real load calc.
What does insulation 'average' mean for an Alabama home?+
Average = typical Birmingham home built 1980-2005 with R-30 attic insulation and double-pane windows. Poor = pre-1980, single-pane, no wall insulation. Good = recent build or renovated with R-49+ attic and tight construction.
Need a real Manual J?
We measure your house room by room, check the duct static, and put the right-size equipment number on paper before any quote.
CALL (205) 994-6402