How the BTU Calculator works
Last data update: 2026-08-24.
Data
The base table is ENERGY STAR's "Room Air Conditioners" sizing table (area in sq ft → capacity in BTU/hr, 14 bands from 100–150 → 5,000 to 2,000–2,500 → 34,000), with its four adjustment rules quoted verbatim in the tool: heavily shaded −10 %, very sunny +10 %, +600 BTU per person beyond two, +4,000 BTU for a kitchen. DOE Energy Saver's 20 BTU/sq ft rule is cited for comparison only.
Formula
base_btu = ENERGY_STAR_TABLE[area] // < 100 sq ft → 5,000; > 2,500 sq ft → "more than one unit"
ceiling = height_ft / 8 // convention (volumetric); not part of the ENERGY STAR table
sun = −0.10 | 0 | +0.10 // ENERGY STAR
top_floor = +0.10 if under an unconditioned attic // convention
people = max(0, people − 2) × 600 // ENERGY STAR
kitchen = 4,000 if kitchen // ENERGY STAR
btu_needed = round_500( base_btu × ceiling × (1 + sun) × (1 + top_floor) + people + kitchen )
Portable mode: the result is presented as a DOE/SACC target; the ASHRAE equivalent shown is btu_needed / 0.65, reflecting the 25–45 % gap in sourced examples (dual-hose at the low end, single-hose at the high end). This is a recognition aid, not a conversion; DOE's SACC formula is SACC = 0.2 × ACC95 + 0.8 × ACC83 with infiltration and duct heat subtracted (81 FR 35242; 10 CFR 430 Appendix CC).
Estimated running watts = btu_needed / EER, with EER 10 (window), 8.5 (portable, ASHRAE basis), 12 (mini-split). These are typical values, not ratings.
Conventions flagged in the UI
Ceiling-height scaling and the top-floor adjustment are not published by ENERGY STAR, DOE, or ACCA in this simple form (ACCA Manual J handles them within a full load calculation). They are labelled "convention" in the result.
Change log
- 2026-08-24 — first release.
Cite this page
GridComfort, "Air Conditioner BTU Calculator — methodology," https://gridcomfort.com/tools/btu-calculator/methodology/, data updated 2026-08-24.