HVAC BTU Estimator
A first-pass size for a conversation with an HVAC contractor — climate band, ceiling, insulation, sun, and people, with every factor on the page.
PRELIMINARY ESTIMATOR ONLY — rule-of-thumb band math, NOT a Manual J load calculation. Final sizing requires an ACCA Manual J by a licensed HVAC contractor; California Title 24 permit documentation requires approved Manual J software. Do not buy equipment off this number.
PRELIMINARY ESTIMATOR ONLY. This page runs the published rule-of-thumb math (a BTU-per-square-foot band with adjustment factors). It is NOT a Manual J load calculation, and it is not a substitute for one. Final equipment sizing requires an ACCA Manual J load calculation by a licensed HVAC contractor — California Title 24 permit documentation requires approved Manual J software. Never buy equipment off a rule of thumb alone.
How the math works
Base load = floor area × a climate band rate: 20 BTU/sq ft for the mild coast (Long Beach), 25 moderate, 30 hot inland, 35 cold — the band is an input, and coastal defaults are set for Long Beach on purpose; cold-climate defaults imported from national sites oversize coastal homes. Then the adjustments: × (ceiling height ÷ 8), × insulation factor (1.10 poor / 1.00 average / 0.90 good), × sun factor (0.90 shaded / 1.00 average / 1.10 full sun), + 400 BTU per occupant past the first two, + 4,000 BTU if a working kitchen is in the space. Tons = BTU ÷ 12,000.
The answer is printed as a range (±15%) because that is the honest precision of this method. A Manual J run models your actual walls, windows, ducts, and infiltration — this estimator cannot see any of those.
A worked example
A 1,500 sq ft Long Beach home, mild-coast band at 20 BTU/sq ft, 8 ft ceilings, average insulation and sun, four occupants: 30,000 base + 800 for the two extra people = 30,800 BTU/hr — 2.57 tons, in a 26,180–35,420 planning range. That frames a 2.5-ton conversation; the Manual J makes the final call.
Questions from the field
How many BTU do I need for 1,500 square feet? On the mild coast, the rule-of-thumb band starts at 20 BTU/sq ft — about 30,000 BTU (2.5 tons) before adjustments. Inland heat pushes the band to 30; that is why the climate input matters more than the square footage.
Is this a Manual J calculation? No. Manual J is the ACCA ANSI standard that models the actual building, and California permits require it from approved software. This is the preliminary estimator you use before that conversation — it says so on the page, on purpose.
How many BTU is a ton of cooling? 12,000 BTU/hr per ton, by definition. A “3-ton” unit is a 36,000 BTU/hr unit.
Why a range instead of one number? Because rule-of-thumb inputs are ranges themselves. Anyone printing a single exact BTU figure from five dropdowns is faking precision the method does not have.
The other calculators
Want us to look at your shop?
The free tools cover the leaks most shops share. The free Snapshot is the one-on-one version: 15 free minutes with our team to pressure-test what you found and map your next step. No pressure either way — you leave with a short list you can act on yourself.
Book Your Free Snapshot — book your free 15-minute call first, then answer a few quick questions, so the call itself is spent on your shop, not on background.