Size a window unit, portable AC, or mini-split the way a tech estimates it: floor area, ceiling height, sun, insulation, bodies in the room, kitchen heat, and climate. The meter answers in BTU/hr and tons — and tells you why oversizing is the classic mistake.
Part of The Talking Toolbox. The meter describes the room; you size the unit. Endless problems, graded within field tolerance — five straight and you think like a tech.
Press New Problem to start the drill.
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The rookie instinct is to oversize "just to be safe." An oversized air conditioner short-cycles: it slams the temperature down, shuts off before it has wrung the humidity out of the air, and leaves the room cold, clammy, and hard on the compressor. Correct sizing beats big sizing every time.
The rule of thumb here — about 20 BTU per square foot, adjusted for the room's real conditions — is how a tech ballparks a window unit or single-zone mini-split. Whole-home equipment deserves the real instrument: an ACCA Manual J load calculation, which accounts for windows, orientation, infiltration, and duct losses room by room.
Roughly 20 BTU per square foot for cooling, adjusted up for high ceilings, sun, poor insulation, occupants, and kitchen heat, and down for shade and tight construction. Manual J is the accurate standard.
Short-cycling: fast cooling, poor dehumidification, a cold-and-clammy room, and premature compressor wear. Size to the load, not to fear.
12,000 BTU per hour — the heat absorbed by melting one ton of ice in 24 hours. A 24,000 BTU mini-split is a 2-ton unit.
No. It is a free educational estimator for room units and single zones. Whole-home systems should be sized by a qualified HVAC professional using ACCA Manual J.
Job-Site Math: The Sovereign Tradesman's Field Companion by Dr. Gene A. Constant teaches this exact math — BTU sizing, the sin of oversizing, CFM and delta-T, with a full answer key — for $3.99 on Kindle. And GENO, the GSU AI tutor, has memorized the entire book: he'll quiz you on any chapter out loud, 24/7, in 83 languages.
Get Job-Site Math — $3.99 EPA 608 Study Guide — $3.99 Kindle Ask GENO to Explain ItEducational tool provided free by Global Sovereign University, a program of The Foundation for Global Instruction (501(c)(3)). Results are rule-of-thumb estimates for room air conditioners and single-zone mini-splits. Whole-home equipment must be sized with an ACCA Manual J load calculation by a qualified HVAC professional, and refrigerant work requires EPA Section 608 certification.