Contractor & trades · free calculator
HVAC tonnage sizing calculator
Manual J shortcut: estimate AC tonnage and BTU requirement for a residential space based on square footage, climate, insulation, and orientation.
AC tonnage
Round up to nearest 0.5 ton
Show the work
- Total BTU/hr cooling load69,600
- BTU/hr per sqft32
- Rounded ton size to install6.0
HVAC sizing — the rule of thumb that's wrong, and the one that's right
The old rule "1 ton per 500-600 sqft" oversizes most homes by 30-50%. A right-sized AC runs longer cycles at lower output, dehumidifies properly, and lasts 5-10 years longer than an oversized unit short-cycling itself to death.
What actually matters
- Climate zone — the design temperature delta drives base BTU/sqft. Hot-humid Florida needs 28-32 BTU/sqft; mixed Ohio is 23-26.
- Envelope — insulation level and window quality. Post-2015 tight homes lose 30-40% less heat than 1980s builds.
- Solar gain — west-facing glass at 5pm in July is brutal. Shaded north-facing rooms are easy.
- Internal loads — bodies (~600 BTU/person), cooking (~1,200 BTU per active kitchen), unvented gas appliances.
The sizing rule
This calc is a Manual J shortcut. For homes in tight bands of envelope/climate the answer is within 10-15% of a full Manual J — close enough for replacement-in-kind decisions. For new construction or a major renovation, always do a full Manual J because the cost of getting it wrong is a 20-year headache.
Why oversized AC fails
An oversized unit hits the thermostat setpoint fast on temperature, but never runs long enough to pull humidity out of the air. You get cold + clammy houses, mold growth, and short-cycling that wears out compressors. Right-sized = longer cycles = drier air = comfort.
Export
CSVPrintable PDFEmbedNot sure which calc you need? Ask →Related calculators