What the tool actually does
This site starts at 22 BTU per square foot, then nudges for sun, envelope, ceiling, and stories. We round to the nearest half-ton because that is how residential equipment is sold. The range under the headline is about ±10%.
A mixed-sun average 1,800–2,000 sq ft one-story house often prints about 3.5 tons. Plenty of tight or shaded houses in that size sit nearer 3. That is the argument. It is not a reason to install 4 “to be safe.”
When 3 tons is the better Florida call
- Manual J latent and sensible loads fit a 3-ton match with honest airflow.
- The house is tighter, shaded, or two-story with less roof per square foot.
- The old 3.5 or 4-ton unit short-cycles and the rooms feel clammy at 74.
When 3.5 tons is honest
- West glass, a hot attic, leaky ducts, or a leaky 1970s envelope push the load up.
- The Manual J number is closer to 3.5 than 3 after real window and duct inputs.
Do not buy either size from this page. Take the range to a contractor and ask to see the load calc. Wrong tonnage is expensive to undo.