Climate zones
Your ZIP code is matched to a state using USPS 3-digit ZIP prefixes, and each state is assigned one of five simplified climate zones. Large states span several climates, so you can always override the zone.
| Zone | Typical area | Cooling BTU/sq ft | Heating BTU/sq ft |
|---|---|---|---|
| Zone 1 — Hot | Florida, Hawaii, far south Texas | 30 | 32.5 |
| Zone 2 — Warm | Texas, Arizona, Gulf states | 27.5 | 37.5 |
| Zone 3 — Mild | California, the Carolinas, Tennessee | 25 | 42.5 |
| Zone 4 — Cool | Mid-Atlantic, Midwest south, Pacific NW | 22.5 | 47.5 |
| Zone 5 — Cold | New England, upper Midwest, Mountain north | 20 | 55 |
Adjustments
- Ceiling height: load × (height ÷ 8 ft), limited to 0.75–2×.
- Insulation: poor × 1.15, average × 1, good × 0.9.
- Sun exposure (cooling only): shaded × 0.9, normal × 1, very sunny × 1.1.
- Occupants (cooling only): +600 BTU for each person beyond two.
- Kitchen (cooling only): +4,000 BTU.
- Mini split rooms: Kitchen +4,000 BTU · Sunroom × 1.3 · Home office +500 BTU · Garage / workshop × 1.25 · Basement × 0.85.
Choosing equipment sizes
- Central AC and heat pumps: tons = BTU ÷ 12,000, matched to 1.5, 2, 2.5, 3, 3.5, 4, 5 tons. A size up to 5% below the load is accepted, because units have some reserve capacity.
- Furnaces: input BTU = heat output ÷ AFUE, matched to 40,000, 60,000, 80,000, 100,000, 120,000, 140,000 BTU.
- Mini split heads: 6,000, 9,000, 12,000, 18,000, 24,000, 30,000, 36,000 BTU.
- Heat pumps in zones 4–5: sized to the heating load but no more than 1.25× the cooling load; the shortfall is shown as backup heat.
Limits
Rule-of-thumb methods can't see your windows, duct leaks, air sealing or local design temperatures. ACCA Manual J, done by a contractor, accounts for those. Use these results to plan, compare quotes and spot a system that looks badly oversized or undersized.