Short answer
A 1,800 sq ft space needs roughly 36,000 BTU (3 tons) of cooling and about a 76,000 BTU furnace in a temperate, average-insulation home. Size up for hot climates, poor insulation, high ceilings, or lots of sun.
Cooling needed
36,000 BTU
Tonnage
3 ton
Furnace (heating)
76,000 BTU
Unit type
Central AC / ducted system
For cooling, 1,800 sq ft at roughly 20 BTU per square foot works out to about 36,000 BTU (3 ton) in a temperate, average-insulation home. For heating, a furnace is sized by climate zone — around 76,000 BTU in a temperate climate, and more in a cold one (50–60 BTU/sq-ft up north).
These are whole-space estimates. Poor insulation, high ceilings, lots of glass, or an extreme climate all raise both numbers — run your home through the calculator to adjust them.
| Factor | Effect on the load |
|---|---|
| Floor area | ~20 BTU per sq ft cooling baseline (ENERGY STAR, 8-ft ceiling) |
| Climate zone | -15% (cold) to +25% (hot) on the cooling load |
| Insulation | Poor +15%, average no change, good -10% |
| Ceiling height | Scales with height ÷ 8 ft — a 10-ft ceiling adds ~25% |
| Sun exposure | Shaded -10% to sunny +10% |
| Occupancy & use | +600 BTU per person over two; +4,000 BTU for a kitchen |
| Climate zone | Cooling BTU | Unit size | Typical unit |
|---|---|---|---|
| Hot (e.g. Phoenix, Miami) | 45,000 BTU | 48,000 BTU · 4 ton | Central AC / ducted system |
| Warm (e.g. Dallas, Atlanta) | 39,600 BTU | 42,000 BTU · 3.5 ton | Central AC / ducted system |
| Temperate (e.g. St. Louis) | 36,000 BTU | 36,000 BTU · 3 ton | Multi-zone mini-split |
| Cool (e.g. Chicago, Boston) | 32,400 BTU | 36,000 BTU · 3 ton | Multi-zone mini-split |
| Cold (e.g. Minneapolis) | 30,600 BTU | 36,000 BTU · 3 ton | Multi-zone mini-split |
Temperate (highlighted) is the baseline for the figures above; set your exact climate, insulation, and sun in the calculator.
Real, widely available units in the 24,000 – 36,000 BTU class. Independent picks — we're not affiliated with any manufacturer or retailer and earn nothing from these links. Always match the unit's BTU to the size above.
Best overall
2–5 zones · up to ~48k BTU
Multi-zone mini-split · DIY · Quick-connect · self-install warranty
Cool several rooms from one outdoor unit, each with its own thermostat, using the same DIY quick-connect line sets — the easiest multi-zone to self-install.
Find on AmazonBest value
2–4 zones
Multi-zone mini-split · Lower cost per zone
A cost-effective multi-head system when you want independent control of several rooms and don’t need DIY line sets.
Find on AmazonBudget pick
2–4 zones
Multi-zone mini-split · heat pump · Cools + heats every zone
Multi-zone heat-pump comfort at a value price, from the same line as our single-zone best-overall pick.
Find on AmazonOne outdoor condenser feeds two or more indoor heads — right when you’re cooling several rooms rather than one big space.
About 36,000 BTU (3 tons) of cooling for 1,800 sq ft in a temperate, average-insulation home — more in a hot climate or with poor insulation.
Roughly a 76,000 BTU furnace in a temperate climate. Cold climates need 50–60 BTU/sq-ft, so the same home can need a larger furnace further north.
3 tons (36,000 BTU), since one ton equals 12,000 BTU. Confirm with a Manual J load calculation before a professional install.