Short answer
A 250 sq ft space needs roughly 5,000 BTU (under 1 ton) of cooling and about a 11,000 BTU furnace in a temperate, average-insulation home. Size up for hot climates, poor insulation, high ceilings, or lots of sun.
Cooling needed
5,000 BTU
Tonnage
0.4 ton
Furnace (heating)
11,000 BTU
Unit type
Window unit
For cooling, 250 sq ft at roughly 20 BTU per square foot works out to about 5,000 BTU (0.4 ton) in a temperate, average-insulation home. For heating, a furnace is sized by climate zone — around 11,000 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) | 6,300 BTU | 8,000 BTU · 0.7 ton | Window unit or single-zone mini-split |
| Warm (e.g. Dallas, Atlanta) | 5,500 BTU | 6,000 BTU · 0.5 ton | Window unit or single-zone mini-split |
| Temperate (e.g. St. Louis) | 5,000 BTU | 5,000 BTU · 0.4 ton | Window unit or single-zone mini-split |
| Cool (e.g. Chicago, Boston) | 4,500 BTU | 5,000 BTU · 0.4 ton | Window unit or single-zone mini-split |
| Cold (e.g. Minneapolis) | 4,300 BTU | 5,000 BTU · 0.4 ton | Window unit or single-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 Up to 12,000 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
8,000–12,000 BTU
Window · inverter · U-shape · ~36 dBA · window still opens
The U-shape puts the compressor outside the glass, so it runs about as quiet as a whisper and you can still open the window. Inverter compressor sips power.
Find on AmazonQuietest
8,000–14,000 BTU
Window · dual inverter · Dual-inverter · Wi-Fi · very quiet
LG’s dual-inverter compressor ramps up and down instead of cycling on/off — quieter, steadier temps, and lower running cost than a fixed-speed unit.
Find on AmazonBudget pick
5,000 BTU
Window · fixed-speed · Cheapest way to cool a small room
A no-frills window unit that reliably cools a small bedroom, office, or dorm up to ~150 sq ft for the lowest upfront cost.
Find on AmazonBest value
8,000–12,000 BTU (SACC)
Portable · inverter · No permanent mount · inverter
When a window mount isn’t an option, this inverter portable is one of the few that actually cools like its rating instead of falling short.
Find on AmazonAbout 5,000 BTU (0.4 tons) of cooling for 250 sq ft in a temperate, average-insulation home — more in a hot climate or with poor insulation.
Roughly a 11,000 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.
0.4 tons (5,000 BTU), since one ton equals 12,000 BTU. Confirm with a Manual J load calculation before a professional install.