Enter your room size and conditions to get the cooling BTU, tonnage, and unit type you need — plus a furnace estimate for heating. Works in both imperial and metric.
Short answer
To size an AC, start from about 20 BTU per square foot, then adjust: +10% for a sunny room, +15% for poor insulation, +25% for a hot climate, +600 BTU per person over two, and +4,000 for a kitchen. A 12×12 room needs about 5,000 BTU; a 1,500 sq ft home about 30,000 BTU (2.5 tons).
Base load
~20 BTU/sqft
temperate
1 ton
12,000 BTU
tonnage unit
12×12 room
~5,000 BTU
window unit
1,500 sqft
~2.5 tons
central AC
Cooling needed
2,900 BTU
Recommended unit
5,000 BTU
Tonnage
0.4 ton
Furnace (heating)
6,000 BTU
Best unit type
Window unit or single-zone mini-split
Alternative: Portable AC
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 AmazonEstimate only, from a simplified ≈20 BTU/sq-ft model with documented adjustments. Confirm with a Manual J load calculation before buying or installing equipment.
| Room size | Estimated load | Unit to buy | Unit type |
|---|---|---|---|
| 150 sq ft | 3,000 BTU | 5,000 BTU | Window unit or single-zone mini-split |
| 250 sq ft | 5,000 BTU | 5,000 BTU | Window unit or single-zone mini-split |
| 350 sq ft | 7,000 BTU | 8,000 BTU | Window unit or single-zone mini-split |
| 450 sq ft | 9,000 BTU | 10,000 BTU | Window unit or single-zone mini-split |
| 550 sq ft | 11,000 BTU | 12,000 BTU | Window unit or single-zone mini-split |
| 700 sq ft | 14,000 BTU | 18,000 BTU | Single-zone mini-split |
| 1,000 sq ft | 20,000 BTU | 24,000 BTU | Single-zone mini-split |
8-ft ceilings, average insulation and sun, temperate climate, two occupants. "Unit to buy" snaps the estimate up to the nearest standard AC size. Adjust the conditions in the calculator for your room.
Every number on this page comes from the same formulas the calculator runs — the standard room-load method behind the published BTU charts:
Last reviewed: July 13, 2026
A BTU (British Thermal Unit) is the amount of heat energy needed to raise one pound of water by one degree Fahrenheit. Air conditioners and furnaces are rated in BTU per hour to describe how much heat they can move: a higher BTU rating means more cooling or heating capacity. For air conditioners, capacity is often given in tons, where one ton equals 12,000 BTU — so a 24,000 BTU unit is a 2-ton AC. Sizing to the right BTU matters: too few leaves the room uncomfortable, while too many makes the unit short-cycle and waste energy.
Start from about 20 BTU per square foot in a temperate climate, then adjust: strong sun (+10%), poor insulation (+15%), a hot climate (+25%), ceilings over 8 ft, each occupant beyond two (+600 BTU), and a kitchen (+4,000 BTU) all raise the load. Enter your exact room above to get the adjusted BTU, tonnage, and recommended unit type.
Divide the BTU by 12,000 — one ton of cooling equals 12,000 BTU. So a 12,000 BTU unit is 1 ton, 24,000 BTU is 2 tons, and 36,000 BTU is 3 tons. The calculator shows both the BTU and the tonnage for your room.
Roughly 20 BTU per square foot for an average-insulation room in a temperate climate. Sunny, poorly insulated, or hot-climate rooms push toward 25 BTU per square foot or more, while a well-shaded, well-insulated room in a mild climate can need a little less.
An oversized AC short-cycles — it cools the air quickly but shuts off before it removes enough humidity, leaving the room cold and clammy. The frequent on/off cycling also wastes energy and wears the compressor faster. Right-sizing matters more than simply buying the largest unit.
Yes — floor area alone is only the starting point. Ceilings above the standard 8 ft add volume to cool, poor insulation adds about 15%, strong direct sun adds about 10%, each person beyond two adds ~600 BTU, and a kitchen adds ~4,000 BTU for appliance heat. The calculator folds all of these in.
A hot climate raises the cooling load by roughly 25% versus a temperate one, so the same room needs more BTU further south. For heating, the reverse is true: cold climates need a larger furnace (about 50–60 BTU per square foot) than a mild one.