The fastest way to estimate air conditioner size is BTU per square foot: plan on roughly 20 BTU per hour for every square foot of space you want to cool. A 500 sq ft room lands near 10,000 BTU, and a 1,000 sq ft area near 20,000 BTU. That single number gets you into the right ballpark for a window unit, a mini-split head, or a central system — but it is a starting point, not a final answer.
This guide gives you the chart, the simple math behind it, the climate map that pushes the number up or down, and the room-by-room adjustments that turn a rough estimate into a usable one.
The Core Rule: 20 BTU per Square Foot
The baseline most contractors and equipment guides use is about 20 BTU/hr per square foot of conditioned floor space for cooling. The practical range runs from roughly 18 to 25 BTU per square foot depending on climate, sun, insulation, and ceiling height.
The other number to memorize is the tonnage conversion: 12,000 BTU/hr = 1 ton of cooling. Air conditioners are sized in tons, so once you have a BTU figure you divide by 12,000 to find the equipment size. Twenty BTU per square foot is why you will often hear the shorthand “about 600 square feet per ton” — 12,000 BTU ÷ 20 BTU/sq ft ≈ 600 sq ft.
The 20 BTU/sq ft figure reflects guidance in the spirit of DOE and ENERGY STAR room-AC recommendations, which map square footage to capacity for typical homes. It assumes 8-foot ceilings, average insulation, and moderate sun. Change any of those and the right number changes too.
BTU per square foot at a glance
Air Conditioner Size per Square Foot Chart
This table applies the 20 BTU/sq ft baseline so you can read your size straight off your square footage. Use it for a quick estimate, then refine with the adjustments further down.
| Space Size (sq ft) | BTU Needed (at 20/sq ft) | Approximate Tons |
|---|---|---|
| 150 | 3,000 | 0.25 |
| 300 | 6,000 | 0.5 |
| 500 | 10,000 | 0.8 |
| 750 | 15,000 | 1.25 |
| 1,000 | 20,000 | 1.67 |
| 1,500 | 30,000 | 2.5 |
| 2,000 | 40,000 | 3.5 |
Small portable and window units are sold in these BTU steps (5,000, 8,000, 10,000, 12,000), so round up to the next available size rather than down. For central systems, equipment comes in half-ton increments (2, 2.5, 3, 3.5 tons), and the same round-up logic applies only after a proper load calculation — more on that below.
Square footage to BTU to tons
How Climate Changes the Number
The biggest reason a flat 20 BTU/sq ft is only a starting point is climate. The hotter and more humid your design conditions, the more cooling each square foot demands.
- Cool / northern zones (upper Midwest, Pacific Northwest, Northeast): trend lower, around 18 BTU/sq ft. Shorter, milder cooling seasons and lower humidity mean less capacity per square foot.
- Moderate zones (much of the central U.S.): the 20 BTU/sq ft baseline fits well.
- Hot, humid southern zones (Gulf Coast, Southeast, desert Southwest): trend higher, 22 to 25 BTU/sq ft. These regions add a heavy latent (humidity-removal) load on top of the sensible cooling load.
This is the same climate-zone logic DOE and ENERGY STAR materials describe in general terms: warmer design temperatures call for more capacity per unit of floor area. If you live where summers are long and sticky, start at the top of the range rather than the middle.
BTU per square foot by climate
Room Adjustments That Move the Number
Beyond climate, a handful of room-specific factors adjust the estimate. These mirror the well-known ENERGY STAR room-air-conditioner adjustments:
- Heavy sun exposure: add about 10% for rooms that are very sunny (large south- or west-facing windows).
- Heavy shade: subtract about 10% for rooms that stay shaded most of the day.
- Extra occupants: add 600 BTU for each person beyond two who regularly occupies the space.
- Kitchens: add roughly 4,000 BTU if the space includes a kitchen, because of heat from appliances and cooking.
- Ceiling height over 8 ft: taller ceilings mean more air volume per square foot, so the load rises proportionally — a 10-foot ceiling adds about 25% over an 8-foot baseline.
Apply climate first to set your per-square-foot rate, then layer these adjustments on the result.
Worked Example
Take a 500 sq ft living room with large west-facing windows that get strong afternoon sun, in a moderate climate.
Step 1 — Base load: 500 × 20 = 10,000 BTU/hr.
Step 2 — Sun adjustment: the room is very sunny, so add 10%. 10,000 × 1.10 = 11,000 BTU/hr.
That points to roughly an 11,000 to 12,000 BTU unit. If two extra people regularly use the room, add 600 BTU each; if it opens into a kitchen, add about 4,000 BTU. Each adjustment nudges you toward the next standard equipment size.
Why BTU per Square Foot Is a Starting Point
BTU per square foot is a screening tool. It treats every 500 sq ft room as identical, when in reality a room’s cooling load depends on far more than floor area:
- Windows — number, size, orientation, and glazing type can swing the load dramatically.
- Insulation — wall and attic R-values change how fast heat enters.
- Air infiltration — leaky homes pull in hot, humid outside air all day.
- Duct losses — ducts running through a hot attic can bleed away a meaningful share of capacity.
- Internal gains — appliances, lighting, and occupants all add heat.
The industry standard that accounts for all of this is a Manual J load calculation, and most codes require it for new installations. It produces a precise peak load for your specific home rather than a one-size-fits-all average.
There is also a real downside to skipping the detail and simply rounding up “to be safe.” Oversizing hurts. An AC that is too large cools the air quickly and shuts off before it has removed enough humidity, leaving rooms that feel cold but clammy. The frequent on-off pattern, called short cycling, wastes energy and wears the compressor out faster. Correct sizing — long, steady cycles — beats raw capacity every time.
For a deeper walk through the full sizing formula, climate factors, and oversizing pitfalls, see our companion guide, How Many BTU Do I Need?
Converting BTU per Square Foot to Tonnage
Once your BTU figure is set, getting to equipment tonnage is one step:
Tons = BTU/hr ÷ 12,000
So 24,000 BTU = 2 tons, 30,000 BTU = 2.5 tons, and 42,000 BTU = 3.5 tons. Because equipment is sold in half-ton steps, you will usually round to the nearest standard size after applying your climate and room adjustments. To go straight from square footage to tons, our AC Tonnage per Square Foot Calculator does the conversion for you.
Use the Free Calculator
BTU Calculator — enter your square footage and conditions to get a sized estimate in seconds.
Plug in your floor area, climate, sun exposure, and ceiling height, and the BTU Calculator returns a tailored BTU figure. To turn that into equipment size, the AC Tonnage per Square Foot Calculator converts square footage and BTU into tons.
FAQ
How many BTU do I need per square foot?
Plan on about 20 BTU per hour per square foot of cooling as a baseline, within a practical range of 18 to 25 depending on conditions. Cool northern climates trend toward 18, moderate climates sit near 20, and hot, humid southern climates run 22 to 25. Then adjust for sun, shade, occupancy, kitchens, and ceiling height.
How many BTU to cool 1,000 square feet?
At 20 BTU per square foot, 1,000 sq ft needs roughly 20,000 BTU/hr, which is about 1.67 tons. In a hot, humid climate move toward 22,000 to 25,000 BTU; in a cool, well-insulated home you may be fine nearer 18,000 BTU. Adjust up for very sunny rooms or extra occupants.
Is 20 BTU per square foot enough?
For most moderate-climate rooms with 8-foot ceilings and average insulation, 20 BTU per square foot is a sound starting estimate. It can fall short in hot, humid climates, sunny rooms, or homes with tall ceilings and poor insulation, where 22 to 25 BTU per square foot is more realistic. Always verify a whole-home system with a Manual J calculation.
Does climate change BTU per square foot?
Yes — climate is the single biggest reason the number shifts. Hot, humid regions need more capacity per square foot (22 to 25) because the AC also has to remove a large amount of moisture, while cool northern regions need less (around 18). The 20 BTU/sq ft baseline reflects a moderate climate.
How do I convert BTU per square foot to tons?
Multiply your square footage by your BTU-per-square-foot rate to get total BTU, then divide by 12,000 to get tons. For example, 1,500 sq ft at 20 BTU/sq ft is 30,000 BTU, which equals 2.5 tons. Equipment is sold in half-ton steps, so round to the nearest standard size after applying your adjustments.