What Size Air Conditioner Do I Need? A Room-by-Room BTU Guide
The short answer to “what size air conditioner do I need” is roughly 20 BTU per square foot of space you’re cooling. A 500 square foot room needs about 10,000 BTU.
That’s the starting point, not the answer. Ceiling height, sun exposure, how many people use the room and what climate you live in can shift that number by 30 percent or more in either direction.
Getting it wrong in either direction causes problems. Here’s the chart, the adjustments that matter, and three worked examples you can copy for your own situation.
The Chart
| Room Size (sq ft) | Room Size (m²) | Base BTU | Typical Space |
|---|---|---|---|
| 100 – 150 | 9 – 14 | 5,000 | Small bedroom, office |
| 150 – 250 | 14 – 23 | 6,000 | Standard bedroom |
| 250 – 300 | 23 – 28 | 7,000 | Large bedroom |
| 300 – 350 | 28 – 33 | 8,000 | Small living room |
| 350 – 400 | 33 – 37 | 9,000 | Living room |
| 400 – 450 | 37 – 42 | 10,000 | Large living room |
| 450 – 550 | 42 – 51 | 12,000 (1 ton) | Studio apartment |
| 550 – 700 | 51 – 65 | 14,000 | Open plan area |
| 700 – 1,000 | 65 – 93 | 18,000 (1.5 tons) | Small home, large open area |
| 1,000 – 1,200 | 93 – 111 | 21,000 | Small home |
| 1,200 – 1,400 | 111 – 130 | 23,000 (2 tons) | Medium home |
| 1,400 – 1,500 | 130 – 139 | 24,000 | Medium home |
| 1,500 – 2,000 | 139 – 186 | 30,000 (2.5 tons) | Larger home |
| 2,000 – 2,500 | 186 – 232 | 34,000 (3 tons) | Large home |
[ca-info] Tons and BTU describe the same thing. One ton equals 12,000 BTU per hour, a unit that dates back to how much cooling a ton of melting ice provides in a day. Window and portable units are usually rated in BTU; central systems and mini splits in tons.
The Five Adjustments That Matter
The chart assumes an average room: eight-foot ceilings, moderate sun, two occupants, average insulation. Here’s how to adjust when yours isn’t.
1. Ceiling Height
The chart is based on floor area, but you’re cooling volume. Standard assumption is eight-foot ceilings.
If your ceilings are higher: multiply by your ceiling height divided by 8. A room with 10-foot ceilings needs 25 percent more capacity than the chart says.
Vaulted or cathedral ceilings need a more careful estimate, since the volume calculation is less straightforward. Err toward the higher figure.
2. Sun Exposure
Heavily sunlit room: add 10 percent. This applies to rooms with large west or south-facing windows, minimal shading, or a top floor under a sun-exposed roof.
Heavily shaded room: subtract 10 percent. North-facing rooms, basements, or spaces shaded by trees or adjacent buildings.
[ca-tip] West-facing rooms are the most demanding, because afternoon sun coincides with the hottest part of the day. If you’re between two sizes on a west-facing room, size up.
3. Kitchens
Add 4,000 BTU for a kitchen or any room where cooking happens regularly.
Ovens, stovetops and refrigerators all release substantial heat. An open-plan kitchen and living area needs this adjustment even though the cooking area is only part of the space.
4. Number of Occupants
The chart assumes two people. Add 600 BTU for each additional regular occupant.
This matters most in bedrooms with more than two sleepers, home offices with multiple workstations, and living areas where a family gathers regularly.
5. Climate
Hot, humid climate (Southeast, Gulf Coast): add 10 to 20 percent. Your system works harder and also needs to remove more moisture.
Hot, dry climate (Southwest): add 10 percent, though moisture removal is less of a concern.
Mild climate (Pacific Northwest, Northeast): the base figure or slightly less is usually adequate.
Other Factors Worth Noting
Insulation quality. A poorly insulated older home may need 10 to 20 percent more. A well-insulated modern build may need less.
Windows. Large window area increases heat gain. Single-pane windows substantially more than double-pane.
Attic space above. A room directly under an uninsulated attic gains considerable heat.
Adjustment Summary
| Factor | Adjustment |
|---|---|
| 10-foot ceilings | +25% |
| 9-foot ceilings | +12% |
| Heavily sunlit | +10% |
| Heavily shaded | −10% |
| Kitchen | +4,000 BTU |
| Each occupant over 2 | +600 BTU |
| Hot humid climate | +10 to 20% |
| Mild climate | −10% |
| Poor insulation | +10 to 20% |
Three Worked Examples
Example 1: Sunny Bedroom
A 200 square foot bedroom, west-facing with large windows, 8-foot ceilings, two occupants, in Texas.
- Base: 200 sq ft → 6,000 BTU from the chart
- Sun exposure: +10% → 6,600
- Climate (hot, humid): +15% → 7,590
- Occupants: two, no adjustment
Result: about 7,600 BTU. The nearest standard sizes are 7,000 and 8,000. Given the west exposure, 8,000 is the safer choice.
Example 2: Open Plan Living and Kitchen
A 500 square foot open area including the kitchen, 9-foot ceilings, moderate sun, four regular occupants, in Ohio.
- Base: 500 sq ft → 12,000 BTU
- Ceiling height (9 ft): +12% → 13,440
- Kitchen: +4,000 → 17,440
- Occupants: two extra × 600 → 18,640
- Climate (moderate): no adjustment
Result: about 18,600 BTU. The nearest standard size is 18,000, which is close enough. A mini split at 18,000 BTU would suit this space.
Example 3: Finished Basement
A 400 square foot basement room, minimal windows, heavily shaded, 8-foot ceilings, two occupants, in Michigan.
- Base: 400 sq ft → 10,000 BTU
- Shading: −10% → 9,000
- Climate (mild): −10% → 8,100
Result: about 8,100 BTU. A 8,000 BTU unit works well here.
[ca-info] Basements often need less cooling than the square footage suggests, because they’re partly underground and stay naturally cooler. They frequently need more dehumidification, though, which is a separate consideration.
What Happens If You Get It Wrong
Both directions cause problems, and oversizing is the more common mistake.
Too Small
The system runs constantly without reaching your setpoint. Energy bills climb because it never gets a break. The compressor runs far more hours than designed, which shortens its life. On the hottest days, the house simply doesn’t get comfortable.
Too Big
Counterintuitively, this is usually worse.
An oversized system cools the air near the thermostat very quickly, satisfies the setpoint, and shuts off. That sounds efficient, but it means:
Poor humidity control. Removing moisture requires sustained runtime as air passes repeatedly over a cold coil. Short cycles don’t allow it, which is why an oversized system leaves a house cold and clammy.
Short cycling. Frequent starts wear the compressor faster than continuous running does.
Uneven temperatures. Brief cycles don’t circulate air long enough to even out room-to-room differences.
Higher cost, twice. More expensive equipment upfront, and higher energy use from repeated startup surges.
[ca-warning] The common instinct to “round up to be safe” causes real problems. If your calculation lands between two sizes, consider the smaller one unless you have a specific reason to size up, like a west-facing room or an unusually hot climate.
When You Need a Professional Load Calculation
The chart and adjustments above work well for individual rooms and single units: window ACs, portable units, and mini splits serving one space.
For a whole-house central system, a proper Manual J load calculation is the right approach. It accounts for insulation R-values, window specifications and orientation, air infiltration rates, duct losses, and local climate data in a way rules of thumb can’t.
[ca-tip] If a contractor sizes your central system by square footage alone without asking about insulation, window types or ceiling heights, that’s a warning sign. Proper sizing requires that information, and getting it wrong locks in years of poor comfort and higher bills.
Sizing Different Equipment Types
Window units: the chart applies directly. Also confirm the window opening dimensions and available electrical capacity.
Portable units: these are less efficient in practice than their ratings suggest, because the exhaust hose radiates heat back into the room and the unit creates negative pressure that pulls warm air in. Size up roughly 10 to 20 percent compared to a window unit for the same space.
Mini splits: the chart applies. Multi-zone systems need each head sized for its own room, with the outdoor unit sized for the combined load.
Central systems: get a Manual J calculation.
Our guide on mini split vs window vs portable units covers the tradeoffs between types.
Frequently Asked Questions
What size air conditioner do I need for 1,500 square feet?
Roughly 24,000 to 30,000 BTU, or 2 to 2.5 tons, depending on ceiling height, climate, sun exposure and insulation. In a hot, humid climate with 9-foot ceilings, the upper end is more appropriate. In a mild climate with good insulation, the lower end.
How many BTU do I need per square foot?
About 20 BTU per square foot as a baseline. That figure assumes 8-foot ceilings, moderate sun, two occupants and average insulation. Adjustments for ceiling height, sun exposure, kitchens, occupancy and climate can move the total substantially.
Is it better to oversize or undersize an air conditioner?
Neither, but oversizing causes more problems in practice. An oversized system cools quickly and shuts off before removing humidity, leaving the house cold and clammy while short cycling wears the compressor. An undersized system runs constantly and struggles on the hottest days, but at least it dehumidifies properly.
How many BTU do I need for a 12×12 room?
A 12×12 room is 144 square feet, calling for about 5,000 to 6,000 BTU as a baseline. Add 10 percent if it gets heavy afternoon sun, and adjust upward in a hot climate.
What size AC unit for a 2,000 square foot house?
Approximately 30,000 to 34,000 BTU, or 2.5 to 3 tons. For a whole-house central system, get a Manual J load calculation rather than relying on square footage, since insulation quality, window area and duct configuration meaningfully change the answer.
How do I convert BTU to tons?
Divide by 12,000. A 24,000 BTU system is 2 tons, and a 36,000 BTU system is 3 tons. The unit originates from how much cooling a ton of melting ice provides over 24 hours.
Does ceiling height affect AC sizing?
Yes, significantly. Standard charts assume 8-foot ceilings. With 10-foot ceilings you’re cooling 25 percent more air volume for the same floor area, and the capacity should increase accordingly.
Do I need a bigger AC for a kitchen?
Yes. Add about 4,000 BTU for any space where cooking happens regularly. Ovens, stovetops and refrigerators release considerable heat, and open-plan kitchens need this adjustment even though cooking occupies only part of the area.
Bottom Line
Start with 20 BTU per square foot from the chart, then apply the adjustments that fit your room: ceiling height, sun exposure, kitchen, occupants and climate.
If your result falls between two standard sizes, lean toward the smaller one unless you have a specific reason not to. Oversizing is the more common mistake and causes worse problems than undersizing, particularly with humidity.
For a whole-house central system, insist on a Manual J load calculation. A contractor who sizes by square footage alone is guessing with your money.