Size correctly.
Air conditioner BTU size for any room. Accounts for climate, insulation, sun exposure, and occupancy so you buy the right unit.
How we calculated this
The baseline formula is 20 BTU per hour per square foot of room area — a well-established rule of thumb for typical homes with average insulation and 8-foot ceilings. This covers most residential rooms in temperate climates.
Ceiling height above 8 feet adds about 5% per additional foot. A room with 10-foot ceilings has 10% more air volume to cool, and the calculator adjusts for that automatically.
Climate adjustment reflects how hard the AC has to work. Cool climates (Seattle, Pacific Northwest) subtract 10% because peak temperatures rarely exceed 85°F. Hot climates (Phoenix, Florida) add 15% because the unit runs harder during prolonged 90°F+ days.
Sun exposure matters because sunny rooms gain significant heat through windows. South-facing rooms or rooms with large windows add 10%. North-facing or shaded rooms reduce 10%.
Occupancy: each person adds about 600 BTU of body heat. The baseline assumes 2 people; add 600 BTU per additional regular occupant (including pets, which contribute about half that).
The result is rounded up to the nearest 500 BTU because AC units are sold in standard sizes: 5,000 BTU (small window), 8,000 BTU (medium window), 10,000-12,000 BTU (large window or small portable), 18,000-24,000 BTU (mini-split), and so on. Round up to the next standard size — undersizing is worse than oversizing within reason, though gross oversizing leads to humidity problems.
The most common HVAC mistake costs $3,000 and makes your house less comfortable
An HVAC technician in Houston described the service call he gets more than any other. A homeowner bought a new air conditioner two summers ago. The contractor installed a 4-ton unit in a house that needs 3 tons, because the contractor said it would cool faster, and the homeowner figured bigger is better. The system does cool fast. Too fast. It reaches the thermostat setpoint in 8 minutes, shuts off, and the indoor humidity is still at 62 percent because the coil did not run long enough to condense moisture out of the air. Six minutes later, the temperature drifts up 2 degrees and the system restarts. This cycle repeats 6 to 8 times per hour, all summer.
The house is 72 degrees and feels clammy. The compressor, rated for 100,000 start-stop cycles over its lifetime, burns through that number in 8 years instead of 15. The homeowner paid $3,000 more for the bigger unit and got worse comfort and a shorter equipment lifespan. Correct sizing is not a nice-to-have. It is the single most important HVAC decision.
How we calculated these numbers▾
The 25 BTU/ft² baseline follows ACCA Manual J simplified residential load calculation methodology. Adjustment factors for sun exposure, occupancy, kitchen heat, and insulation quality are from the same source. SEER rating energy costs use 2025 EIA average residential electricity rates ($0.16/kWh) and assume 1,200 cooling hours per year (typical for zones 3-4). Equipment efficiency from the AHRI certified ratings directory.
How to size your AC: the 25 BTU rule
The baseline calculation is straightforward. Multiply your conditioned square footage by 25. That gives you the approximate BTU per hour of cooling capacity needed. A 2,000 square foot house needs roughly 50,000 BTU/hr, which is about 4 tons (1 ton equals 12,000 BTU/hr). Residential systems come in half-ton increments: 1.5, 2, 2.5, 3, 3.5, 4, and 5 ton units.
What changes the number
The 25 BTU/ft² baseline assumes average insulation, standard windows, moderate climate, and 8-foot ceilings. Real houses deviate from this in every direction. A well-insulated house with double-pane low-E windows in a mild climate might need only 18 BTU/ft². A poorly insulated house with single-pane windows facing south in Phoenix might need 35.
The calculator at the top of this page applies these factors automatically based on your inputs. If you are doing the math manually, start with area times 25, then apply each applicable factor. Round up to the next available unit size, but never go more than half a ton above the calculated number. That half-ton buffer accounts for the imprecision of simplified load calculations without crossing into the oversizing danger zone.
Composite illustration based on typical project dimensions, regional contractor pricing, and 2026 material costs. Not a specific real project.
SEER ratings: what efficiency actually costs you
SEER (Seasonal Energy Efficiency Ratio) measures how much cooling a system produces per watt of electricity consumed over a typical cooling season. Higher SEER means lower electricity bills. The current federal minimum for new installations is 14 SEER2 in northern states and 15 SEER2 in southern states. Systems from 10 to 15 years ago were commonly 10 to 13 SEER.
The diminishing returns kick in above 18 SEER2. Going from 14 to 16 saves about $60 per year. Going from 16 to 20 saves another $90. Going from 20 to 25 saves about $40. Meanwhile, the equipment cost jumps $1,000 to $2,000 per SEER point at the high end. For most homeowners, 16 SEER2 is the sweet spot: meaningfully better than minimum, with a payback period under 8 years.
Window unit vs mini-split vs central: which system type
Window unit | Mini-split | Central AC | |
|---|---|---|---|
| Cooling capacity | 5,000–25,000 BTU | 9,000–48,000 BTU | 18,000–60,000 BTU |
| Installed cost | $200–800 | $3,000–7,000 | $3,500–8,000 |
| Efficiency | 8–12 CEER | 16–22 SEER2 | 14–20 SEER2 |
| Noise level | 50–60 dB (noticeable) | 20–40 dB (whisper quiet) | 50–70 dB (outdoor unit) |
| Best for | Single room, renters, budget | Additions, ductless, zone control | Whole house with existing ducts |
If you have ducts, central AC or a heat pump is the default. If you don't, mini-splits are far better than window units for anything other than temporary or budget cooling.
For whole-house cooling, central AC or a ducted heat pump is the standard if your house already has ductwork. If you are replacing an aging system, the heat pump calculator can help you compare heat pump vs traditional AC sizing. A heat pump does both heating and cooling in one unit, which often pencils out better than replacing AC and furnace separately.
Mini-splits deserve serious consideration for homes without ductwork, for additions, and for rooms that are consistently too hot or too cold. They are 30 to 50 percent more efficient than window units, nearly silent indoors, and allow zone-by-zone temperature control. The installed cost ($3,000 to $7,000 for a single-zone system) is higher than window units but lower than adding ductwork to a house that does not have it. For homes where insulation upgrades have reduced the heating and cooling load, a mini-split may be all you need. For hot water sizing, the water heater calculator determines tank or tankless capacity by household size.
Sources
- Energy Star — Room Air Conditioners — Baseline 20 BTU per sq ft recommendation
- ACCA Manual J (Residential Load Calculation) — Industry reference for HVAC sizing
Frequently asked
What size AC do I need for a 200 sq ft bedroom?
A 200 sq ft room in a moderate climate needs about 4,000-5,000 BTU, which means a standard 5,000 BTU window unit works. Bump up to 6,000 if the room has lots of sun or is in a hot climate. The calculator above gives you the exact number for your specific room.
What happens if my AC is too big?
An oversized AC will cool the room quickly but turn off before removing enough humidity, leaving the room cold and clammy. It also cycles on and off more frequently, which wastes energy and wears out the compressor. Size correctly, not bigger.
What happens if my AC is too small?
Undersized units run constantly without reaching the set temperature on hot days. They waste energy, burn out faster from continuous operation, and can't keep up during peak heat. If you're between sizes, round up to the next standard size.
Does this work for mini-split systems?
Yes — same BTU calculation. Mini-splits are commonly 9,000 / 12,000 / 18,000 / 24,000 BTU. Pick the nearest size above your calculated need. Multi-zone systems should calculate each zone separately and sum.
Does this work for central air?
Central AC is sized in tons (1 ton = 12,000 BTU). A whole-house calculation is more complex because it includes duct losses, multiple rooms, and heat gain through the attic. For rough sizing, add up individual rooms and divide by 12,000 to get tons. For accurate sizing, a pro should do a Manual J load calculation.
How does climate affect sizing?
AC sizing is based on the design temperature — the peak summer temperature your unit must handle. Hot climates have higher design temps (95-105°F), so the unit must move more heat. Cool climates rarely exceed 85°F, so smaller units suffice. The 10% / 15% adjustments in the calculator roughly cover this.
Does the calculator cover heating BTU?
Not directly — heating sizing requires factoring in insulation, windows, and outdoor design temperature. For a quick estimate, cooling BTU is usually 70-80% of heating BTU for the same space, but use a heat pump calculator for accurate heating sizing.
Should I size for worst-case or average?
For peak summer days. Use the calculator's default hot-climate setting if you experience prolonged 90°F+ days. Undersizing to save money on cooler days backfires during July-August heat waves, when the unit runs nonstop and still can't catch up.
Was this calculator helpful?