Quick answer: Start with 20 BTU per square foot of room area, then adjust up or down for sun exposure, ceiling height, occupancy, and what's happening in the room. A 300 sq ft bedroom needs roughly 6,000 BTU; add ~10% for a sun-drenched room, subtract ~10% for a shaded one, add 600 BTU per person beyond two, and add 4,000 BTU if it's a kitchen. Oversizing is a real mistake — a too-big unit cools fast, shuts off, and leaves the room clammy.
The actual math, step by step
The baseline formula the industry uses is simple:
Room square footage × 20 = base BTU
Measure length × width in feet. A 15 ft × 20 ft living room is 300 sq ft, so 6,000 BTU base. Then apply the adjustments:
| Situation | Adjustment |
|---|---|
| Room is heavily shaded | −10% |
| Room gets strong direct sun | +10% |
| Ceilings above 8 ft | +10% per extra foot (roughly) |
| More than 2 people regularly in the room | +600 BTU per extra person |
| Kitchen | +4,000 BTU |
| Room opens to other unconditioned spaces | Count that area too |
So that 300 sq ft living room, west-facing with afternoon sun and four people on the couch most evenings: 6,000 × 1.10 = 6,600, plus 1,200 for the two extra people = 7,800 BTU. You'd shop a 8,000–10,000 BTU class unit.
- Efficient Cooling for Large Rooms: This 14,000 BTU (ASHRAE) portable air conditioner quickly cools spaces up to 700 sq. ft., making it perfect for living rooms, bedrooms, and
- 4-in-1 Functionality for Year-Round Comfort: The portable AC unit is designed for all seasons, this unit functions as an air cooler, heater, fan, and dehumidifier, ensuring
- Exceptional Cooling: This powerful, 10,000/9,800 BTU through the wall air conditioner with 10600/8600 BTU supplemental heat (not meant to be used as a primary heat source in the
- Cooling Flexibility: With 8-way directional louvers, 3 fan speeds, and 4 operation modes you can customize your cooling to fit your specific needs
The BTU number on the box may not be the real one
This is where most people get burned, specifically on portable units.
Portable air conditioners are often advertised with an ASHRAE BTU rating and a much lower SACC (Seasonally Adjusted Cooling Capacity) rating. SACC accounts for the heat the exhaust hose leaks back into your room and the air the unit pulls out of the space. It is the honest number. Look at the Midea 12,000 BTU ASHRAE (8,000 BTU SACC) Portable Air Conditioner, Cools up to 350 Sq.Ft., with Dehumidifier & Fan mode, Control with Remote, Amazon Alexa & Google Assistant, Easy-to-use and Install — it states both numbers plainly, 12,000 ASHRAE against 8,000 SACC, and rates the room coverage at up to 350 sq ft. That gap is normal for the category, not a defect.
Also notice how differently manufacturers translate the same headline BTU into room coverage. The Air Choice 14000 BTU Portable Air Conditioner, 4-in-1 Air Conditioner Portable for Room Up to 700 Sq.Ft. with Remote, Auto Swing, Quiet AC Unit as Heater Dehumidifier Fan, 24H Timer, Window Kit, White claims 14,000 BTU ASHRAE for up to 700 sq ft, while the SereneLife 14000 BTU Portable Air Conditioner - 115V Compact Standing Air Conditioner for Room Up to 450 Sq. Ft., 3-in-1 Floor AC Unit with Remote Control - Universal Casters, Window Mount Kit lists the same 14,000 BTU for up to 450 sq ft. Same headline number, a 250 sq ft difference in claimed coverage. Trust your own square-footage math over the marketing area, every time.
Window and wall units are simpler — their BTU ratings are measured on a stricter basis and the stated room coverage tends to track the 20-BTU-per-sq-ft rule closely.
Specification comparison
| Product | Rated BTU | Stated room coverage | Type | Modes | Noise rating | Price |
|---|---|---|---|---|---|---|
| GE Profile Inverter Window Air Conditioner Unit 10,000 BTU… up to 450 sq ft. | 10,000 | Up to 450 sq ft | Window (inverter) | Cooling + 3 fan speeds, quiet mode | As low as 43 dB | Check price on Amazon |
| GE Profile Inverter Window Air Conditioner Unit 12,000 BTU… up to 550 sq ft. | 12,000 | Up to 550 sq ft | Window (inverter) | Cooling + 3 fan speeds, quiet mode | As low as 44 dB | Check price on Amazon |
| Whirlpool 10,000 BTU Wall Air Conditioner | 10,000 / 9,800; 10,600 / 8,600 BTU supplemental heat | Up to 450 sq ft | Through-the-wall | 4 operation modes, 3 fan speeds, Eco, Sleep | Not stated | Check price on Amazon |
| Midea 12,000 BTU ASHRAE (8,000 BTU SACC) Portable | 12,000 ASHRAE / 8,000 SACC | Up to 350 sq ft | Portable | Cool, fan, dehumidify | Not stated | See the product list on this page |
| Air Choice 14000 BTU Portable | 14,000 ASHRAE | Up to 700 sq ft | Portable | Cool, heat, fan, dehumidify | Not stated | Check price on Amazon |
| SereneLife 14000 BTU Portable | 14,000 | Up to 450 sq ft | Portable | Cool, fan, dehumidify | Not stated | Check price on Amazon |
A worked example with a real unit
Say you've done the math and landed at 9,400 BTU for a 450 sq ft bedroom with average sun. That points straight at the 10,000 BTU class, and the GE Profile Inverter Window Air Conditioner Unit 10,000 BTU with Energy Efficient AC Cooling up to 450 sq ft., Ultra Quiet, SMART Home Capable, Includes Remote and Install Kit, White is rated for exactly that ceiling — 10,000 BTU, up to 450 sq ft, with a 15.0 CEER rating the listing describes as 37% more efficient than the DOE minimum for comparable capacity, and noise as low as 43 dB.
Inverter units deserve a note here. Because they modulate output rather than slamming between full-on and off, a slight overshoot in BTU is less punishing than it would be on a fixed-speed compressor — the unit just runs slower. That doesn't make sizing irrelevant, but it widens the margin for error.
Who this sizing approach is for
- Anyone buying a single-room unit — window, wall, or portable — where you can measure the space yourself.
- People replacing a unit that "never kept up" or "made the room feel damp," which are the two classic symptoms of undersizing and oversizing respectively.
- Renters deciding between a portable and a window unit, where the SACC gap changes which BTU number you should be comparing.
Who should skip this and get a load calculation
- Anyone cooling a whole house or multiple connected rooms with one system. Square-footage rules of thumb fall apart across floors and open plans; you want a Manual J load calculation from an HVAC contractor.
- Homes with unusual envelopes — single-pane glass walls, uninsulated attics, converted garages, historic buildings. The 20 BTU/sq ft baseline assumes reasonably modern insulation.
- Anyone sizing for a server closet, grow room, or workshop with heat-generating equipment. Equipment heat load is calculated separately and it's usually the dominant term.
Honest trade-offs
Oversizing costs you comfort, not just money. A too-large unit hits the thermostat setpoint before it has run long enough to pull moisture out of the air. The room reads 72°F and feels swampy. It also short-cycles, which is hard on a compressor.
Undersizing costs you the compressor. A unit that runs continuously and never reaches setpoint on a 95°F day is being asked to do something it can't. Better to be 10% over than 10% under, if you must choose — and on an inverter model, being slightly over is genuinely low-risk.
Portable units are structurally less efficient. The exhaust hose radiates heat back into the room and the single-hose design creates negative pressure that pulls warm air in from elsewhere. That's why SACC exists. Portables win on installation freedom and lose on cooling per BTU. Go in knowing that.
Wall units require a sleeve. The Whirlpool 10,000 BTU Wall Air Conditioner, up to 450 Sq Ft., White fits sleeves 24.5"–27" wide and needs a 230V/20A outlet — so sizing is only half the problem. If you don't already have a sleeve and the right circuit, the install cost can exceed the unit.
What to check on the listing before you buy
Listings change, and specs get revised between production runs. Before you commit:
- Current price and variant. Multiple BTU sizes often share one listing page. Confirm the price shown matches the capacity you selected — the wrong dropdown choice is the most common ordering error in this category.
- Voltage and plug type. The Whirlpool wall unit specifies 230V/20A; the SereneLife specifies 115V. Confirm what your outlet actually is before ordering, not after.
- Physical dimensions against your opening. Window sash height, sleeve width, or floor clearance. The SereneLife lists 18.5" × 13.9" × 30"; the Whirlpool lists 20.3" D × 24.2" W × 14.5" H.
- Window kit range. The Air Choice unit's long panel covers 37.4"–50" and the short one 25.6"–37.4". Measure your window before assuming it fits.
- Warranty terms and what's included. The GE Profile models list an install kit and remote; the Midea lists exhaust hose, window kit, and remote with batteries. Verify current inclusions on the page.
- Go read the recent reviews yourself, filtered to the specific capacity variant you're buying. Sizing complaints are the most informative ones in this category.
Air Choice 14000 BTU Portable Air Conditioner — consider instead if…
…your BTU math landed high and you want one appliance covering the whole year. The Air Choice 14000 BTU Portable Air Conditioner, 4-in-1 Air Conditioner Portable for Room Up to 700 Sq.Ft. with Remote, Auto Swing, Quiet AC Unit as Heater Dehumidifier Fan, 24H Timer, Window Kit, White runs as cooler, heater, fan, and dehumidifier, with a 62°F–86°F temperature range, 90° oscillation, sleep mode, and a 24-hour timer. Note the 700 sq ft claim is on the ASHRAE figure; size conservatively against it. $419.99.
SereneLife 14000 BTU Portable Air Conditioner — consider instead if…
…humidity is as much your problem as heat. The SereneLife 14000 BTU Portable Air Conditioner - 115V Compact Standing Air Conditioner for Room Up to 450 Sq. Ft., 3-in-1 Floor AC Unit with Remote Control - Universal Casters, Window Mount Kit states moisture removal up to 1.42 liters per hour, with 420 m³/h airflow, two fan speeds, R32 refrigerant, and 115V operation. At $366.73 it's the lowest-priced option here.
GE Profile Inverter Window Air Conditioner Unit 12,000 BTU — consider instead if…
…your calculation exceeded 10,000 BTU and you want window-unit efficiency rather than a portable's losses. The GE Profile Inverter Window Air Conditioner Unit 12,000 BTU with Energy Efficient AC Cooling up to 550 sq ft., Ultra Quiet, SMART Home Capable, Includes Remote and Install Kit, White covers up to 550 sq ft at a 15.0 CEER rating, as low as 44 dB, with SmartHQ app control and three fan speeds. Listed at $438.00 — currently below its 10,000 BTU sibling.
Whirlpool 10,000 BTU Wall Air Conditioner — consider instead if…
…you have an existing wall sleeve and want the window back. The Whirlpool 10,000 BTU Wall Air Conditioner, up to 450 Sq Ft., White covers up to 450 sq ft, adds 10,600/8,600 BTU supplemental heat (not intended as a primary heat source in the coldest months), and offers 8-way louvers, Eco and Sleep functions, a 24-hour timer, and a quick-access washable filter. $569.99.
FAQ
Is it better to buy a bigger BTU unit than I need, just in case? No. Oversizing causes short-cycling and leaves the room humid because the unit doesn't run long enough to dehumidify. Aim within about 10% of your calculated number. Inverter models tolerate slight overshoot better because they modulate output instead of cycling on and off.
Why does a 14,000 BTU portable cool less area than a 12,000 BTU window unit? Because they're measured differently, and portables lose capacity to the exhaust hose and to air infiltration. Compare portables on their SACC rating when it's published — the Midea listing states 8,000 SACC against 12,000 ASHRAE — and compare window units on their own rating.
How do I count an open-plan space? Include any area the cool air will spill into. If your 300 sq ft living room opens to a 150 sq ft dining area with no door, calculate for 450 sq ft. Air doesn't respect the floor plan you drew.
Does ceiling height really matter? Yes — you're cooling volume, not floor area. The 20 BTU/sq ft rule assumes roughly 8-foot ceilings. Add about 10% for each additional foot, and more if you have a vaulted ceiling where hot air stratifies at the top.
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