How Do You Choose the Right BTU for Your Room Size?

Start with floor area, not with the biggest number on the box

Sizing a room air conditioner begins with the floor, not with the unit. Measure the length and the width of the room in feet, turning inches into fractions of a foot: 9 inches is .75 ft, 6 inches is .5 ft, 3 inches is .25 ft. Multiply the two figures. A room 14 ft 9 in long and 10 ft wide comes out at 147.5 sq ft.

For an L-shaped or otherwise awkward room, split the floor into rectangles and triangles, work out each area, and add them together. ENERGY STAR's sizing guidance uses that approach. Frigidaire's size guide makes the same starting point clear: the unit needed depends on the size of the room to be cooled, and capacity is expressed in BTU per hour.

Write the square footage down before shopping. Every later step, the table lookup and each adjustment, hangs on that one number.

Look the square footage up in the capacity table

With the area in hand, read the recommended cooling capacity. The ENERGY STAR chart below is calculated for an 8-foot ceiling and gives capacity in BTU per hour.

The bands meet at their edges, so a room sitting exactly on a boundary can go either way once the adjustments below are applied. A 250 sq ft room tops out the 6,000 BTU band; a 300 sq ft room tops out the 7,000 BTU band.

Apply the four adjustments

The table figure is a baseline for an average room. ENERGY STAR lists four adjustments that move it up or down, and each one reflects something the room itself does to the cooling load.

Work through them one at a time on paper, starting from the table figure and applying only the conditions the room actually has. A sunny kitchen used by three people takes the sun increase, the kitchen addition and the occupant addition on top of its table figure; a shaded spare bedroom with two regular occupants takes the shade reduction and nothing else.

Round down, because bigger is not better

ENERGY STAR's guidance says to round the final total down to the nearest thousand and look for that number on the unit. Rounding down matters because an oversized unit is not a better unit. ENERGY STAR states plainly that bigger is not always better: an air conditioner that is too large cools the room before it has had a chance to remove humidity, so the air ends up cool and clammy. The Department of Energy's purchasing guidance makes the same point, noting that oversizing leads to excessive on-off cycling and poorer humidity removal.

Two failure modes follow from getting this wrong, and each has its own explainer: What Happens When a Room AC Is Too Big and Signs Your Room AC Is Too Small or Struggling.

Window units and portables do not use the same number

The table gives a BTU-per-hour figure, and that is the number to match on a window unit: window air conditioners are compared on the BTU figure their listing states. Portable air conditioners work differently on paper. For a portable, the figure to use is the seasonally adjusted cooling capacity, the Department of Energy figure for portables, which appears on the listing as a separate number from the BTU figure. Rank and compare portables on that figure only. When a portable's listing does not give one, the maker does not list it, and that unit's BTU figure cannot be lined up against another portable's SACC.

Never convert between the two and never estimate one from the other. What BTU and SACC Ratings Mean on a Room AC goes through the difference in more detail, and Portable vs Window AC: Which Fits Your Room covers the installation side.

When the table is not enough

The chart assumes an 8-foot ceiling. ENERGY STAR notes that a room with higher ceilings may call for more capacity, and Frigidaire's size guide points out that general sizing guidelines do not account for vaulted ceilings or for how well the room is insulated.

Insulation and air sealing carry real weight. Frigidaire's support article on a room air conditioner that is not cooling says the unit will struggle to hold temperature if new air keeps entering the room, and recommends closing windows, doors, side curtains and the window framing the unit sits in. The same article notes that large heat-producing appliances in the room make a cooler temperature harder to hold. Why Some Rooms Are Harder to Cool Than Others covers sun, glass and insulation in more depth.

Match the number to the room's job

Once the capacity is settled, how the room is used narrows the field. For a bedroom, window units of 5,000 to 8,000 BTU are compared at best window air conditioner for bedroom. Rooms up to 250 sq ft, or units of 6,000 BTU or less, are gathered at best air conditioner for small room. Spaces that need 12,000 BTU or more are covered at best air conditioner for large room, and 10,000 BTU units at best 10000 BTU air conditioners.

The sizing table sets the target; the buying pages show which units in a given capacity band the catalog holds. Window AC Buying Checklist: Size, CEER, Features walks through the rest of the decision once the capacity is fixed.

Check the fit once the unit is running

Frigidaire's support article states that a working unit drops the temperature of the air it pulls in by 17 to 20°F before blowing it back into the room, so cold air at the vent is a sign the unit is doing its job. The same page suggests checking that nothing blocks airflow at the front or the back of the unit, that the side louvers are clear, and that the louvers are angled so air is not deflecting off a wall or ceiling back into the unit.

Whynter's portable air conditioner troubleshooting page lists the room-side causes for a unit that runs but does not cool the room: open windows or doors, too many people or heat-producing appliances, direct sunlight, and a unit switched on late in the day after the room has already heated up. It suggests closing curtains and starting the unit earlier rather than waiting for the hottest part of the day.

A worked example, end to end

Take a living room in the 300 up to 350 sq ft band with a large west-facing window, used by two people, not a kitchen. The table puts 300 up to 350 sq ft at 8,000 BTU. The room is very sunny, so the sun increase applies on top of that figure. Two regular occupants means no occupant addition. Round the total down to the nearest thousand and look for that number on the unit.

Now take the same room used as a kitchen with three regular occupants. Start again from the table figure for 300 up to 350 sq ft, add the sun increase, add 4,000 BTU for the kitchen and 600 BTU for the third person, then round down. Same floor area, a very different unit, because the room does different work. Cooling a Bedroom With a Room AC shows the same method applied at the small end of the range.