How Many BTU Do I Need?

Find your room's floor area in the table below for a base BTU/hr figure, published by Energy Star for sizing a room air conditioner. That base figure assumes ordinary sun exposure, no kitchen use, and up to two occupants — for a room that's sunny, shaded, a kitchen, or more crowded than that, the BTU calculator applies the published adjustments for you automatically. This page is the lookup table itself, plus a worked example for sizing several separate room units across one home.

Energy Star room air conditioner sizing table

This is the published base table room-AC capacity is sized from — find the first row whose area is above your room's. A room of exactly 350 sq ft falls in the next row, because ENERGY STAR prints the bands as "350 up to 400".

ENERGY STAR, Room Air Conditioners (sizing chart) publishes this table directly; the calculator implements it verbatim. From there, adjust the base figure: add 10% for a very sunny room, subtract 10% for a heavily shaded one, add 4,000 BTU flat for kitchen use, and add 600 BTU for each occupant beyond two. The calculator's FAQ covers why each adjustment works the way it does, and what to do when your result lands between two retail unit sizes.

A full example: sizing three rooms in one apartment

Central air isn't always an option, and a common real question is how to size several separate window or through-wall units for one home rather than a single room. Take a small apartment with three rooms, each getting its own unit: a 200 sq ft living room with ordinary sun, a 140 sq ft bedroom with ordinary sun, and a 120 sq ft sunny kitchen. The living room falls in the "up to 250 sq ft" band, 6,000 BTU/hr, with no adjustments. The bedroom falls in the "up to 150 sq ft" band, 5,000 BTU/hr, also with no adjustments. The kitchen also falls in the "up to 150 sq ft" band — base 5,000 BTU/hr — but it's sunny, adding 10% (5,500), and it's a kitchen, adding a flat 4,000 BTU (9,500 BTU/hr total). Each room needs its own correctly sized unit rather than one average figure — buying three 7,000 BTU units because that's the rough average across the three rooms would undersize the kitchen and oversize the bedroom. Summed, the apartment needs about 20,500 BTU/hr of total cooling capacity across three separate units, which is also a useful figure for estimating the combined electrical load and running cost, even though each unit is bought and installed separately.

The same table in kilowatts and tons

BTU per hour is the US retail unit, but other markets label cooling capacity in kilowatts, and larger US systems are described in tons. The conversions are fixed: 1 kW is about 3,412 BTU/hr, and 1 ton of cooling is exactly 12,000 BTU/hr. The rows below convert the base figures so you can compare them with a unit labeled in either:

When comparing a metric label, match it against the cooling capacity, not the power the unit draws from the wall. Both are given in watts or kilowatts on many spec sheets, and the electrical input is typically much smaller than the cooling output. Going the other way, a unit rated at 2.5 kW of cooling delivers about 2.5 × 3,412 ≈ 8,530 BTU/hr: enough for the up-to-350 sq ft row, but short of the 9,000 BTU/hr the 400 sq ft row calls for before any sun or kitchen adjustment.

Why a flat BTU-per-square-foot rule misleads

The last column above shows something a single rule of thumb hides: the table doesn't use one fixed figure per square foot. A room just under 150 sq ft gets about 33 BTU/hr per square foot, while a 2,500 sq ft space gets about 14. Small rooms carry a larger share of wall, window and people heat for their size, and room air conditioners are sold in steps that start around 5,000 BTU/hr. Multiplying every room by one fixed number therefore tends to undersize small bedrooms and oversize large open spaces. For a single room, start from the table, not from a per-square-foot shortcut.

Rooms that sit on a band edge

The table works in steps, so a room a few square feet over a boundary jumps to the next capacity. A bedroom of 12 ft 4 in × 12 ft 2 in is about 150.1 sq ft, just past the 150 sq ft line, which moves the base figure from 5,000 to 6,000 BTU/hr. That makes careful measuring worth the effort near an edge. Measure the floor wall to wall at floor level, leave out closets whose doors stay shut, and use feet and inches rather than rounding each side up to the next foot, which on a small room can add 10% or more to the area. If the room still lands within a few square feet of a boundary after careful measuring, the adjustments decide it: a shaded room just over the line is well served by the smaller size, and a sunny one by the larger.

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