BTU Calculator

Enter a room's length and width in feet, say how much sun it gets, how many people regularly occupy it, and whether it is a kitchen, and the calculator returns the cooling capacity a room air conditioner should have, in BTU per hour, along with the same figure in tons of cooling. The recommendation implements the sizing chart published by ENERGY STAR, the EPA-run efficiency labeling program whose room air conditioner guidance is the de facto consumer standard. Pick the unit rated closest to the number rather than one comfortably above it; oversizing has a cost covered further down the page.

How the sizing works

Everything starts from floor area, which is length times width. ENERGY STAR's chart, headed "Area To Be Cooled" against "Capacity Needed (BTUs per hour)", maps that area to a base capacity in fourteen bands: rooms of 100 up to 150 square feet get 5,000 BTU/h, 150 up to 250 get 6,000, and the bands step up to 34,000 BTU/h for rooms of 2,000 up to 2,500 square feet. The calculator implements the table as an exact lookup, so a 160 and a 240 square foot room land on the same 6,000 BTU/h row, and outside the chart's 100 to 2,500 square foot span it reports an error rather than extrapolating.

Four adjustments then modify the base figure, each taken from the same ENERGY STAR guidance. If the room is very sunny, capacity rises by 10 percent; if it is heavily shaded, it falls by 10 percent. If more than two people regularly occupy the room, each additional person adds 600 BTU/h, and if the room is a kitchen, 4,000 BTU/h is added, because people and cooking are heat gains the machine has to overcome. ENERGY STAR lists the four adjustments without saying how they combine, so applying the percentage to the base before the flat additions is this calculator's stated convention rather than an official rule.

capacity = base × sun + 600 × (occupants − 2) + kitchen

Here sun is 1.10 for very sunny, 0.90 for heavily shaded and 1.00 otherwise, the occupant term applies only beyond two people, and kitchen is 4,000 BTU/h or nothing. The second output converts the result into tons:

tons = capacity ÷ 12,000

A worked example

Take an open-plan living room and kitchen measuring 20 by 24 feet, used by a family of four. The floor area is 20 × 24 = 480 square feet, which falls in the chart's 450 up to 550 band, so the base is 12,000 BTU/h. The kitchen adds 4,000, bringing the running total to 16,000, and four regular occupants are two more than the chart already assumes, adding 2 × 600 = 1,200. The recommendation is 17,200 BTU/h, or 17,200 ÷ 12,000 = 1.43 tons of cooling.

A smaller case shows why the chart beats the popular shorthand of about 20 BTU per square foot. A heavily shaded 10 by 12 foot office is 120 square feet, and the shorthand would suggest a 2,400 BTU/h unit. The chart instead assigns 5,000 BTU/h, trimmed 10 percent for shade to 4,500, because 5,000-class machines are the smallest size commonly sold and the chart's bands are deliberately non-linear; the shorthand agrees with them only mid-range.

What a British thermal unit is

The U.S. Energy Information Administration defines the Btu as the quantity of heat required to raise the temperature of one pound of liquid water by one degree Fahrenheit, at the temperature where water is densest, around 39 degrees Fahrenheit. That is about 1,055 joules; NIST's tables put the international-table Btu at 1,055.056 joules. A BTU on its own is an amount of energy, while the number on an air conditioner is a rate, BTU per hour, which is why this page writes cooling figures as BTU/h. A 6,000 BTU/h unit moves 6,000 Btu of heat out of the room every hour, about 1,758 watts of cooling at 0.2930711 watts per BTU/h.

The name is older than air conditioning. Heating historians trace the underlying bookkeeping, rating a fuel by how much water its heat could warm and by how many degrees, to the early nineteenth-century engineer Thomas Tredgold, and the phrase British thermal unit appears in W.J.M. Rankine's steam-engine literature by 1859. Histories of the unit put it in general engineering acceptance by the 1890s, well before machines pushed heat the other way.

The Brooklyn printing plant where modern air conditioning began

Air conditioning did not begin with comfort. In the summer of 1902, Willis Carrier, a 25-year-old Cornell graduate working for the Buffalo Forge Company, designed what is regarded as the first modern air-conditioning system for the Sackett-Wilhelms Lithographing and Publishing Company in Brooklyn; Carrier's corporate history dates the drawings he submitted to the plant to July 17, 1902. The problem was humidity, not heat: the U.S. Department of Energy's history of air conditioning describes moisture wrinkling magazine pages, and the corporate account adds that swings in humidity misaligned the colors of the plant's multicolor printing and left wavy edges that jammed the presses. Carrier's answer drew the air across chilled coils so that moisture condensed out, and the installation was supplemented by an ammonia compressor the following spring. Carrier did not invent cooling as such, since the DOE timeline credits John Gorrie with an ice-machine patent back in 1851; what was new in 1902 was engineered control of humidity. In 1906 Carrier received a patent for his "Apparatus for Treating Air", and the same corporate history credits the engineer Stuart W. Cramer with coining the term air conditioning in a May 1906 address to cotton manufacturers.

Domestic cooling took decades longer. According to the same DOE history, Frigidaire introduced a split-system room cooler for homes in 1929, and H.H. Schultz and J.Q. Sherman filed a patent for an air conditioner that sat on a window ledge, with units reaching the market in 1932 at prices few households could contemplate. The window unit only became ordinary after the Second World War. DOE reports 43,000 sold by 1947, the first time homeowners could have air conditioning without expensive alterations to the house, and by the late 1960s, the same timeline records, most new American homes were built with central air. The sizing chart exists because of that postwar wave: ordinary households suddenly needed a quick answer to how big a unit to buy.

Why cooling is sold by the ton

A ton of refrigeration is defined as 12,000 BTU/h, which NIST's conversion tables put at 3,516.853 watts, and the number is not arbitrary. Before mechanical refrigeration, cold was a commodity cut from frozen lakes, and one engineering reference notes that American ice merchants of the era described capacity as how many tons of ice melt per day. The arithmetic that fixed the standard: melting ice absorbs a latent heat of 144 Btu per pound, so a 2,000 pound ton takes 2,000 × 144 = 288,000 Btu, and spreading that over 24 hours gives 12,000 Btu per hour. ASHRAE's terminology entry describes it the same way: a rate of 12,000 Btu/h roughly matching the heat of melting one ton of ice from and at 32 degrees Fahrenheit. The worked example's 17,200 BTU/h recommendation is therefore a 1.43 ton load, and dividing by 12,000 is all the tons output does.

Oversizing, undersizing and other assumptions

Rounding up generously is the wrong instinct for air conditioners. ENERGY STAR's sizing guidance is blunt that bigger is not always better: a unit that is too large cools the room before it has a chance to remove the humidity, leaving it damp and clammy. Moisture leaves the air only while air is moving across the cold coil, and a too-large machine satisfies the thermostat so quickly that it shuts off before much water has condensed out. A correctly sized unit runs longer and drier. Undersizing fails differently, with the unit running flat out on hot days without ever reaching temperature.

A few working assumptions are built in. The chart is ENERGY STAR's guidance for room and window air conditioners, not a substitute for a professional load calculation; central systems and heat pumps need a full ACCA Manual J assessment. The chart also has no ceiling-height input and assumes typical rooms, so a loftier room holds more air than its floor area implies. The BTU used throughout is the international-table value of 1,055.056 joules, and the occupant field accepts one to ten people, the first two of whom the chart already accounts for. High ceilings, poor insulation and very hot climates are all reasons to treat the result as a starting point and ask a professional before buying.

This sizes room and window air conditioners only; central systems need a professional load calculation. See the site disclaimer.

Frequently asked questions

How many BTU do I need per square foot?

There is no single per-square-foot figure, because ENERGY STAR's sizing chart is deliberately non-linear. A 120 square foot room gets 5,000 BTU per hour, roughly 42 per square foot, while a 480 square foot room gets 12,000, about 25 per square foot. The popular shorthand of around 20 BTU per square foot only matches the chart in the middle of its range, so look the area up rather than multiplying.

What is a BTU in air conditioning?

A British thermal unit is the heat needed to raise one pound of liquid water by one degree Fahrenheit at the temperature where water is densest, about 39 degrees Fahrenheit, per the U.S. Energy Information Administration's definition. It equals about 1,055 joules. An air conditioner's rating is a rate, BTU per hour, so a 6,000 BTU unit moves 6,000 BTU of heat out of the room each hour.

Is a bigger air conditioner always better?

No. ENERGY STAR's sizing guidance warns that a unit that is too large will cool the room before it has a chance to remove the humidity, leaving it with a damp, clammy feeling. A unit matched to the room runs longer per cycle and pulls more moisture out of the air while it does, which is a large part of comfort. Size to the chart, then pick the nearest available capacity.

What does a ton mean for an air conditioner?

One ton of cooling is 12,000 BTU per hour, which NIST lists as 3,516.853 watts. The name comes from the ice trade: ice absorbs 144 BTU per pound as it melts, so melting a 2,000 pound ton takes 288,000 BTU, and spread over 24 hours that is 12,000 BTU per hour. A 17,200 BTU per hour recommendation is therefore about a 1.43 ton load.

Do sun and shade really change what size AC I need?

Yes. ENERGY STAR's guidance says to increase capacity by 10 percent if the room is very sunny and reduce it by 10 percent if the room is heavily shaded. It also adds 600 BTU per hour for each regular occupant beyond two and 4,000 BTU per hour if the room is a kitchen, because people and cooking are heat sources inside the room.

Can I use this calculator for central air conditioning?

No. The chart behind it is ENERGY STAR's guidance for room and window air conditioners only. Central systems and heat pumps should be sized with a full load calculation such as ACCA Manual J, which accounts for the whole building rather than floor area alone.