What Size Air Conditioner Do I Need?

If you are asking what size air conditioner do I need for one room, there is a quick way to get a sensible first answer, and then a short list of adjustments that keep that answer honest. This guide walks through both, with a worked example you can copy for your own room. If you would rather skip the arithmetic, our AC BTU calculator does the same math in seconds — but it helps to understand what it is doing, because a common sizing mistake is simply buying the biggest unit on the shelf.

The short answer: start with 20 BTU per square foot

For a reasonably insulated room with 8-foot ceilings, allow about 20 BTU of cooling for every square foot of floor area. Measure the room’s length and width in feet, multiply them together, and multiply that area by 20. A 150-square-foot bedroom therefore starts at about 3,000 BTU before adjustments, and a 400-square-foot living room starts at about 8,000 BTU. Insulation changes the rate. For a home with average, older insulation, use up to 25 BTU per square foot instead. For a poorly insulated room, or a hot room with a lot of sun and glass, use a figure in the low 30s. Those are the same bands our calculator uses, because insulation and sun decide how fast heat gets back into the room you have just cooled.

One honesty note before we go further: rules of thumb like this one are starting points, not engineering. They treat every wall, window and door as average, and real rooms are rarely average in every way. A proper ACCA Manual J load calculation is the professional standard for whole-home systems, and it is what a good installer will run before quoting for central air. For a single room unit, though, the rule below — used with its adjustments — is a reasonable and widely used way to land on the right shelf size.

The adjustments that move the number

The base figure assumes a normally shaded room, standard ceilings and two people. Adjust it for the room you actually have:

• Very sunny room: add 10%. Large, sun-facing windows pour heat in all afternoon, and the unit has to remove that heat as well as the room’s own warmth.
• Heavily shaded room: subtract 10%. A room that rarely sees direct sun starts cooler and stays cooler.
• A kitchen: add 4,000 BTU. Ovens, hobs and other cooking heat are a real extra load, and kitchens are the one room where the standard rule badly understates what you need.
• Extra people: add 600 BTU for each regular occupant beyond the first two. People give off heat, and a living room that usually holds five people is a warmer room than the same space used by two.
• Taller ceilings: for ceilings above 8 feet, scale the figure up roughly in proportion to the extra volume. A 10-foot ceiling means about a quarter more air to cool, so multiply by 10 divided by 8.

These adjustments stack, which is why two rooms of identical size can honestly need different units. If you would like the stacking done for you, enter your room in our free AC BTU calculator and it will apply the same rates and adjustments step by step.

Worked example: a sunny 15 by 20 foot living room

Take a living room that measures 15 by 20 feet, gets strong afternoon sun, and usually has four people in it in the evening. The math runs like this:

• Step 1 — base: 15 × 20 = 300 square feet, and 300 × 20 = 6,000 BTU.
• Step 2 — sun: the room is very sunny, so add 10%. That takes 6,000 to 6,600 BTU.
• Step 3 — people: four regular occupants is two more than the first two the base figure assumes, so add 2 × 600 = 1,200 BTU. The total is now 7,800 BTU.
• Step 4 — what to buy: choose the nearest standard size at or above your figure, which here is an 8,000 BTU unit.

Notice what the example does not do: it does not jump two sizes “just in case.” Rounding 7,800 up to the 8,000 BTU unit is sensible rounding; rounding it up to 12,000 BTU would create the short-cycling problems described below. If you prefer to read a size straight off a table instead, our BTU chart for room size lists the standard size for every room area.

The metric version: square meters and kilowatts

The same rule works in metric. At the base rate it comes to about 215 BTU per square meter, and at the average-insulation rate of 25 BTU per square foot it comes to about 270 BTU per square meter. Metric units are usually sold by kilowatts rather than BTU, and the conversion is simple: kilowatts equal BTU divided by 3,412. So the 8,000 BTU unit from the worked example is about 2.3 kW (8,000 ÷ 3,412). On that basis, a typical 20-square-meter bedroom in the UK lands around 2.0 to 2.5 kW, depending on its insulation and how much sun it gets — a useful sanity check against the sizes on a retailer’s page.

Why oversizing backfires — and undersizing fails

Bigger is not better

An oversized air conditioner cools the air so quickly that the thermostat switches it off after only a few minutes. Those short run cycles are the problem. The unit has barely started removing moisture before it stops, so the room ends up cool but clammy, because the unit cools before it dehumidifies. Frequent starting and stopping also costs more to run than a steady, longer cycle, and it wears the compressor and controls faster. You pay more for the bigger unit, more to run it, and you get a less comfortable room for your money.

Too small fails on the days that matter

An undersized unit has the opposite fault: it runs almost constantly and still loses ground. On a mild day it may cope, which is why the mistake goes unnoticed in spring. On the hottest afternoons of the year — exactly when you need it most — it runs flat out and the room never quite reaches the temperature you set. The fix is not to overshoot wildly, but to aim honestly: apply the adjustments, then buy the nearest standard size at or just above your figure.

For a whole-home central system, stop at this point and get professional help. A Manual J calculation by a professional, working room by room from your home’s construction, windows and climate, replaces all of these rules of thumb, and it is the standard a system that costs thousands should be sized by.

Which units does this sizing apply to?

Window units

The rule applies directly to window air conditioners, which are rated honestly in BTU and cool one room. Measure, adjust, and buy the nearest standard size, making sure the unit physically fits the window and the electrical circuit can support it.

Portable units

Portables need a warning. A portable unit is less effective per rated BTU than a window unit, because its hot exhaust hose sits inside the room and the single-hose design pulls warm air back in from the rest of the home. Many portables now carry a lower DOE or SACC rating alongside the headline number, and that lower figure is the honest one. Buy by the SACC figure, not the bigger number on the front of the box, or the room will feel under-cooled.

Mini-split systems

The same math sizes each indoor head of a ductless mini-split: work out the load for the room that head serves, apply the adjustments, and choose the nearest size. Mini-splits modulate their output better than single-speed window units, so they forgive small sizing errors more gracefully, but a badly oversized head will still short-cycle in a small bedroom.

Whichever type you choose, size first and shop second. Run your own room through our AC BTU sizing calculator, note the adjusted figure, and compare models against that number rather than against the largest rating your budget allows.

Frequently asked questions