What Size Air Conditioner Do I Need? Buying Guide

Buying an air conditioner sounds simple until you're standing in the aisle staring at a wall of boxes, each shouting a different BTU number, and none of them telling you which one is right for your room. Buy too small and it runs all day without ever catching up. Buy too big and it blasts cold air, snaps off, and leaves the room cold, damp, and weirdly clammy. Getting the size right is the whole game.

The good news is you don't need an HVAC certification to nail it. A little arithmetic and a tape measure will get you within a few hundred BTUs of the correct unit, and that's close enough. This guide walks you through the sizing math in plain English, then covers the specs worth caring about, how to pick between the main types, and where to actually put the thing so it earns its keep.

How to size it: the BTU rule, plainly

BTUs (British Thermal Units) measure how much heat the unit can pull out of a room per hour. More BTUs means more cooling power. The trick is matching that power to your space, and the honest truth is that bigger is not better here.

Start with the room's square footage and multiply by about 20 BTU per square foot. That's your baseline. Then adjust for the things that make a room harder or easier to cool: add 10% if the room bakes in direct afternoon sun, or subtract 10% if it's well shaded. Add 600 BTU for each person beyond two who's regularly in the room, since bodies throw off heat. And if you're cooling a kitchen, add a flat 4,000 BTU because the stove and oven are basically small furnaces.

Here's a worked example. Say you have a 300-square-foot sunny living room where three people usually hang out. Baseline: 300 x 20 = 6,000 BTU. It's very sunny, so add 10%: 6,000 x 1.10 = 6,600. One person over two: add 600, giving 7,200 BTU. You'd shop for a unit rated around 7,000 to 8,000 BTU and not lose a minute of sleep over the gap. Round to the nearest common size and you're done. Whatever you do, resist the urge to "round way up" for good measure. An oversized unit cools the air so fast that it hits the target temperature and shuts off before it has run long enough to wring the humidity out. It short-cycles on and off, and the room ends up cold but clammy, which is exactly the miserable feeling you were trying to escape.

The specs that actually matter beyond size

Once size is settled, efficiency is the next number to check, because it's the one you pay for every month. Room units are rated by CEER (Combined Energy Efficiency Ratio) and central and mini-split systems by SEER2, the updated federal test standard that's been in force since 2023. Higher is better on both. A more efficient unit costs a bit more up front and quietly hands the money back on your power bill every summer, so if you'll run it a lot, spend up.

Noise is the spec buyers underrate and then regret. A bedroom AC that reads great on paper but roars at 55-plus decibels will have you turning it off at night. Look for a quoted decibel rating and favor units with a genuine low or "sleep" fan speed. Inverter compressors, which ramp up and down instead of slamming fully on and off, run noticeably quieter and steadier than old-school single-speed units, and they dodge the short-cycling problem entirely.

After that, it's about the features you'll actually use: a real thermostat rather than just "low/medium/high," a washable filter you'll clean instead of dread, a solid remote or app control, and a dehumidify mode for muggy days. Skip paying a premium for gimmicks you'll never touch.

Choosing between the main types

Window units are the default for a reason. They're the cheapest way to cool a single room well, they install yourself in an afternoon, and BTU for BTU they're more efficient than portables. The catch is they eat a window and stick out the side of your house, and they don't suit casement or sliding windows without a special kit.

Portable units are the compromise you make when a window unit won't work, either because the window is the wrong shape, the rental rules forbid it, or you want to wheel cooling from room to room. Be clear-eyed about the tradeoff: they're louder, less efficient, and take up floor space. Single-hose models especially fight themselves by exhausting cooled air out the window, which sucks warm air back in through every gap. If you go portable, a dual-hose model is worth seeking out.

Mini-splits are the long-game play. They're the quietest and by a wide margin the most efficient, thanks to inverter compressors that sip power and hold a rock-steady temperature. They cost the most and need professional installation, so they only make sense if you own the place and plan to cool a room, or a whole home, for years. If this is a rental or a stopgap, a window unit is the smarter money.

Placement, setup, and squeezing out efficiency

A correctly sized unit in a bad spot still disappoints. Mount or place it on the shady side of the room if you can, and keep the outdoor half of the unit (or a portable's exhaust hose) clear of hot air pooling against a wall. For window units, seal the gaps around the frame with the included foam and a bit of weatherstripping; the cold air you paid for shouldn't be leaking straight back outside.

Aim the airflow across the length of the room, not into a wall or a curtain three feet away. Cold air needs a clear runway to circulate. Keep the unit away from lamps, TVs, and other heat sources that sit near its thermostat, or it'll misread the room as warmer than it is and overwork.

The rest is cheap upkeep with a real payoff. Clean or replace the filter every few weeks during heavy use, since a clogged filter chokes airflow and drags efficiency down fast. Pair the AC with a ceiling or box fan and you can set the thermostat a couple degrees higher for the same comfort, because moving air feels cooler. And use the timer or a smart plug so you're not cooling an empty room all afternoon, which is the easiest money you'll ever save.