Air Conditioner Sizing: The BTU Chart, With Its Assumptions Showing
Air conditioner sizing has a failure mode in each direction, and they're not symmetric. Undersize and the unit runs flat-out on hot days and falls a few degrees short — annoying, obvious, fixable. Oversize and the unit blasts the room to temperature in ten minutes and shuts off before it's removed any meaningful humidity, leaving you 68°F and clammy in a room that feels like a cave. The second failure is the common one, because 'bigger to be safe' feels prudent right up until August. Here's the honest math.
The chart everyone starts with
| Unit size | Typical space | Honest caveat |
|---|---|---|
| 5,000 BTU | 100-150 sq ft | Small bedroom, home office; the $150 workhorse class |
| 6,000 BTU | 150-250 sq ft | Standard bedroom size |
| 8,000 BTU | 250-350 sq ft | The most-sold size — large bedroom, small living room |
| 10,000 BTU | 350-450 sq ft | Living rooms, studios |
| 12,000 BTU | 450-550 sq ft | Open living areas, small apartments |
| 14,000-15,000 BTU | 550-700 sq ft | Large open plans; check your circuit before buying |
| 18,000-24,000 BTU | 700-1,500 sq ft | Whole-floor territory — usually 230V; consider a mini-split instead |
The rule behind the chart is roughly 20 BTU per square foot, and it assumes 8-foot ceilings, decent insulation, moderate sun, and one or two occupants. Portable AC buyers: this chart is in SACC (the DOE's honest delivered-cooling number), not the ASHRAE sticker BTU. A portable sold as '14,000 BTU' typically delivers 8,000-9,500 BTU SACC and belongs in the 8,000 BTU row — sizing a portable off its ASHRAE number is the single most common way people end up with a unit that 'doesn't work'. Our SACC guide covers the difference in full.
Why oversizing backfires: the humidity half of cooling
Comfort is temperature plus humidity, and an air conditioner handles humidity as a side effect of runtime: moisture condenses on the cold evaporator coil only while the compressor runs. A correctly sized unit runs long cycles — twenty, thirty minutes — and wrings real water out of the air as it goes. An oversized unit satisfies the thermostat in eight minutes and shuts down with the room's moisture mostly still in the room. The result is the signature complaint of every oversized AC: the thermostat says 68°F, the room feels like a damp basement, and the owner responds by setting it colder, which makes it clammier. In muggy climates this isn't a nuance — an 8,000 BTU unit that runs steadily will out-comfort a 12,000 BTU unit that short-cycles, every single day of a Gulf Coast summer.
Short-cycling also wears the machine (compressor starts are the hard part of its life) and wastes electricity (a compressor's least efficient seconds are its first ones). Fixed-speed units — which is nearly every traditional window AC and budget portable — have exactly one output and no way to adapt, so the sizing decision is the comfort decision. When your space falls between chart rows on a fixed-speed unit, take the smaller size. The hot-day worst case is 'runs continuously and holds most of the setpoint', which beats 'cold and clammy' comfortably.
Inverter units change this calculus, and it's their quietly best feature: a variable-speed compressor can throttle down to 30-40% of rated output and just cruise, so a modestly oversized inverter unit still runs the long, humidity-controlling cycles that a fixed-speed unit can't. If you want capacity headroom — for a heat wave, a party, a future room — buy it in an inverter unit (the Midea U, LG's Dual Inverter line, Frigidaire's Gallery inverters), where the headroom is nearly free. On a fixed-speed unit, headroom is the thing you'll complain about in July.
The adjustments that actually matter
Sun is the big one: a room with large south- or west-facing glass gains heat all afternoon, and the standard guidance — add 10% for a sunny room, subtract 10% for a fully shaded one — is if anything conservative for a west wall in Phoenix. Kitchens add a steady 4,000 BTU while cooking, which is why the classic advice for a kitchen-adjacent living area is one size up. Each regular occupant beyond two adds about 600 BTU — people are 100-watt heaters — which matters for a home office with three monitors and a desktop tower more than the chart admits.
Ceilings and construction: BTUs cool air volume, not floor area, so a 10-foot ceiling means multiplying the square footage by 1.25 before consulting the chart. Top-floor rooms under an uninsulated roof run 10-20% hotter than the same room a floor down. And insulation cuts both ways — a tight 2010s build can genuinely use the row below its square footage, while a 1920s walk-up with original single-pane windows needs the row above and will still run long cycles on the hottest days. If your room has two or more of these penalties stacked, that's when the bigger size is legitimate; one penalty alone usually isn't.
The electrical fine print
Through 12,000 BTU, window and portable units run on an ordinary 115V/15A outlet — but they want it mostly to themselves. An 8,000-12,000 BTU unit draws 6-11 amps, and sharing a 15-amp bedroom circuit with a space heater, a hair dryer, or a microwave is how breakers learn to trip. At 14,000-15,000 BTU you're at the edge of what a shared 115V circuit tolerates, and most 18,000 BTU-and-up window units require a dedicated 230V circuit — the plug on the cord will be a shape your wall doesn't have, and that's the product telling you it needs an electrician. Check the plug type in the listing photos before you buy a big unit; 'I didn't know it needed 230V' is a weekly return-desk story.
Extension cords deserve their own warning: manufacturers prohibit them, and the prohibition is real — an undersized cord under a 10-amp continuous load gets hot enough to matter. If the cord doesn't reach, the answer is a different outlet or an appliance-rated 12-gauge cord as a last resort, never the household cord that came free with a lamp.
Units mentioned in this guide
Frigidaire
Frigidaire FFRA051WAE 5,000 BTU
This is the default answer whenever someone on r/Frugal asks 'cheapest way to cool a bedroom?' — and it's been the answer for years because it just works. Two mechanical dials, no remote, no Wi-Fi, no circuit board to fry: you twist the thermostat to a number between 1 and 7 and it cools a 150 sq ft room hard. That simplicity is also why these routinely outlive fancier units; there's almost nothing to break, and the compressor is the same basic hardware Frigidaire has been shipping for a decade. The honest downsides are exactly what you'd expect at this price. It's loud for its size — a droning hum plus a distinct compressor click when the thermostat cycles, which light sleepers either learn to love as white noise or can't tolerate at all. The thermostat is imprecise, so rooms can overshoot cold overnight, and there's no timer to prevent it. The included side curtains and mounting hardware are notoriously chintzy — reviewers universally recommend adding foam sealing. Compared to the GE AHTC05AA it's a coin flip on specs; the Frigidaire tends to be a few dollars cheaper and slightly easier to find. If you need a set-and-forget cooler for a small bedroom, dorm, or camper, this is the one to buy. If noise matters, spend up for an inverter unit.
The 12K dual inverter is the sweet spot of LG's window lineup and arguably the best large-room window AC you can buy on specs: 550 sq ft of coverage, CEER 15.0, and the same 44 dB near-silence as its smaller sibling, running on a standard 115V outlet. That last part matters — most competing 12K units with this efficiency either don't exist or push you toward 230V hardware. Owner sentiment follows the LG inverter pattern: people are stunned by how quiet a foot-and-a-half of living-room glass can get, electric bills drop noticeably versus the fixed-speed unit it replaced, and the variable-speed compressor's long, low-power runs dehumidify better than cycling units. The failure threads are worth knowing: the most common complaints are ThinQ Wi-Fi pairing frustration, occasional refrigerant whoosh/gurgle sounds, and — as with all LG inverters — a small but vocal group with resonance hums their walls amplify. At 83 pounds it's a genuinely awkward install, and at ~$559 it's a big check for a window unit; the fixed-speed Midea/GE 12Ks run $150 less. But if this is cooling your main living space eight hours a day, the efficiency math and the silence make it the easy recommendation over any conventional 12K.
The middle child of the Midea U line is the pick for larger bedrooms and small living rooms — 10,000 BTU rated for about 450 sq ft, with the same variable-speed inverter and CEER 15 efficiency as its siblings. Owner sentiment tracks the 8K almost exactly: people buy it for the quiet and stay for the power bill, with the compressor loping along at low speed instead of slamming on and off. Reddit threads consistently note it holds temperature better than the LG dual-inverter units it competes with, though LG's app is better. The same caveats apply, and they're not small: this platform was part of Midea's June 2025 recall (1.7M units, slow basepan drainage, documented mold cases), so a used or old-stock unit needs the free drain-plug fix, while current 2026 stock ships with the revised design. You still want to pull and rinse the basepan yearly. Install friction is the other recurring complaint — the bracket, the foam panels, the weight. A few owners also report the display and Wi-Fi module failing before the compressor ever hiccups. If you're between sizes, most people who went 10K over 8K for a 300-350 sq ft room say the unit just runs at an even lower, quieter speed — a legitimate strategy with an inverter.