An air conditioner that’s too big for your room doesn’t cool it faster — it actually leaves the space colder on the thermostat and clammier in the air, because it blasts down to temperature and shuts off before it’s run long enough to pull the humidity out. Getting the BTU number right is the single biggest factor in whether a new unit leaves you comfortably cool, or cold, damp, and short-cycling itself into an early replacement.
The Quick Verdict
- The industry-standard chart is based on square footage and an 8-foot ceiling — that’s your starting point, not your final answer
- Four adjustments (sun exposure, occupancy, ceiling height, kitchen use) can shift the real number by 1,000–2,000 BTU in either direction
- Bigger is not safer: an oversized unit cools the air fast but doesn’t run long enough to remove humidity, leaving the room feeling cold and damp instead of just cool
- If your calculated number falls between two available unit sizes, round up to the next size — never down — but stop at the first step up
- Central air conditioning is sized in tons (1 ton = 12,000 BTU) using a room-by-room Manual J load calculation, not the square-footage chart below — square footage alone is a rough starting point for a contractor’s quote, not the final number
The Official BTU Sizing Chart
This chart, published by ENERGY STAR and the U.S. Department of Energy, is the baseline every room air conditioner (window, portable, and single-zone mini-split) sizing recommendation traces back to. It assumes a standard 8-foot ceiling and average sun exposure — the adjustments below are what turn this into your actual number.
| Area to Be Cooled (sq ft) | BTUs Needed |
|---|---|
| 100–150 | 5,000 |
| 150–250 | 6,000 |
| 250–300 | 7,000 |
| 300–350 | 8,000 |
| 350–400 | 9,000 |
| 400–450 | 10,000 |
| 450–550 | 12,000 |
| 550–700 | 14,000 |
| 700–1,000 | 18,000 |
| 1,000–1,200 | 21,000 |
| 1,200–1,400 | 23,000 |
| 1,400–1,500 | 24,000 |
Past roughly 1,500 square feet, a single room unit stops being practical — you’re into multi-zone mini-split or central air territory, which is a different calculation entirely (more on that below).
Square Footage Alone Isn’t Enough: 4 Adjustments
- Heavily shaded room (little direct sun): subtract 10% from the chart number
- Very sunny room (south- or west-facing windows): add 10%
- More than two people regularly in the room: add 600 BTU for each additional person
- Used as a kitchen: add 4,000 BTU — cooking appliances add a real heat load the chart doesn’t account for
Do the Math: A Worked Example
Say you’re sizing a 320-square-foot family room with south-facing windows that gets strong afternoon sun, and it’s regularly used by four people, not just the two the chart assumes.
- Start with the chart. 320 sq ft falls in the 300–350 range → 8,000 BTU.
- Adjust for sun. It’s a sunny room → add 10%: 8,000 × 1.10 = 8,800 BTU.
- Adjust for occupancy. Four regular occupants is two more than the chart’s baseline of two → add 600 BTU × 2 = 1,200 BTU: 8,800 + 1,200 = 10,000 BTU.
That family room needs a 10,000 BTU unit — two full sizes above what the square footage alone suggested. Skipping the adjustments here would have meant buying an 8,000 BTU unit that runs constantly on hot afternoons and never quite catches up.
What Oversizing Actually Costs You
An air conditioner cools in two ways at once: it lowers the temperature, and it pulls moisture out of the air as it runs. A unit sized correctly for the room runs in longer, steady cycles that do both jobs. An oversized unit blasts the room down to the thermostat’s set temperature in a few minutes, then shuts off — long before it’s run enough of a cycle to dehumidify. The room hits the number on the thermostat but still feels muggy, and the compressor wears out faster from all the extra starts and stops.
| BTU Size | Typical Room | Est. Annual Energy Use* | Approx. Annual Cost** |
|---|---|---|---|
| 6,000 | Small bedroom/office | ~330 kWh | ~$59 |
| 8,000 | Large bedroom | ~390 kWh | ~$70 |
| 10,000 | Small living room | ~500 kWh | ~$90 |
| 12,000 | Medium living room | ~600 kWh | ~$108 |
| 14,000 | Large room/small apt | ~700 kWh | ~$126 |
| 18,000–24,000 | Whole small apartment | ~1,215–1,350 kWh | ~$219–$243 |
*Based on ENERGY STAR-certified model averages at each size. **At the 2026 U.S. average residential rate of about 18¢/kWh — rates run from around 11¢ in North Dakota to over 40¢ in Hawaii, so check where your state falls with our appliance cost by state breakdown before assuming the national number applies to you. For your exact unit and usage hours, the Appliance Running Cost Calculator plugs in your own rate instead of relying on averages.
Central Air Is a Completely Different Calculation
Everything above applies to a single room: a window unit, a portable unit, or one zone of a mini-split. Whole-house central air is sized in tons — 1 ton equals 12,000 BTU — using a room-by-room Manual J load calculation that factors in insulation quality, window count and orientation, duct layout, and your climate zone, not just total square footage. A contractor’s rule of thumb like “one ton per 600 square feet” is a starting point for a conversation, not a substitute for the calculation — it can be off by half a ton or more in an older, poorly insulated house or a hot, humid climate. If a quote is based on square footage alone with no Manual J worksheet attached, it’s worth getting a second opinion before committing to a system that’ll be just as prone to short-cycling as an oversized window unit, at a much higher price to fix.
Check Your Exact Number
The chart and the adjustments above get you close by hand, but the AC BTU Calculator — free at thehomepilots.com/tools — runs all four adjustments for you and gives you one final number instead of doing the arithmetic yourself. If you’re weighing a window unit against upgrading a central system’s controls first, our Climate & Energy buying guide lays out which upgrade to tackle first, and the Smart Thermostat Savings Calculator shows what better scheduling is worth on top of correct sizing. Once you have your number, HomePilot Picks rounds up the window and portable units we’ve actually tested at the common sizes, so you’re not guessing on brand and features too. The same adjust-the-baseline-number approach works for other appliances too — see our Water Heater Size Calculator, or check whether induction actually saves you money in your state.
The One Rule That Beats the Chart
If the size you’re eyeing in the store is more than one step above your calculated number, that’s the oversizing mistake happening in real time — a bigger unit at checkout always feels like the safer bet, but it’s the one that leaves you cold and clammy instead of just cool. Trust the math, not the sticker.
