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Instead of providing the maximum capacity of a particular model, airliner manufacturers often provide official typical capacities, like 280-366 seats for A350 series and 350/400seats for Boeing 777x series.

The question is: How do they generate this number?

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These numbers are based on mixed-class configurations and use of floor space for other things. The A380 for instance can theoretically be configured with 853 "economy" seats, but no-one I know of has bought one configured that way. Typical first-business-economy configuration is 525, and Emirates seats just 489 because its first-class section is basically staterooms, and it has showers and a bar taking up floor space (it also has an excellent and large business class instead of using the back half of both decks for coach).

When designing an aircraft's seating capacity, especially for long-haul flights, total cabin weight (passengers and cargo) must be taken into account. The more stuff you're hauling, the less weight allowance you have for fuel, and therefore your range is reduced. Therefore, increased room per passenger on long-haul flights isn't just for passenger comfort, it's often a necessity to reduce total cabin weight and thus be able to max out the fuel load to get between the airports non-stop. Operators of shorter flights don't care as much because you can pack them in like sardines and still have plenty of fuel to get them where they're going.

Therefore, dimensions of various classes of seat differ by airline, aircraft and route (Lufthansa, for instance, operates three different A380 configurations based on the routes they run). The typical coach seat is between 17 and 19 inches wide (including half the armrest), and has a "pitch" (space between seats which determines the legroom and ability to recline) of between 31 and 35 inches.

For business class in three-class planes it gets a bit better; seat width averages about 21" and a pitch of about 38" (more for planes outfitted with lay-flat seats). First class ranges widely by aircraft type and airline (it's truly "first-class" in aircraft with three-class configuration; in two-class what's called "first" is closer to business class), but typically offers room to spare especially on long-hauls.

Coupled with required/desired aisle widths and additional amenities that widen the seats or replace them in certain areas of the cabin, it's fairly academic to count up how many seats you can fit aboard a particular airframe.

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  • $\begingroup$ I don't think most airliners have their passenger capacities limited by weight. As I commented on another answer, the Boeing 777-300ER has a maximum payload of about 400,000 pounds or more, assuming fuel for a long flight. That's the weight of roughly 2,000 people - but nobody would say the plane has a capacity of 2,000 passengers, or anywhere near that number. The evacuation time mentioned by user6396 has a major effect on how tightly the seats can be packed. $\endgroup$ – Nate Eldredge Aug 1 '15 at 16:08
  • $\begingroup$ Fair point, it's a bigger deal for the jumbos. The A380 can carry over 800 passengers, but doing so with full fuel as well will exceed MTOW, and as the economics of a jumbo typically only make sense for long-haul, nobody's bothered ordering an all-economy A380. $\endgroup$ – KeithS Aug 1 '15 at 16:13
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The FAA average passenger weight (adult male) is 200 LBS in the summer and 205LBS in the winter, you can extrapolate seats from that number. It is assumed that there is one passenger to a seat. You would also need to assume that the worst case is a plane full of adult winter males (205LBS) plus luggage. The document linked goes over how to calculate average baggage weights and what some rough percentages are on that.

It should be noted that people who are above standard size may be required to buy 2 seats. This is generally handled on a case by case basis it seams.

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    $\begingroup$ It seems unlikely that these capacities are produced purely based on weight. For instance the Boeing 777-300ER has a maximum payload of roughly 400,000 pounds for a long flight - but it would be absurd to say it has a capacity of 2,000 passengers. $\endgroup$ – Nate Eldredge Aug 1 '15 at 16:05

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