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How to approximate the mass of passengers is well explained, but balance of the aircraft is concerned more with where they sit. Other answers have hinted that assigned seating locations are one method for calculating loadsheet balance, but this is not always an option.

Certain passenger airlines (such as Southwest) do not assign seats, allowing passengers to select whatever seat they like during boarding. Since passengers must be accounted for in load balance calculations, how do these airlines know how passenger mass is distributed? Are there strategies besides knowing exact locations to determine whether cargo balance will be within the envelope? Do they simply have flight attendants record where passengers are seated?

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  • $\begingroup$ The FAA (in the US) will allow carriers to use assumed summer and winter weights for each person (170lb and 175lb, if I recall). Smaller carriers, like charter carriers with small planes, generally have to use an "actual weight" -or- "reported weight + 10lbs" policy. I actually carried a scale in some of our smaller charter planes (C210, C310) to weigh passengers. Most passengers just opted to state a weight and let me add 10lbs... $\endgroup$
    – acpilot
    Commented Nov 1, 2017 at 23:35
  • $\begingroup$ Also Related: Is passenger weight distribution important on a commercial flight? and How does a commercial airliner measure its weight mass? $\endgroup$
    – Ron Beyer
    Commented Nov 2, 2017 at 3:12
  • $\begingroup$ @RonBeyer Those didn't ask or answer the question directly which is how do you know where people sit without assigned seats. The question doesn't ask about what the weight is either. $\endgroup$
    – user71659
    Commented Nov 2, 2017 at 3:38
  • $\begingroup$ @user71659 Pretty sure the last sentence of this question asks how the airlines know how passenger weight is distributed... Balance deals with the specific locations of mass to determine CG... It doesn't matter where people sit, what matters is where the CG is (cumulative weight/location) and the total mass. $\endgroup$
    – Ron Beyer
    Commented Nov 2, 2017 at 4:09
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    $\begingroup$ @RonBeyer The related questions were the inspiration for this question. They establish that passenger weight is important to balance, but only briefly address how sentient self-directed mass is addressed in pre-flight calculations. While cumulative CG is all that eventually matters, the location of masses in the aircraft most certainly affects where that cumulative point is. $\endgroup$
    – user9394
    Commented Nov 2, 2017 at 6:28

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The cabin crew simply counts the seated passengers (or alternatively empty seats) by zone (a block of rows), writes it down and hands the slip to the flight crew. It is common to do this on commuter planes even with assigned seating since passenger balance matters more on small aircraft and people may move.

As pointed out by SKYbrary:

In practice, low cost airlines, which routinely offer free seating, also operate the majority of their flights with a high load factor, so that identifying any unoccupied seats will be a fairly simple matter.

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