Can you give me the load limit on a 4x6 pine board 18' long used for a swing set? Thank you

  • 1
    Hi, Joan, and welcome to Stack Exchange. Are you asking about the weight of the board, or the load capacity of the board? Oct 23, 2017 at 16:04
  • Also, will the beam span 18 feet, or is it like a typical swingset where part of it cantilevers?
    – isherwood
    Oct 23, 2017 at 16:05
  • Is it supported at the ends (18 foot span) or somewhere else? i.e. legs 12 feet apart with 3 feet sticking out at the ends, or 3 sets of legs 9 feet apart, or two sets of legs 18 feet apart? Beam calculators are not too hard to find, but for a swing set you might want to increase the safety factor (amount by which you exceed the expected load when calculating.) Grade of the lumber also matters.
    – Ecnerwal
    Oct 23, 2017 at 16:07
  • ...I'm betting there's a 90+ percent chance OP is asking how much weight can hang from the 4"x6"x18' pine beam. +1 so she can upload a helpful drawing or picture or something.
    – elrobis
    Oct 23, 2017 at 16:53
  • The beam is 18' long supported on each end, no support in middle. Not sure of the grade of lumber. I just need to know worst case how much weight can the swings support. Sorry I don't have a picture or more info. Thanks for any help.
    – Joan
    Oct 24, 2017 at 15:05

2 Answers 2


Assuming it’s “Lodgepole Pine” (Idaho Pine and Ponderosa Pine is slightly less) and it’s grade is a No. 2 and better (no loose or missing knotholes), then a 4x6 spanning 18’ will support about 105 lbs. loaded at the mid-span (slightly higher if it’s loaded at third points or further from the center point).

  • Is there any resource that lists maximal admissible workload of various beam/planks? In case I want to check a similar-yet different configuration.
    – ebosi
    Aug 21, 2018 at 15:54
  • 2
    @ebo I’ve seen structural tables on the internet. Maybe Google “Structural beam tables”.
    – Lee Sam
    Aug 21, 2018 at 16:21
  • as a non-native speaker, thanks for the keyword!
    – ebosi
    Aug 21, 2018 at 16:29

Better idea instead of the 4x6 use 2 2x8 boards bolted of nailed together. Ive done this on a 18 foot span and it holds 3 large adults (over 200 lbs each) on swings without any flex or bowing. Outdoors i would use a good grade of treated lumber to prevent decay from the elements and longer life.

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