I need to make a 7.5' circle 12 inches wide (OD 7.5', ID 6.5'). Since there is no lumber sold in 8'x8' pieces, and I don't have room to set up 2 4x8s side by side, I thought another approach might be to buy 12" wide boards and cut them in trapezoidal shapes of a certain length, and join them side to side to form a Dodecagon that is 12" wide.

I'm unsure of what angle to use for the sides of each of the 12 pieces, as well as how long each one should be on the long side and the short side.

Can anyone help?

  • 2
    Lol I wondered why on earth you would need that. Then, I saw your profile pic. Feb 5, 2019 at 17:35
  • 1
    Yes, it going to be part of a basement size model railroad. ;0)
    – Dan Brewer
    Feb 5, 2019 at 17:36

1 Answer 1


Here is a diagram of what you need. Cut each piece exactly the same with a 75 degree angle on the ends. If they are all the same it will go together alright even if you do not make the outside length of each piece be 23.2937.

If you use planed 1x12 pine boards they will be more like 11.25 inches wide.

Dimensions are in inches.

enter image description here

  • 1
    At first 75 seemed like way too much. Then I realized really what you get is each of the intersections is (90 - 75) = 15 x 2 = 30, and 30 x 12 = 360 = full circle. So this should work. Feb 5, 2019 at 18:00
  • 1
    I'd be willing to be that dimensions are in any unit. :)
    – isherwood
    Feb 5, 2019 at 18:32
  • Thanks Michael! I think I will use ripped plywood at 12" wide, and this should work great!
    – Dan Brewer
    Feb 5, 2019 at 20:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.