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We built Montessori curvy plywood balance boards. They are 36" x 11" deep by 3/4" thick. They are made of 3 layers of maple veneer plywood. I'm unsure of the exact classification of the plywood as I purchased it a few months ago.

These balance boards are glued together and bent over a form. We have tested them with adults around 180lbs with no issue. But parents would like to know the weight capacity before purchasing and I'm unsure of how to get an accurate estimation.

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    I think the only way you're going to be able to determine the load capacity will be to destructively test one and then multiply by an appropriate safety factor. There are just too many variables involved in manufacturing plywood to get any kind of accurate estimate.
    – SaSSafraS1232
    Nov 24, 2020 at 22:10
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    Hi, welcome to StackExchange. That's a very nice looking finished piece! But I absolutely concur with the above, I doubt there's any way to even estimate this in the abstract, because so much depends on the quality of the plywood. Additionally, if you found any voids while cutting or machining the ply I think you should also take it that it may vary a surprising amount piece to piece.
    – Graphus
    Nov 25, 2020 at 6:35
  • I also concur that destructive testing is probably the best way to go. However, be aware that high quality (as you're likely to get with a maple veneer) 3/4" plywood is going to be very strong. I'd imagine that an adult could stand in the center of that for quite some time without witnessing any significant deflection. As an example, we've got some cabinets in our kitchen that someone made 40+ years ago. The paint we applied is wearing off, but the cabinets themselves show no signs of wear despite decades of pots/pans/plates/glasses/bowls/etc.
    – FreeMan
    Nov 25, 2020 at 13:56
  • I want to concur, can I concur. Please, Please let me concur. It should be noted that I did not concur in concurrence with the previous concur-ing. I came to the party late as I am not always up on current events. Perhaps I need a balance board because I seem to be imbalanced.
    – Alaska Man
    Nov 25, 2020 at 18:58

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You will be making these in batches of some sort, even if it is an evening of 2-3 items. Semi-regularly you pick one at random and test it using a test platform of some kind. Ideally you want to know the safe deflection mass, the mass under which it deforms and stays deformed, and the complete failure mass.

Keep track of this and then you can see how your small changes in material and manufacturing affect the overall safety and strength.

Your major worry is if there is a sudden failure mode that results in injury under slightly beyond "normal" circumstances. The only way to be sure is to destructively test each batch at random.

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