How thick should plywood shelves be?
The short answer: it depends on how long the shelf is and how much it holds. Doubling the thickness makes a shelf about eight times stiffer, while doubling the length makes it much more flexible. Type your numbers into the shelf sag calculator to see the sag and the thickness you need.
Why shelves sag
A shelf is a beam. It bends under its load, and wood keeps bending slowly over the years under a constant weight. That slow extra sag is called creep, and it is bigger in MDF and particleboard than in plywood or solid wood. A shelf that looks fine when you load it can be visibly bowed a few years later.
The four things that matter
- Thickness: the strongest lever. Stiffness grows with the cube of the thickness.
- Span: the distance between supports. Stiffness falls quickly as it grows.
- Depth: a deeper shelf is stiffer, but it also holds more load.
- Material: plywood and solid wood are stiffer and creep less than MDF or particleboard.
A simple limit
A common rule is that a shelf should not sag more than 1/300 of its length over time. On a 36" shelf that is about 1/8". The calculator uses that limit and also shows the longest shelf and the thinnest board that stay within it for your load.
How to stop a long shelf from sagging
- Shorten the span with a middle support, a divider or a vertical.
- Use thicker board.
- Add a front stiffener: a strip of wood glued under or on the front edge.
- Switch to a stiffer material.
For the plans on this site
The bookcases and shelving use shelves that cross the sides at half-laps. Their length is up to you, so check the span with the calculator, and add a division if a shelf is long and loaded.
Frequently asked questions
How long can a 3/4 inch plywood shelf be?
It depends on the load, the depth and how the shelf is held. For a shelf of books, about 30 inches between supports is a common limit. The calculator gives you the number for your case.
Does MDF sag more than plywood?
Yes, MDF and particleboard are less stiff and creep more over time than plywood, so the same shelf sags more.
How do I stop a long shelf from sagging?
Shorten the span, use thicker board, or add a middle support or a front stiffener.