Then we can find the maximum width shelf that will give an initial sag of 0.02” per foot. And let’s assume a typical 30# per linear foot load. Let’s assume, however, that changing the size of the shelf, and how much load or weight you need it to support, is not an option.įor the examples below to give a sense of how much each design change can help, let’s focus on a 10” deep shelf, made of 3/4” thick plywood shelf. If your design sags too much, there are a few simple solutions, and a few more involved design changes. Select the material, enter a few numbers, and voila, it estimates the total and per foot sag, and even makes a judgement call, summarizing the results as one of Excessive, Borderline, or Acceptable. I’ve come across a few calculators online, but the Sagulator seems to be the best one so far for ease of use, flexibility, and options that allow for common solutions that woodworkers use to counteract sagging shelves. But don’t worry - these are online calculations that hide the messy math, at least to start out with! Of course, if you’re loading your shelves like that coffee table above, all bets are off! The Sagulatorīut - what if the design calls for something outside these “small” shelf guidelines? Higher weight capacity, or longer or narrower shelves? Then, it’s time to break out a calculator. A 6” deep adjustable shelf will probably not get loaded more than about 20# per foot - made of any of those materials, a 30” span would still be fine. But fortunately, as you get narrower, it’s less likely that you’ll get to 35# per foot. Narrower shelves aren’t as strong, however. Deeper or shorter shelves? Or anticipating less weight? Sag definitely won’t be a problem for those small adjustable shelves. Make it of poplar - the stiffest of those three materials - and you can make a narrower adjustable shelf, down to about 8”, still without any problems. And a typical shelf load to plan for is 20-40# per foot - libraries often plan for 35#.Ī 30” wide adjustable shelf that is 10” deep, made of any of these three (plywood, poplar, pine) materials, will have an acceptable sag for a load of up to 35# per foot, or about 90# for the entire shelf.

What’s “small”? The most frequently used quality materials for custom-built wood shelves are 3/4” thick plywood, poplar, or pine definitely avoid the melamine-laminated composite shelving and MDF. However, for “small” shelves, sag is not really a problem and one can just ignore considerations of sag for typical loads. But with a bit more planning and design consideration (and better materials), sagging can be wholly avoided.

This is a consequence of many factors in that $50 hand-me-down bookcase. Unfortunately, however, adjustable shelves are more prone to sagging than fixed shelves, as anyone who has loaded a cheap Ikea bookshelf with all of their college textbooks can attest. Many cabinet designs call for the flexibility of adjustable shelves. So a good target for initial allowable sag is 0.02″ per foot or less. And over many years, the sag of a shelf tends to increase by as much as 50% as it is stressed repeatedly in the same direction. You will probably notice deflection far before it gets to the sorry state of that coffee table! 1/32″ (0.03″) per running foot, or 3/32″ (0.09″) for a 3′ wide bookshelf or table, is typically visible.

Bookshelf 2 ft wide