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The Sagulator

Calculates Shelf Sag

The Sagulator helps you design shelves by calculating shelf sag (deflection) given type of shelf material, shelf load, length, width, and thickness. See the notes below for usage tips.

1. Species/material used for shelf
Imported Woods
Sheet stock

2. Shelf Load  - see note 3 below
    Weight lbs    kg
Load Distribution Uniform load  Center load

3. Dimensions - specify as decimals or fractions
Width in    cm

(same units as inputs)


1. The eye will notice a deflection of 1/32 inch per running foot, or 3/32" (0.09") for a 3' wide bookshelf.

2. Once you have a maximum allowable sag figure in mind, you can design your shelf by tweaking the material types and dimensions. Many bookshelves have a depth of 8" to 12", a width of 24" to 36", and a thickness of 3/4" to 1".

3. A fully loaded bookshelf weighs 20-25 pounds (9-11 kg) per running foot, or 60-75 pounds (27-34 kg) for a 3' wide shelf. 

4. You can enter fractional dimensions as decimals (8.75), or as conventional fractions (8 3/4). If you use fractions, just make sure to leave a space between any leading whole number and the fraction.

5. This calculator can also be used to measure beam deflection. You can specify either center or uniform loading along the length of the beam.

6. The deflection calculations are based on average wood stiffness properties measured in controlled laboratory conditions (U.S. Forest Products Lab). Your mileage may vary.



Thanks to Harry A. "Steve" Stephenson, Autocad Operator, with H & H Associates, Inc. in Nashville, TN for providing advice and formulas. Another very helpful resource was Bruce Hoadleys book - Understanding Wood: A Craftsman's Guide to Wood Technology.


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