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Coming from the Sagulator?
WoodBin's Sagulator works out how far a shelf sags, and its Shrinkulator how much a board shrinks or swells. If you have used them, this page shows where each of their inputs lives on Woodworking Math, what works differently, and what isn't here yet. Every statement about WoodBin's tools comes from their own pages.
Woodworking Math has no connection with WoodBin. The Sagulator and Shrinkulator are WoodBin's calculators, named here only so you can find your way around.
What you get here
- The sag at first and after years, both against the sag people notice, with the source of each figure.
- The source of every stiffness: the 2021 Wood Handbook for wood, makers' own data sheets for panels, named on the result.
- A "Why these numbers?" panel that walks through the beam equation with your numbers.
- Your inputs kept in the page address and your browser, so a reload or a shared link opens with the same shelf.
- Any browser, phone included, and the calculators work offline after the first visit.
What isn't here yet
- A center (point) load: the load is spread along the shelf.
- A shelf fixed to its sides: the shelf rests on its supports.
- OSB, waferboard, melamine and glass shelves.
- A total-load entry: the load is per foot or per meter.
- An adjustable fiber saturation point for wood movement.
The Sagulator, input by input
The Sagulator's inputs and behavior as its page describes them, and where each one is here.
| In WoodBin's tool | Here | Status | What to know |
|---|---|---|---|
| Shelf material: wood species, plywood, MDF, particleboard grades, OSB, waferboard, melamine and glass 1 | Partly | Every wood species with a 12% stiffness in the Wood Handbook, plus MDF, particleboard (grade M-2), multi-ply plywood and one maker’s veneer-core plywood. No OSB, waferboard, melamine or glass yet: no makers’ data sheets have been read for them. | |
| Span, depth and thickness, in inches, centimeters or millimeters 1 | Here | Inches (shop fractions like 3/4 or 1 1/16 work) or millimeters, following the site-wide inch/mm choice; switching converts what you typed. | |
| Load per foot or in total, in pounds or kilograms 1 | Partly | Load per foot (lb/ft) or per meter (kg/m) only, with a books preset of 30.5 lb/ft from two published weighings. No total-load entry yet. | |
| Uniform load or center load 1 | Partly | Uniform load only: the load spread along the shelf. No center (point) load yet. | |
| Shelf fixed (attached to sides) or floating (not attached) 1 | Nothing here | Not here | The shelf is treated as resting on its supports, the floating case. There is no fixed-ends option yet. |
| Optional edging strip: material, width and thickness 1 | Here | A solid-wood strip of any species glued to the front edge, flush with the top, with its height and thickness; it may hang below the shelf. | |
| Computes initial sag only; notes wood sags about 50% more over time; target 0.02 in per foot or less 1 | Here, differently | Shows the sag when first loaded and after years under load (twice the first, from the Wood Handbook’s creep section, p. 5-39), each against about 1/32 in per foot, the sag two published sources say people notice. | |
| Stiffness: average clear, straight-grained wood, mainly from the U.S. Forest Products Laboratory 1 | Here, differently | Wood from the 2021 Wood Handbook (Tables 5-3b and 5-5b). Panels from makers’ own data sheets: the median of three makers for MDF, of four for particleboard. The source is named on the result for whatever you pick. |
The Shrinkulator counterpart
WoodBin's Shrinkulator estimates how much a board shrinks or swells. Its inputs as its page describes them, and where each one is here.
| In WoodBin's tool | Here | Status | What to know |
|---|---|---|---|
| Species shrinkage from the Wood Handbook (1999 edition, FPL-GTR-113) 2 | Here, differently | The 2021 edition (FPL-GTR-282): its Table 13-5 movement coefficients between 6% and 14% moisture content, and its shrinkage equation outside that range or for species the table lacks. | |
| Initial and final moisture content, or relative humidity at an assumed 70 °F 2 | Here | Moisture content, or temperature and relative humidity worked to equilibrium moisture content by the Handbook’s equation (30–130 °F). | |
| Radial and tangential change for any initial dimension, in any unit 2 | Here | Pick flatsawn or quartersawn and give the board’s width in inches or millimeters. | |
| Adjustable fiber saturation point (default 28%) 2 | Partly | Not adjustable: the Handbook’s 30%, or 22% for the three species its Table 13-5 footnote names (northern white-cedar, western redcedar, redwood). |
Why the numbers may differ
Both calculators work a shelf as a beam, so for the same wood, size and load the first sag should land close. Three things can move it: the stiffness each one uses for your material (here, the 2021 Wood Handbook for wood and makers' sheets for panels), the Sagulator's fixed-shelf and center-load options, which this calculator doesn't have, and the long-term figure, which here is twice the first sag from the Handbook's creep section rather than the 50% the Sagulator's page gives as a rule of thumb.
Open the shelf sag calculator or the wood movement calculator to try your own numbers.
Sources
- WoodBin: The Sagulator, read 2026-11-07.
- WoodBin: The Shrinkulator, read 2026-11-07.