Wood Movement Gap Calculator

Wood Movement Gap Calculator for furniture and floors. Enter species, grain, width and the room humidity range to see the seasonal swing, the fastener play and the gap to plan for.

Wood Movement Gap Calculator for Tabletops and Panels

Dry wood keeps swelling and shrinking across its width as the seasons change. The Wood Movement Gap Calculator works out that seasonal swing for a finished piece, then turns it into the play a tabletop fastener needs, the slot for a case side or the gap around a frame panel. It is built for furniture and cabinet makers and for anyone laying a solid wood floor.

Pick the project, species and grain, then enter the width across the grain in inches and the moisture of the wood when you build. Choose indoors, where the tool uses your room's driest and most humid relative humidity, or outdoors under a roof, where it uses monthly data for the nearest US city.

From Room Humidity to Wood Moisture

Wood settles at a moisture content set by the air around it, its equilibrium moisture content. The tool converts room humidity and temperature to that moisture with the sorption equation in the USDA Forest Products Laboratory Wood Handbook.

The values match the EMC table in the FPL guide Air Drying of Lumber. The default range of 30% to 50% humidity comes from the National Wood Flooring Association. Brett Miller of the NWFA explains in Wood Floor Business that it matches the 6% to 9% moisture most wood is dried to.

How Much a Board Moves

The width change comes from the green-dimension equation in Chapter 13 of the 2021 Wood Handbook.

$$\Delta D=\frac{D_I(M_F-M_I)}{30(100)/S_T-30+M_I}$$

D_I is the starting width, M_I and M_F are the starting and ending moisture, and S_T is the species' total tangential shrinkage in percent. Quartersawn boards use the radial shrinkage instead. The 30 is the fiber saturation point, which the 2021 Wood Handbook drops to 22 for western redcedar and redwood.

With the defaults, a 36 inch flatsawn red oak top built at 8% swings 0.339 inch between 30% and 50% humidity. Quartersawn, the same top moves 0.152 inch. Flatsawn white oak moves 0.420 inch and quartersawn white oak 0.215. Rift sawn uses the average of the two shrinkage values, a middle estimate rather than a published figure.

The fourth card also gives movement per point of moisture. For northern red oak the 2021 Wood Handbook puts the flatsawn coefficient at 0.00304 inch per inch per 1% moisture. Several wood movement calculators and flooring guides still use an older figure of 0.00369, which inflates the default top's swing to about 0.408 inch.

Build Moisture Decides Which Way It Moves

The Wood Handbook's advice is to build at the moisture the wood will see in service. The second card finds the balanced build point, where the piece swells and shrinks by equal amounts. For 30% to 50% humidity at 70 °F that is about 7.7%.

Building off that point shifts the movement one way. A reader of Fine Woodworking's wood movement article asked whether building halfway between summer and winter halves the allowance. It does, as long as the build moisture really sits in the middle.

Build too wet and the piece only shrinks. A 36 inch red oak top built at 12% and moved into a room at 30% humidity loses about 0.634 inch. The tool warns when the build moisture falls outside the range the piece will see.

Turning Movement Into Gaps and Slots

Each project type turns the swing into a different fitting. A tabletop fixed at its center moves half the swing at each edge, so buttons, figure-8 fasteners or slotted screws at the default edges need 0.169 inch of play. The edges move 0.069 inch outward in the humid season and 0.101 inch inward in the dry one.

A case side fixed along its front edge needs rear screw slots as long as the whole swing. The third card also says how far back in the slot to set the screw, 59% of the way at the defaults, since the side shrinks more than it swells from 8%.

A panel in a frame gets a gap at glue-up so it cannot push the frame open in summer. Groove depth beyond the tongue has to cover half the swing on each side. For floors, the card gives the dry-season gap per board, and the Plank Calculator counts the boards for the room.

Humidity control changes all of these. The same red oak top in a room that runs from 20% to 60% humidity swings 0.713 inch, more than twice as much as at 30% to 50%.

Outdoor Projects Under a Roof

Outdoors, the tool reads the monthly equilibrium moisture for the nearest city from Table 13-1 of the 2021 Wood Handbook. The Width Month by Month table shows the piece's width through the year.

The swing depends heavily on climate. A covered 36 inch red oak panel moves about 0.220 inch a year in St. Louis, where outdoor moisture stays between 12.3% and 14.3%. In Phoenix it moves 0.507 inch, and in Seattle 0.540 inch.

These values assume the wood stays dry. Rain wets wood well past the monthly figures, up to fiber saturation, so exposed wood moves more.

Wood Movement Questions

Does a finish stop wood movement?

A finish slows movement but does not stop it. The 2021 Wood Handbook says protective coatings can retard dimensional changes in wood but do not prevent them.

The same chapter notes that daily humidity changes are usually small and of little consequence, while seasonal changes are the ones to design for. Plan the full gaps and slots the calculator gives, finished or not.

Which direction does wood move?

Almost entirely across the grain. The FPL Air Drying of Lumber guide puts shrinkage along the length at only 0.1% to 0.2% from green to oven dry, so length is ignored for normal wood.

Across the grain, flatsawn boards move most in width and quartersawn boards move least. For the much larger shrinkage from green to dry, use the Green Wood Shrinkage Calculator.