How Do I Calculate Wood Movement for My Project?
Wood movement due to moisture is a risk when you’re working with wood.
Trying to stop it is usually pointless.
But you can minimize that risk by understanding wood movement and taking it into account when building your project. The Forest Product Laboratory’s Wood Handbook provides the equations and charts you’ll need to do this. We’ve included them here with some simple explanations for how to use them.
On this page, you can expect to learn:
- Factors affecting wood movement
- Wood movement formula and charts
- How to minimize the impact of wood movement
Before we start calculating wood movement, let’s look at what factors play into it.
Factors affecting wood movement
Wood either absorbs or releases moisture to match the moisture content (MC) of its environment (a term known as the equilibrium moisture content or EMC). If it absorbs or releases enough moisture, the wood moves by expanding or contracting, respectively.
This movement often occurs during seasonal changes, when the relative humidity levels shift significantly. And it can lead to warping, shrinkage, or other types of unwanted issues.
But aside from moisture itself, other factors play into how much a board of wood will move. These factors are:
- Wood species
- Grain direction
- Board width
The formula for wood movement (which we’ll look at below) takes all these into account.
Wood species
Different wood species have different densities, which affect how they respond to moisture.1 In general, softwoods tend to move more than hardwoods, though there are definitely exceptions to this rule.
To take wood species into account, the wood movement formula requires a wood movement value specific to the species you’re using.
Grain pattern
Wood tends to lose more moisture from certain dimensions than others. The greatest amount of change usually happens tangential to the growth rings. The change can be anywhere from 5 to 15%. Change in the radial dimension is much less at 2–8%, while longitudinal change is negligible.2
This means that the way the wood is cut will affect how much it moves. Flatsawn boards will move nearly twice as much as quartersawn boards.3
For this reason, charts with movement values include both the tangential movement value and the radial movement value—tangential applying to flatsawn wood and radial applying to quartersawn wood.
Board width
The wider a board is, the more it’ll move. In fact, a 10-inch board will move twice as much as a 5-inch board.4
The reason for this movement goes back to the grain pattern. Wood moves more radially and tangentially than longitudinally. Even when the wood is quartersawn rather than flatsawn, you’ll notice more movement in the width and hardly any in the length.
The width of the board will also be part of the wood movement equation.
We’ll turn to that next.
Wood movement formula and charts
The Wood Handbook provides a formula to predict the dimensional change in a piece of wood based on the expected change in moisture content. Rather than a formula with symbols, here’s a simple version:
Dimensional change = Wood width × wood movement value × expected change in MC (%)
Wood width
The wood width can be in whatever measurement system you choose, whether metric or imperial. The result will reflect that. If you put in wood width in inches, the dimensional change will be in inches.
Wood movement value
The wood movement value is different for each species and depends on whether the wood is quartersawn or flatsawn.
The following chart from the Wood Handbook gives movement values for both. CR is the radial coefficient, so you would use that for quartersawn wood. CT is the tangential coefficient, which you would use for flatsawn wood.
Expected change in MC (%)
The last value you’ll need for the formula is the expected change in moisture content. We recommend inputting the greatest amount of MC change that could occur in a year in your location.
The chart below, also from the Wood Handbook, helps you to estimate the change based on the EMC in different regions of the United States:
For example, if you were building a cabinet in Boise, Idaho, your greatest possible MC change would be 7.9%. That’s the lowest possible MC of 6.9 subtracted from the highest possible MC of 14.8.
Putting it all together
Now, let’s go through the same steps using the illustration of building cabinets out of solid wood in Boise.
The cabinet door is western white pine and needs to be 24 inches wide. The wood is quartersawn.
Looking at the movement value chart, you find the radial coefficient for western white pine: 0.00141. And we already know the possible MC change is 7.9%.
So, let’s plug all the numbers into the formula:
Dimensional change = 24 inches × 0.00141 × 7.9
Dimensional change = 0.267336
The possible dimensional change is a little over a quarter inch, so you’ll account for this amount when cutting the wood for the cabinet door.
How to minimize the impact of wood movement
Though it’s good to account for wood movement using the above formula and charts, here are some other steps to prevent problems:
Acclimation
As we’ve already mentioned, wood absorbs or releases moisture to meet the EMC of its environment. This will happen regardless of what you do, so it’s best to allow it to happen before you build something with the wood or install wood floor planks.
Determine the EMC of the final location with an EMC chart. Then, use a wood moisture meter to measure the MC of the wood. Once your moisture readings match or nearly match the EMC, you’re set to go.
And remember, just because your wood has been kiln dried doesn’t mean it’ll be at the moisture content you need. Testing is the only way to know for sure.
Environmental control
If the woodworking project or flooring will end up indoors, it won’t experience as much movement as an outdoor project.
To prevent as much movement as possible, seek to keep the environment consistent year-round. If you control the inside levels with an HVAC system or humidifiers/dehumidifiers, your wood is much less likely to move with seasonal temperature and humidity fluctuations.
Extra space for movement
If your project will be in an area or space with major fluctuations, wood movement is bound to happen. You may not be able to prevent it, but you can work with it.
This means leaving space for movement, based on your calculations with the dimensional change equation. You may need to cut a little more off the wood to make room for expansion or include expansion gaps around flooring materials.
If you’re woodworking, use fasteners that allow for movement, especially if you’re working with wider pieces of wood.
For example, if you’re building a tabletop, choose figure-8 fasteners, Z clips, or shop-made fasteners that will allow some movement without damaging the table.5 WOOD magazine offers strategies for working with wood movement when making table joinery and drawers, too.
Don’t let wood movement catch you off guard
Wood movement can be subtle, but it can lead to not-so-subtle problems down the road.
Thankfully, those problems don’t have to catch you off guard. Working with wood movement will minimize the risk. Use the dimensional change equation and leave some extra space in case the wood expands.
But beyond that, be sure the wood you’re working with has dried sufficiently and reached the EMC. This way, movement will be minimal, and you can enjoy a successful, damage-free project.
For more information on dealing with wood movement,
- Becksvoort, Christian, “Stop Guessing at Wood Movement,” Fine Woodworking, Nov./Dec. 2006. (↑)
- Moisture and Wood, NWFA, 2017, p. 8. (↑)
- Hagstrom, Carl, “A Finish Carpenter’s Guide to Understanding Moisture Movement In Wood,” WOODWEB, Dec. 15, 2009. (↑)
- Ibid. (↑)
- “Dealing with Wood Movement,” WOOD, July 13, 2021. (↑)
