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Planning For Wood Movement: Easier Than You Think (Equation and Calculator)

December 04, 2021 8 min read

Wood movement can sound like an existential threat for someone getting started in woodworking.

The way people talk about it, you’d think a jewelry box might explode and level your shop.

I’m here to tell you, it’s going to be ok.

Wood movement is absolutely something to keep in mind, but it’s not an otherworldly force shrouded in mystery. No need to offer sacrifice to the wood gods.

In fact, I’m going to show you that wood movement is:

  • Predictable
  • Easy to calculate
  • Not as bad as you think

All the information here is based on a 500 page handbook from the US Forest Service’s Forest Products Laboratory. It goes into painful detail on wood as an engineering material, using 30 years of data to boil wood movement down to a precise science.

If you really want to nerd out, you can check out the handbook. But I’m going to cover everything you need to know for woodworking here.

WOOD MOVEMENT 101

WHAT WOOD MOVEMENT IS

Wood movement is the dimensional change in lumber caused by changes in the surrounding environment’s relative humidity and temperature.

All wood goes through these dimensional changes. But as long as your lumber is dried and acclimated to your shop (more on this later), you’ll only see movement across seasons — not in the span of a day.

The biggest shifts are typically between summer and winter. Of course, wood will be subject to a lot more movement outdoors as opposed to indoors.

WHAT CAUSES WOOD MOVEMENT

In a word, moisture.

Wood is hygroscopic — meaning it tends to absorb moisture from the air.

This makes wood act like a kitchen sponge: it expands when wet and contracts when dry.

In the simplest terms: as moisture increases, wood expands. As moisture decreases, wood contracts.

HOW WOOD MOVES

Wood tends to expand and contract across the grain — meaning along the width of your board or perpendicular to the long grain.

This is an important point.

As long as movement across the grain isn’t inhibited, you have nothing to worry about.

Let me give you an example.

I made a houndstooth dovetail joint for a video a while back that caused an uproar amongst the wood movement doomsdayers. One look at the razor-thin dovetails, and they said it would detonate into an explosion of walnut and maple shrapnel.

But the thing is, the grain all went the same way — meaning the joint would expand and contract together. Almost 3 years later, that joint’s still holding strong.

Wood movement becomes a concern with joints where the grain direction isn’t matched — like when attaching a tabletop to a base.

KEY POINTS ABOUT WOOD MOVEMENT

  • Caused by changes in moisture content of wood
    • As moisture increases, wood expands
    • As moisture decreases, wood contracts
  • It occurs across seasons, not days (biggest change from summer to winter)
  • Concentrated ACROSS the grain of a board — the width, not the length

HOW TO CALCULATE WOOD MOVEMENT

You can calculate wood movement quickly, easily, and accurately with this formula (and no specialty tools):

(Width of board) x (Radial or Tangential Coefficient of Wood) x (Change in Moisture)

Ok, that looks like a nightmare without context. But the only variable you actually have to measure is the width of your board.

And if you’re reading this, you’re probably pretty good with a tape measure.

For the other two variables — I have a couple of pages from the Forest Products Lab handbook that literally give you the answers. It almost feels like cheating.

Let’s go through each of this equation’s variables, so it all makes sense.

WIDTH OF THE BOARD

Whether you’re working with a monolithic slab or several boards glued together, measure across the grain of the wood from one side to the other. Again, the width, not the length.

You can use inches, millimeters, scraps of exotic hardwood you’ll never use but can’t bring yourself to throw away — it doesn’t matter.

Just know the result of this equation will be in whatever unit you use here.

RADIAL AND TANGENTIAL COEFFICEINT

This is the equivalent of that guy that seems super scary and intimidating but is actually a sweetheart.

To find the radial or tangential coefficient, all you need to know is:

  • Whether your board is plain sawn vs. rift/quarter sawn
  • What species of wood your board is

Depending on how your board was originally sawn from the tree, you use either the tangential or radial coefficient.

Lumber is usually either plain (flat) sawn, rift sawn, or quarter sawn.

You can tell them apart by looking at the end grain. In plain sawn lumber, the grain lines run horizontally across the width of a board. In rift or quarter-sawn wood, they run vertically.

The way lumber is sawn affects how it will expand and contract over time. Rift or quarter sawn wood changes dimension radially, while plain sawn boards do it tangentially.

Because of that, you use the tangential coefficient for plain sawn lumber, and the radial coefficient for rift or quarter sawn lumber.

And this is where the Forest Products Lab handbook gets really useful.

This chart lists wood species, along with their tangential coefficients (CT) and radial coefficients (CR).

Find the species of wood you're using, determine how it’s sawn, look at the corresponding column — and done.

For example, say you’re making a tabletop out of quarter sawn walnut.

Find walnut in the list of species, look at the radial coefficient column, and you’ll see 0.0019.

That’s the second variable in our formula for calculating wood movement.

You’ll notice for each species, the coefficients in the flat sawn column are higher than in the rift/quarter sawn column. That’s because rift or quarter sawn lumber is more stable and subject to less movement over seasons.

If the species of wood you’re using isn’t on this list, find a wood with a similar janka rating and use those coefficients.

This chart is based on 30 years worth of data that the Forest Products Lab got by literally watching wood move — let's be grateful we didn’t have to do it.

Key Points:

  • For plain sawn lumber, use the tangential coefficient; for rift or quarter sawn lumber, use the radial coefficient
  • In plain sawn lumber, grain lines run horizontally along width of board; in rift or quarter sawn lumber, grain lines run vertically to width of board
  • To find your board’s coefficient, find your species of wood on the Change Coefficient Chart, and use the number in the corresponding column (CR or CT)

CHANGE IN MOISTURE

Still with me?

Change in moisture might seem daunting, but the Forest Products Lab has another cheat sheet for us. They were definitely the kids everyone copied off back in middle school.

This chart shows wood’s equilibrium moisture content (EMC) — or the moisture level where wood is at equilibrium with its environment and won’t lose or gain moisture.

All you need to know for this chart is where you live and what month it is.

Find your city or country. If it’s not on there, look for a place close by with a similar climate. Then find the month during which you’re building your project.

That number is your starting moisture content.

Next, find the month with an EMC that’s the furthest away from your current month. Then, calculate the difference between these two numbers.

That’s your change in moisture, the third variable in our wood movement equation.

For example, my shop’s in Santa Barbara. Let’s say I’m building my table in November. Looking at the chart, I can see the EMC of wood in Santa Barbara in November is 12.1%.

I also see July is the most extreme difference from November, with an EMC of 15.3%. I take the difference (just ignore negative numbers for now) and get 3.2%.

15.3% (EMC Jul., Santa Barbara) – 12.1% (EMC Nov., Santa Barbara) = 3.2% (change in moisture content)

That’s our third and final variable for calculating wood’s seasonal dimensional change.

And if you’re wondering how accurate this chart is, I tested it against a humidity meter — and the difference was only 0.1%.

My hat’s off to you, Forest Products Lab.

EXAMPLE: CALCULATING WOOD MOVEMENT

Ok, time for a concrete example.

Say I’m building a table out of quarter sawn walnut. The tabletop is 40 inches across, and it’s November in Santa Barbara.

Let’s plug the variables we calculated above into our equation:

40 inches (width of board) x 0.0019 (radial coefficient of walnut) x 3.2 (% change in moisture) = 0.243 inches.

That means that from November to July, my tabletop will expand just under ¼ inch — and only ⅛ inch in either direction.

For a tabletop that’s nearly 3 ½ feet wide, that ain’t bad…

Especially since the EMC chart we used is based on wood being outdoors.

If your furniture’s inside, where moisture tends to be a lot more stable across seasons, the change in dimension will be even less.

So if you plan for that ¼ inch of movement, you’ll be 100% fine.

With this formula, there’s literally zero guesswork involved in calculating wood movement. It’s just copy and paste math.

HOW TO PLAN FOR WOOD MOVEMENT

Here are a couple of tips for dealing with the inevitable.

1. LET LUMBER ACCLIMATE TO YOUR SHOP

Before you start milling the lumber you just picked up, it’s best to let it sit. For air dried lumber, a couple days will do. For kiln dried, you’ll want to wait a couple of weeks.

This makes sure your wood has a chance to adapt to the new environment and get all its twisting and warping out before you start working with it.

2. MILL YOUR LUMBER... THEN WAIT

If you really want to be 100% sure nothing’s going to twist out of shape on you, a little patience goes a long way.

Milling lumber can sometimes reveal new sources of tension — so a board you milled to perfection the night before could suddenly be bent out of shape the morning after.

That’s why it’s good to let milled lumber sit overnight before actually shaping your stock — and also worth milling a few extra pieces just in case.

3. PAY SPECIAL ATTENTION TO TABLETOPS

One of the biggest mistakes newer woodworkers make is not accounting for wood movement when attaching a tabletop to a base.

You need to fasten the tabletop in a way that lets it expand and contract freely. There are a few ways to do this:

  • If you’re adding a breadboard, use a glued middle dowel but two unglued dowels on either side, with grooves for them to move across the grain (1/8 inch for the example above)
  • Use figure 8 fasteners or z-clips to fasten a tabletop to its base - these are designed to shift with seasonal wood movement
  • Drill pilot holes bigger than your screws and secure them with a washer so they have space to move (again, ⅛ inch of space in our example above)

TAKEAWAY: IT'S GOING TO BE OK

Yes, wood movement is real — but it’s measurable and easy to predict.

So the next time turdburglar6969 comments on Youtube that a piece of furniture will detonate by Christmas — don’t sweat it.

And to make things even easier for you, I put together this Wood Movement Calculator you can download for free.

It contains all the data from the Forest Products Lab handbook, so you just need to type in a few variables and let it do the rest.

Additonally, we have created our own Wood Movement Calculator (free to use on our website) so you can calculate wood movement for your next project on the fly without having to do any of the tedious calculations.

Got any wood movement horror stories to share? Leave them in the comments below!

For company questions or customer service, you can email us at support@kmtools.com.

As always, STAY SAFE IN THE SHOP.

Jonathan Katz-Moses
Jonathan Katz-Moses


8 Responses

Jonathan Katz-Moses
Jonathan Katz-Moses

February 21, 2022

Thanks for the comments, everyone!

@Ryan: Tell that to some jewelry box lids I’ve built haha. I think 1/4" moves just as much as any other thickness. Nominal thickness is included in our wood movement calculator >>> https://kmtools.com/pages/wood-movement-calculator

@Alan: Click the link above!

Don
Don

December 07, 2021

It’s worth recognizing that if you’re building that table in Santa Barbara for a client in Cheboygan, you’d use the humidity extreme for the latter in your calculation.

This is just a hypothetical example—nobody in Cheboygan actually buys furniture. I just like saying “Cheboygan.”

Russ
Russ

December 07, 2021

Thanks for doing all the hard work. This was really helpful!

Ryan
Ryan

December 07, 2021

Love this article. I do have one question about thickness. I’ve heard it said that once wood gets around 1/4" it acts like a veneer and basically doesn’t move. This leads me to wonder how thickness plays into the equation. Wouldn’t a 2" thick table top move more than a 1"? Was there anything in your research that alluded to thickness?

Jorrit
Jorrit

December 06, 2021

turdburglar6969, I hate that guy.
But great stuff again JKM!

michael
michael

December 05, 2021

“And if you’re reading this, you’re probably pretty good with a tape measure.”
O fawk, that hit me right in the funny bone.

Thanx for this Jonathan, massive help. You sir, are a rock star.

Richard A Boykin
Richard A Boykin

December 05, 2021

Super useful! Many thanks.

Alan
Alan

December 05, 2021

This article is awesome. Well organized and written. I’ve downloaded the data but if the bolded items in “I put together this Wood Movement Calculator you can download for free” were supposed to be links, they are not active. I also did not find an actual calculator. Am I missing something.
Thanks
Alan

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