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Woodworking Math

Wood movement calculator

How much will a tabletop, a panel or a drawer front grow and shrink across the grain? Pick the species, the width and how the board was sawn, then where its moisture content is now and where it will go. Either one can be given as the air it will live in.

Units

How it was sawn
Now
Later

Swells by

0.271 in

about 9/32 in

6% → 12% MC · width becomes 12.271 in

One source: USDA Forest Products Laboratory, the authority for species properties it measured

Why: White oak swells by the Handbook's own movement equation for this moisture change.

Why these numbers?

  1. The situation

    The Wood Handbook gives white oak a tangential dimensional change coefficient of 0.00376 per 1% change in moisture content (Table 13-5).

    One source: USDA Forest Products Laboratory, the authority for species properties it measured , sources [1]

  2. What that means

    A flatsawn board moves across its width with the tangential value.

    From your input: how the board was sawn

  3. So

    Between 6% and 14% moisture content the Handbook's Eq. 13-3 applies: change = width × coefficient × (12% − 6%).

    Arithmetic: exact by definition , sources [1]

  4. The number

    12 in board: +0.271 in.

    Arithmetic: exact by definition

How we source these numbers

Hardness, weight and uses: White oak wood.

How the movement is worked out

Between 6% and 14% moisture content, wood shrinks and swells in close to a straight line, so the USDA Wood Handbook gives each US species a dimensional change coefficient (Table 13-5): the fraction of its width a board gains or loses for each 1% change in moisture content. The change is width × coefficient × change in moisture content (Eq. 13-3). Outside that range, or for a species with no coefficient, the Handbook works it from the green-to-oven-dry shrinkage instead (Eq. 13-4).

Flatsawn boards move with the tangential figure, quartersawn boards with the radial one, which the Handbook's tables show is smaller for every species. When you aren't sure, the calculator uses the tangential figure, as the Handbook suggests for unknown ring orientation. Air given as temperature and humidity is turned into a moisture content with the Handbook's equation for equilibrium moisture content (Eq. 4-5), the same one behind the EMC calculator.

Tropical woods

231 tropical woods come from their sheets in the Forest Products Laboratory's Tropical Timbers of the World (Chudnoff, 1979 edition). The sheets print no dimensional change coefficient, so the movement is always worked from the sheet's radial and tangential shrinkage, green to oven-dry, with the Handbook's Eq. 13-4. A sheet whose shrinkage is printed only to 12% moisture content or to air-dry doesn't fit that equation, so the wood is listed here instead of guessed.

76 tropical woods left out: the chart needs radial and tangential shrinkage from green to oven-dry

Printed to 12% moisture content, not to oven-dry: 33. No shrinkage from green to oven-dry on the sheet: 28. Printed to air-dry, not to oven-dry: 12. Printed to 15% moisture content, not to oven-dry: 3.

  • Sajo (p. 69): No shrinkage from green to oven-dry on the sheet
  • Cedro-rana (p. 83): No shrinkage from green to oven-dry on the sheet
  • Cupressus lusitanica (p. 105): No shrinkage from green to oven-dry on the sheet
  • Carbonero (p. 239): No shrinkage from green to oven-dry on the sheet
  • Quebracho (p. 273): No shrinkage from green to oven-dry on the sheet
  • Trophis spp. (p. 305): No shrinkage from green to oven-dry on the sheet
  • Afzelia (p. 321): Printed to 12% moisture content, not to oven-dry
  • Nkobakoba (p. 337): Printed to air-dry, not to oven-dry
  • Rhodesian-teak (p. 339): Printed to 12% moisture content, not to oven-dry
  • Brachylaena Hutchinsii (p. 347): Printed to 12% moisture content, not to oven-dry
  • Mtundu (p. 349): Printed to 12% moisture content, not to oven-dry
  • Okwen (p. 351): Printed to 12% moisture content, not to oven-dry
  • Burkea (p. 353): Printed to 12% moisture content, not to oven-dry
  • African Canarium (p. 355): Printed to 12% moisture content, not to oven-dry
  • Muirungi (p. 359): Printed to air-dry, not to oven-dry
  • Cassipourea malosana (p. 361): Printed to air-dry, not to oven-dry
  • Mtambara (p. 367): Printed to 12% moisture content, not to oven-dry
  • Cordyla (p. 375): Printed to 12% moisture content, not to oven-dry
  • Musine (p. 377): No shrinkage from green to oven-dry on the sheet
  • Cynometra alexandri (p. 381): Printed to 12% moisture content, not to oven-dry
  • Adjouaba (p. 383): No shrinkage from green to oven-dry on the sheet
  • African Blackwood (p. 385): No shrinkage from green to oven-dry on the sheet
  • Eyoum (p. 389): No shrinkage from green to oven-dry on the sheet
  • Ekebergia (p. 397): Printed to 12% moisture content, not to oven-dry
  • Kosipo (p. 401): Printed to air-dry, not to oven-dry
  • Erythroxylum manii (p. 409): No shrinkage from green to oven-dry on the sheet
  • East African Satinwood (p. 411): No shrinkage from green to oven-dry on the sheet
  • Mafu (p. 413): Printed to 12% moisture content, not to oven-dry
  • Kamassi (p. 417): No shrinkage from green to oven-dry on the sheet
  • Gossweilerodendron balsamiferum (p. 419): Printed to 12% moisture content, not to oven-dry
  • Mutenye (p. 423): No shrinkage from green to oven-dry on the sheet
  • Mbarika (p. 433): Printed to air-dry, not to oven-dry
  • Muwa (p. 435): Printed to 12% moisture content, not to oven-dry
  • African Pencil Cedar (p. 437): Printed to 12% moisture content, not to oven-dry
  • African-walnut (p. 447): Printed to 12% moisture content, not to oven-dry
  • Musizi (p. 449): Printed to 12% moisture content, not to oven-dry
  • East African Camphorwood (p. 471): Printed to 12% moisture content, not to oven-dry
  • Olea hochstetteri (p. 475): Printed to 12% moisture content, not to oven-dry
  • Muninga (p. 491): Printed to 12% moisture content, not to oven-dry
  • Mueri (p. 499): No shrinkage from green to oven-dry on the sheet
  • Odoko (p. 503): Printed to 12% moisture content, not to oven-dry
  • Brown Sterculia (p. 511): Printed to 12% moisture content, not to oven-dry
  • Niangon (p. 517): Printed to 12% moisture content, not to oven-dry
  • Tieghemella heckelii (p. 527): No shrinkage from green to oven-dry on the sheet
  • Sugar-plum (p. 533): No shrinkage from green to oven-dry on the sheet
  • Vitex (p. 535): Printed to 12% moisture content, not to oven-dry
  • Black Wattle (p. 543): No shrinkage from green to oven-dry on the sheet
  • Amoora (p. 555): No shrinkage from green to oven-dry on the sheet
  • Neem (p. 565): Printed to air-dry, not to oven-dry
  • Chengal (p. 567): No shrinkage from green to oven-dry on the sheet
  • Tawa (p. 569): No shrinkage from green to oven-dry on the sheet
  • Pipli (p. 573): No shrinkage from green to oven-dry on the sheet
  • White Cypress-pine (p. 575): Printed to air-dry, not to oven-dry
  • Berangan (p. 583): No shrinkage from green to oven-dry on the sheet
  • Cinnamon Wood (p. 593): No shrinkage from green to oven-dry on the sheet
  • Geronggang (p. 599): Printed to air-dry, not to oven-dry
  • Kekatong (p. 603): Printed to air-dry, not to oven-dry
  • Dacrydium spp. (p. 605): Printed to 12% moisture content, not to oven-dry
  • Jongkong (p. 607): Printed to 12% moisture content, not to oven-dry
  • Keranji (p. 611): Printed to 15% moisture content, not to oven-dry
  • Simpoh (p. 613): Printed to air-dry, not to oven-dry
  • Magas (p. 623): Printed to 12% moisture content, not to oven-dry
  • Gubas (p. 631): Printed to 12% moisture content, not to oven-dry
  • Tembusu (p. 645): Printed to air-dry, not to oven-dry
  • Queensland-maple (p. 647): Printed to 12% moisture content, not to oven-dry
  • Mengkulang (p. 659): Printed to 15% moisture content, not to oven-dry
  • Burma Lancewood (p. 661): Printed to 15% moisture content, not to oven-dry
  • Ranggu (p. 669): Printed to 12% moisture content, not to oven-dry
  • Tasmanian-myrtle (p. 693): No shrinkage from green to oven-dry on the sheet
  • Binuang (p. 695): Printed to 12% moisture content, not to oven-dry
  • Nyatoh (p. 697): No shrinkage from green to oven-dry on the sheet
  • White Seraya (p. 699): Printed to 12% moisture content, not to oven-dry
  • Merkus Pine (p. 707): No shrinkage from green to oven-dry on the sheet
  • Putat Paya (p. 709): Printed to 12% moisture content, not to oven-dry
  • Sandalwood (p. 723): No shrinkage from green to oven-dry on the sheet
  • Turpentine (p. 743): Printed to air-dry, not to oven-dry

What it can't tell you

These are averages for clear wood. The Handbook gives shrinkage a coefficient of variation of about 15% (p. 4-11), so one board can move noticeably more or less. Treat the answer as a planning figure.