Green Wood Shrinkage Calculator

Green Wood Shrinkage Calculator for fresh-cut boards and bowl blanks. Enter green size, species and grain to see the dry size, the green saw size for full thickness and dry weight.

Green Wood Shrinkage Calculator for Boards and Bowls

The Green Wood Shrinkage Calculator predicts the dry size of wood you measured wet. It is built for people who start with fresh-cut stock, such as a sawyer milling a backyard log, a turner roughing a bowl or a builder working green oak.

Enter the actual green width and thickness in inches, not a nominal size. Pick the species, the grain orientation and the moisture content the piece will live at. The hero shows the dry size, and the cards cover shrinkage, sawing size, a weight target and seasonal movement.

The math follows Equation 4-9 of the USDA Forest Products Laboratory Wood Handbook, Chapter 4. Wood keeps its green size until it dries below the fiber saturation point, which averages about 30% moisture content. Below that, it shrinks in a nearly straight line to oven-dry.

$$S_x=S_0\left(1-\frac{x}{FSP}\right)$$

S0 is the green to oven-dry shrinkage from Handbook Table 4-3, x is the target moisture content and FSP is the fiber saturation point. For a flat-sawn Northern red oak board, tangential S0 is 8.6%. Drying to 8% gives 6.31% width shrinkage, so an 8 × 2 inch green board ends near 7.50 × 1.94 inches.

The Handbook uses tangential values for the width of flat-sawn stock and radial values for quartersawn stock. The tool swaps the two when you pick Quartersawn. For boards with mixed or unknown ring angles, the Handbook suggests the tangential figure, which is the larger and safer number.

One detail the linear formula hides is the moisture gradient. The Handbook notes that a drying board's surface falls below fiber saturation first, so the outer layer starts shrinking while the core is still wet. A board can lose width while its average moisture still reads above 30%.

The first card gives a range around the width loss. The Handbook puts shrinkage variability at a coefficient of variation near 15%, and higher in commercial lumber with imperfect ring angles. About two of every three boards should fall inside that band, so plan for some pieces outside it.

Sawing Logs with the Green Wood Shrinkage Calculator

The second card tells a sawyer how thick to cut green stock so it still grades full after drying. It measures against the NHLA Standard Kiln Dried Rule. Rough kiln-dried stock from 3/8 to 1-3/4 inches may run 1/16 inch scant, and 2 inches and thicker may run 1/8 inch scant.

That makes the minimum for kiln-dried 4/4 15/16 inch. The table shows what happens to 4/4 sawn at exactly 1.000 inch green and dried to 8%. The last column is the green thickness needed for a full inch after drying, quartersawn.

Every flat-sawn board clears 15/16 inch. Quartersawn red oak, sugar maple, white oak and beech do not, because thickness on a quartersawn board shrinks at the tangential rate. The late Gene Wengert made the same point in his Wood Doctor column, noting quartersawn stock loses about twice as much thickness as flat-sawn.

Those figures cover drying only. Sawing variation and planing allowance come on top, which is why sawyers on the WOODWEB sawing forum often cut 4/4 at 1-1/8 inches. To plan the finished size after planing, use the Rough to Finished Lumber Calculator.

Using Weight to Tell When It Is Dry

The third card turns the target moisture into a scale reading. Moisture content in the Wood Handbook is water mass divided by oven-dry wood mass. The oven-dry wood never changes, so weight tracks moisture exactly.

$$W_t=W_g\times \frac{1+M{C}_t/100}{1+M{C}_g/100}$$

In words, divide the green weight by one plus the starting moisture, then multiply by one plus the target moisture. A 6 lb red oak blank at 80% should weigh 3.60 lb at 8%, so it must lose 2.40 lb of water. Weigh it weekly and treat it as dry once the readings stop falling near that number.

Starting moisture defaults to Wood Handbook Table 4-1, which lists 80% for red oak heartwood and 69% for its sapwood. The Handbook stresses that green moisture varies within and between trees. It lists no figure for butternut, Pacific madrone, live oak, Eastern white pine or the imported species, and the tool flags those as estimates.

For a real starting value, Chapter 13 of the Handbook describes the oven method. Cut a full cross-section 1 inch long, at least 20 inches from the end of the board. Weigh it, dry it at 214 to 221°F until the weight holds steady over 3-hour checks, then apply the moisture formula.

Rough-Turning Green Bowls

Switch What You Are Drying to Rough-Turned Bowl and the inputs become rim diameter and wall thickness. A face-grain bowl shrinks across the grain but barely along it, so the rim dries oval. The tool uses the tangential rate across the grain, which gives the largest oval and errs on the safe side.

Turners follow a convention of roughing the wall to about 10% of the diameter, so a 10 inch bowl gets a 1 inch wall. Members of the American Association of Woodturners forum and Woodworker's Journal both describe it. It is a shop rule, not a published standard.

The tool checks the rule against shrinkage. It trues the outside to the short axis of the dried oval and the inside to the long axis, then reports the wall left to re-turn.

$$R=\frac{2F+D(L-A)}{2L}$$

R is the rough wall needed, F is the finished wall and D is the green rim diameter. A and L are the fractions of size kept across and along the grain. The table applies it to a 10 inch face-grain bowl dried from green to 8% with a 3/8 inch finished wall.

Every species needs less than the 1 inch the 10% rule gives. The difference covers stress warp and off-center movement, which the formula cannot predict. With madrone, only about 0.17 inch of that margin remains, while mahogany keeps nearly half an inch.

End-grain bowls behave differently. With the pith centered, the rim shrinks radially toward the pith and tangentially around the rings, so the bigger risk is a split from the pith rather than an oval. Read the end-grain oval as a worst case for an off-center pith. Once the blank is dry, the Lathe Speed Calculator helps set a safe speed for an out-of-round piece.

Seasonal Movement After Drying

Drying to target is not the end of movement. The fourth card uses the moisture band from Wood Handbook Table 13-2 for your setting and reports how much the dry piece keeps growing and shrinking. For interior furniture in most of the US, that band is 6 to 10%.

Chapter 13 estimates these small in-service changes with a dimensional change coefficient. It multiplies the starting size by the coefficient and by the change in moisture content, with coefficients based on the size at 10% moisture.

$$\Delta D=D_I\times C_T\times (M_F-M_I)$$

The 2021 Handbook lists Northern red oak at a tangential coefficient of 0.00304. Older flooring articles still quote 0.00369 from an earlier edition, which overstates the swing by about a fifth. The default red oak board moves about 0.09 inch across its width between 6 and 10%.

The Handbook adds that a straight-line estimate tends to understate real movement by about 5%. For dry stock that will change moisture in service, the Wood Shrinkage Calculator covers kiln-dried lumber. The Wood Movement Gap Calculator turns movement into gaps for panels and tabletops.

Shrinkage Puzzles from the Sawmill and Lathe

Does green wood shrink in length?

Very little in normal wood. The Wood Handbook puts green to oven-dry longitudinal shrinkage at 0.1% to 0.2% for most species, so an 8 foot board loses about 1/8 inch of length. The tool uses 0.15% for the along-grain axis of a bowl.

Reaction wood and juvenile wood from near the pith are the exceptions. The Handbook reports they can shrink 2% lengthwise, nearly 2 inches on the same 8 foot board. When those zones sit on one side of a board, the uneven shrinkage shows up as bow, crook or twist.

Can I trust a moisture meter on green wood?

Not for the starting reading. Chapter 13 of the Wood Handbook gives a reliable range of about 6% to 30% for resistance meters and 4% to 30% for dielectric meters. Readings above 30% should be treated as qualitative only.

Green wood sits well above that ceiling, so use the Table 4-1 value or an oven-dried sample for the starting moisture. Once the piece drops below 30%, a pin meter becomes useful. The Handbook suggests driving the pins one-fifth to one-fourth of the thickness to read the average rather than the drier surface.

How long does a rough-turned bowl take to dry?

Turners report wide ranges. Woodworker's Journal suggests about three months for a blank roughed to a 10% wall. One AAW forum member kept a bowl bagged for four months, then checked it with a meter every couple of months until it read 10%.

Another member in the same thread reported an 18 inch bowl going from soaking wet to dry in five months. Time depends on species, wall thickness and storage, so the weight target is a better finish line than the calendar. For an estimate by thickness and method, see the Wood Drying Time Calculator.

What moisture should green wood reach before I build with it?

It depends on where the piece will live. Wood Handbook Table 13-2 recommends an 8% average for interior furniture and woodwork in most of the US, 6% in the dry Southwest and 11% along the damp, warm coast. Exterior siding and trim average 12% in most areas.

Framing is looser. Chapter 13 says softwood framing is usually dried to an average of 15%, not to exceed 19%, while hardwood for furniture, cabinets and millwork is dried to 6% to 8%. Those four settings, plus 15% framing, are the presets in the Target Moisture field.