Woodworking calculator
Wood Screw Withdrawal Strength Calculator
A wood screw resists being pulled straight out by the threads biting into the surrounding fibers, and the denser the wood the harder it grips. This calculator uses the published USDA Forest Products Laboratory equation to estimate the ultimate withdrawal load for a screw set in side grain, based on the wood specific gravity, the screw shank diameter, and how deep the threads penetrate, so you can size fasteners and joints with real numbers instead of guesswork.
How it works
Withdrawal resistance is the force needed to pull a fastener straight out along its axis. The Wood Handbook gives an empirical formula for wood screws in side grain: the ultimate load per inch of thread penetration equals 15700 times the specific gravity squared times the shank diameter in inches. Specific gravity enters as a square, so a dense oak at 0.65 holds roughly four times more than a soft pine near 0.35 for the same screw.
Two practical points follow. First, the result is an ultimate or failure value, the load at which the joint tears apart. For a working design you should divide it by a safety factor, commonly five or six, to get an allowable load. Second, penetration matters linearly: only the threaded length embedded in the holding member counts, not the part passing through the top board or the unthreaded shank.
A correctly sized pilot hole, roughly 70 percent of the root diameter in hardwoods, lets the screw seat fully without splitting the wood and reaching the rated grip. End-grain withdrawal is much weaker, around 75 percent or less of side grain, and is generally avoided in load-bearing joints.
Worked example
A #8 screw (0.164 in) with 1 in of thread in Red Oak (G = 0.70): 15700 x 0.70² x 0.164 = 1262 lb ultimate withdrawal. Design for a fraction of that.
Frequently asked questions
What is screw withdrawal strength?
It is the axial force required to pull a screw straight out of the wood along its length. The threads anchor into the fibers, so denser woods and deeper penetration both raise the load the joint can take before it fails.
Why does wood density matter so much?
Specific gravity appears as a square in the formula, so it has an outsized effect. A wood twice as dense holds roughly four times as well, which is why oak and maple grip far better than pine or cedar for the same screw.
Is the calculated load safe to design to?
No, the formula gives an ultimate failure load, not a safe working value. Divide the result by a safety factor of about five or six to get an allowable load that accounts for variability in wood, moisture, and installation.
Does a pilot hole reduce holding power?
A properly sized pilot hole does not meaningfully reduce withdrawal strength and it prevents splitting, especially in hardwoods. Drill roughly 70 percent of the screw root diameter so the threads bite fully without cracking the wood.
How does end grain compare to side grain?
Screws driven into end grain hold much less, often around 75 percent or less of the side grain value, and the grip is less reliable. For load-bearing connections, favor side grain or add mechanical reinforcement.
Related calculators
Sources
These calculators are for planning and estimation. Engineering results (shelf sag, wood movement) use published average material properties; real boards vary by grade, grain, moisture and defects. Verify load-bearing designs with a professional.