Screw withdrawal force
"Will this screw hold the shelf?" is a question that depends on the wood, the screw, and how deep it goes. This calculator uses the USDA FPL withdrawal equation to estimate pull-out force for common species and screw types.
Calculator
The withdrawal formula
USDA FPL's standard equation for wood-screw withdrawal is:
p = 285 × G^1.5 × D × L
where p is load per inch of threaded embedment (lb), G is specific gravity, D is screw diameter (in), L is embedment length (in). The calculator applies species, thread type, grain orientation and pilot-hole factors to this baseline.
Why drywall screws fail in furniture
Drywall screws have a smooth shank near the head (to prevent paper tear) and a brittle heat treatment (to avoid snapping when driven into metal studs). They have only ~85% the withdrawal strength of a wood screw, and they snap under shock load. Use wood screws or deck screws for anything load-bearing.
End grain: halve the numbers
A screw going into end grain holds about half as much as the same screw going into side grain. There's no long-grain fibre to clamp onto. For end-grain fastening, use a longer screw, a dowel-and-glue sandwich, or a different joint geometry entirely (a through-bolt is much stronger).
Limitations
- Equation is for static load. Cyclic / impact loads are far worse.
- Doesn't model wood moisture. Wet wood holds screws worse; kiln-dried wood holds better.
- Doesn't model screw corrosion — outdoor screws need to be exterior-rated.
- Doesn't account for thread engagement with a pilot hole that's too large (slips) or too small (splits).
Frequently asked questions
Should I drill a pilot hole?
For hardwoods, always — splitting is the bigger risk than the slight loss of holding. For softwoods, optional. The calculator assumes "screw cuts its own hole" if you select "no pilot". For a correctly sized pilot, the loss is ~15% — usually worth it for the no-split guarantee.
Are longer screws always stronger?
Yes — withdrawal force scales linearly with embedment depth. But past about 2/3 of the receiving piece's thickness, you risk breaking out the other side, and the practical returns diminish. Most furniture uses 1″–1.5″ embedment.
What about screws near the edge?
Rule of thumb: stay at least 2× the screw diameter from the edge. Closer and you'll split the wood. Closer than 1× and the calculator's number is unreliable.