Fastener Strength Calculator
How much a screw will hold where you are putting it, which of three different failures comes first, and the safe load rather than the load it gives at.
- Situation
- A shelf on brackets, 60 lb on 4 fasteners
- Fastener
- #10 wood screw, 2 1/2" long, ≈1 19/32" of thread in the wood
- Into
- Spruce-pine-fir stud, face or edge, specific gravity 0.40
This fastener is pulled out and pushed sideways at once. Each is checked on its own; a connection doing both is weaker than either check alone suggests, so leave more margin than the numbers do.
The safe load is a sixth of the average test result, raised a tenth for ordinary long-term loading, which is how published practice turns a laboratory figure into one you can rely on. The figure beside it is where that path actually lets go.
The three ways it gives
the safe load is what to rely on; the load it gives at is where that path actually lets go in a test. The smallest safe load is what the connection can do.| What gives | Safe load | Gives at | Of the safe load |
|---|---|---|---|
| The wood lets go of the threadaverage load it pulls out at | 139 lb | 756 lb | 36% |
| The wood crushes and the fastener bendsload at which it yields, mode IIIs | 160 lb | 383 lb | 9% |
| The fastener itself snapstensile strength of the shank at its root | 178 lb | 970 lb | 28% |
Where the load goes
why a shelf pulls out at the top and not the bottomHow this is worked out
A screw in a piece of wood can give way in three quite different ways, and which one comes first depends on the screw, the wood and how deep it goes. The thread can lose its grip and pull straight out. The wood can crush around the shank while the screw bends, which is what happens when the load is sideways. Or the screw itself can snap, which is what actually happens to a thin screw driven deep into a dense hardwood. This works all three out and reports the smallest, because that is the one that decides.
Pulling out is the easiest to predict. The Forest Products Laboratory fitted it to the specific gravity of the wood, the diameter of the screw and how much thread is buried, and those fits are what this uses. Sideways is the hard one. The screw and the wood yield together in one of several patterns, each a different balance of the wood crushing and hinges forming in the steel, and the governing load is the smallest across all of them. That calculation is the American Wood Council's general dowel equations, and this implementation reproduces every value in their own published example, which is the only reason to believe it.
The example on this page is 60 lb of shelf sitting 10" out from the wall on 4 #10 screws into a spruce stud. The lever does the damage: each screw in the top row is pulled out with 50 lb while carrying only 15 lb of the weight itself. The connection is safely holds 166 lb. The same screws into white oak would hold 213 lb.
Two numbers come out of each path and they are not the same thing. The safe load is a sixth of the average test result, raised a tenth for ordinary long-term loading, which is how published practice turns a laboratory figure into one you can rely on day after day. Beside it is where that path actually lets go. Showing only the safe load makes a sound connection look alarming; showing only the load it gives at makes a marginal one look fine.
How to use it
- Pick what you are hanging. The geometry it asks for is the geometry it needs.
- Say how far the weight sits out from the wall and how far apart the top and bottom fasteners are. These two decide how hard the top row is pulled, and they matter more than the screw does.
- Pick the screw and its length, and say what it passes through on the way in.
- Say what it is going into. If you do not know what the studs are, spruce-pine-fir is the usual answer and the weakest of the common three.
- Read the margin. Below one, the connection is over what it should safely carry, and the table says which of the three is the problem.
What it assumes
- This is for hanging things on walls and ceilings. A connection carrying a floor, a roof or people is an engineer’s work, and no calculator replaces that.
- Fasteners into solid wood only. A hollow-wall or masonry anchor holds what its maker tested it to hold, which is a published figure for that product and not something a formula gives; read the packet.
- The wood is at ordinary indoor moisture, at room temperature, under a load that has been there a while. Wet service, high temperature, treated lumber and short-term loads are not adjusted for.
- Edge distances, end distances and spacing are assumed to be enough that the wood does not split first. If they are not, the wood splits and none of this applies.
- Each fastener is checked on its own. Where many share a load, published practice reduces the group’s capacity below the sum of its parts, and that reduction is not applied here.
- Pull-out and sideways load are checked separately. A fastener doing both at once is weaker than either check alone suggests.
- The bracket itself, and the fastener head pulling through it, are not checked. Neither is the wall behind the stud.
Sources
- Wood Handbook, chapter 8: ultimate withdrawal from side grain for wood screws (eq. 8-10b), lag screws (eq. 8-14b) and nails (eq. 8-1b); withdrawal from end grain at about three quarters of side grain; one sixth of the average maximum as the allowable for long-term loading, raised 10% for normal duration; screw shank diameters by gauge (Table 8-9) and a root diameter of about two thirds the shank; lag screw base metal at 45,000 psi yield and 77,000 psi ultimate — USDA Forest Products Laboratory, FPL-GTR-282 (2021) (checked 2026-10-03)
- Technical Report 12, General Dowel Equations for Calculating Lateral Connection Values: Table 1-1 for the yield mode equations, Table A1 for dowel bearing strength, Table A2 for fastener bending yield strength, and example 3.1, whose published values this implementation reproduces — American Wood Council (checked 2026-10-03)
- Wood Handbook, chapter 5, Table 5-3b: specific gravity by species at 12% moisture, and chapter 4, Table 4-3: volumetric shrinkage, used together to put specific gravity on the ovendry basis the bearing equations want — USDA Forest Products Laboratory, FPL-GTR-282 (2021) (checked 2026-10-03)
- NDS Table 12.3.1B, the reduction terms that turn a yield load into a reference design value: 2.2 below 0.17 in., 10D + 0.5 to 0.25 in., and above that 4, 3.6 and 3.2 by yield mode. The half-inch row is confirmed by reproducing example 3.1 of Technical Report 12 — American Wood Council (checked 2026-10-03)