Wood Screw Weight Limit Calculator: Pull-Out and Shear

This wood screw weight limit calculator estimates how much load a wood screw holds when it is pulled straight out and when it is loaded sideways in shear. It uses the equations in the American Wood Council’s National Design Specification, so the result depends on the screw size, the species’ specific gravity, the thickness of the board the screw passes through and how far it enters the second board. The figures are reference design values for one screw, not failure loads, and a safety note below explains what they leave out.

Press Calculate to see the math with your numbers.

How the wood screw weight limit calculator works

A screw can fail two ways, and the calculator reports both so the weaker direction for the real load is visible. The numbers are the reference design values of the American Wood Council’s National Design Specification for Wood Construction (NDS), 2018, Chapter 12, for one wood screw in side grain.

Pull-out (withdrawal)

NDS Equation 12.2-2 gives the withdrawal value per inch of threads:

W = 2,850 × G² × D (lb per inch of thread penetration)

Here G is the species’ specific gravity and D is the screw’s diameter in inches. The capacity is W multiplied by the penetration p into the receiving board. Because G is squared, a dense species holds a screw much better: moving from G = 0.50 to G = 0.67 raises the value by about 80% (0.67² ÷ 0.50²). The equation is the one behind NDS Table 12.2B, which prints 117 lb/in. for a #8 screw at G = 0.50.

Sideways load (shear)

For a load across the screw, the NDS uses the yield limit equations of Table 12.3.1A. Each equation describes one way the joint can give way (the wood crushes under the screw, or the screw bends, or both), and the joint is limited by the lowest result:

• Dowel bearing strength of the wood: F_e = 16,600 × G^1.84 psi for screws under 1/4 in. (NDS Table 12.3.3).
• Screw bending yield strength F_yb: 100,000 psi up to D = 0.142 in., 90,000 psi up to 0.177 in., 80,000 psi up to 0.236 in., 70,000 psi above that (note 2 of NDS Table 12L).
• Reduction term R_d = 2.2 for diameters up to 0.17 in., and 10D + 0.5 between 0.17 and 0.25 in. (NDS Table 12.3.1B).
• The diameter in the yield equations is the screw’s root diameter D_r (NDS Section 12.3.7.1), and the bearing length in the receiving board is the penetration minus one screw diameter, because the tapered tip is taken as 2D long (Section 12.3.5.3).
• Modes I_m, I_s, II, III_m, III_s and IV, as printed in Table 12.3.1A for a single-shear connection, with both boards the same species. Open “How is this calculated?” under the calculator to see every mode worked with your numbers.

Diameters and root diameters are from NDS Appendix Table L3 (rolled-thread screws).

Species values

The specific gravities are the assigned values in NDS Table 12.3.3A, based on oven-dry weight and volume.

Checked against the NDS tables

The results were compared with the printed NDS tables. For sideways load, all 490 entries of Table 12L that the calculator can reproduce (screws #6 to #14, ten species groups, side members from 1/2 to 1-3/4 in., penetration 10D) agree to within 1 lb, and the largest difference is 1.4%. A #8 screw through a 3/4 in. board in Douglas fir-larch gives 88 lb, as the table does. The small differences come from the table rounding its dowel bearing strengths to the nearest 50 psi. For pull-out, 70 entries of Table 12.2B agree to within about 1 lb per inch of thread, such as 163 lb/in. for a #10 screw at G = 0.55.

Worked example: a #8 screw in Douglas fir-larch

A #8 screw (D = 0.164 in.) goes through a 3/4 in. board and 1.64 in. (10D) into a second Douglas fir-larch board, G = 0.50.

• Withdrawal: W = 2,850 × 0.50² × 0.164 = 116.9 lb/in., times 1.64 in. = 192 lb
• Sideways: F_e = 16,600 × 0.50^1.84 = 4,638 psi, F_yb = 90,000 psi, R_d = 2.2, D_r = 0.131 in. The lowest yield mode (III_s) gives 88 lb
• Four such screws sharing the load: 767 lb in withdrawal and 353 lb sideways, if the load is shared equally and the screws are spaced as the NDS requires

Changing the species shows the effect of density: the same screw in southern pine (G = 0.55) gives 232 lb in withdrawal and 100 lb sideways, and in red oak (G = 0.67) it gives 344 lb and 120 lb.

Safety factor and what is not included

NDS reference design values are allowable-stress values. They are set below the load that would actually pull a screw out or bend it, so the result is already a working load rather than a failure load, and it should not be treated as a number to load up to. The NDS then multiplies reference values by adjustment factors (NDS Table 11.3.1) for load duration, wet service, temperature and similar conditions, and this calculator applies none of them: it assumes dry service and normal load duration.

Also outside the calculator:

• Penetration. The NDS minimum penetration is 6 times the diameter (Section 12.1.5.6). The calculator shows 6D and the ratio of your penetration to it; a ratio under 1.0 means the sideways value does not apply.
• End grain. Wood screws must not be loaded in withdrawal from end grain (Section 12.2.2.3).
• Lead holes. Pre-drilled holes of about 70% of the root diameter are specified for withdrawal in wood with G between 0.5 and 0.6, and about 90% for G above 0.6 (Section 12.1.5.2).
• Screw strength. The screw’s own tensile strength limits withdrawal for long penetrations in dense wood (Section 12.2.2.5), and the calculator does not check it.
• Spacing and edge distance. Several screws only add up when they are spaced as Section 12.5 requires.

Use the result to compare screws and species. For anything that holds a person, a heavy cabinet or a structure, have the connection designed by an engineer.

Related tools

The wood load capacity calculator covers boards and shelves, the wood screw shear strength calculator looks up tabulated joint values, and the wood screw length calculator picks a screw length for two boards. For fastener choices in general, see woodworking fasteners and project basics.

Frequently asked questions

How much weight can one wood screw hold?

It depends on the screw size, the wood and the direction of the load. A #8 screw with 1.64 in. of penetration into Douglas fir-larch has an NDS reference value of about 192 lb in pull-out and 88 lb sideways through a 3/4 in. board. Those are allowable values with dry service and normal load duration assumed, so they are not failure loads.

Is a screw weaker in pull-out or in shear?

Neither is always weaker. In the example above the sideways value is lower than the pull-out value, but a long thread in light wood can reverse that, and a thin side board lowers the sideways value. Check the direction the load really acts. A shelf or cabinet hung on a wall loads its screws in both ways at once.

What safety factor should be applied to a screw load?

The NDS reference design value already sits below the failure load, and the standard then applies adjustment factors for conditions such as wet service and load duration. Because the calculator applies none of those, stay below the printed result, and when a failure could hurt someone, have the connection engineered rather than adding an informal margin.