Wood Screw Shear Strength Calculator
Estimate how much sideways (shear) and pullout load a group of wood screws can carry, using NDS design values.
Screw Weight Limit Calculator (Shear & Pullout)
This is different from the Wood Screw Length calculator on our main wood load capacity page. That one sizes a single screw; this one estimates how much load a group of screws in a joint can safely carry, checking both shear (sideways force) and pullout (withdrawal force) and using whichever is lower. Enter design values for your screw and wood from the table below; the defaults are a #8 screw in spruce-pine-fir.
Formula & Example
Shear limit = n × shear/screw × condition × load-type ÷ safety factor (pullout limit uses the same formula with the pullout value); the design limit is the lower of the two, and the recommended load is 80% of that.
NDS Screw Values by Wood
Withdrawal is the NDS reference value W = 2850 × G² × D in pounds per inch of thread in the receiving piece (G is specific gravity, D is the screw diameter in inches; the NDS gives no withdrawal value for screws in end grain). Sideways values Z are from NDS Table 12L for a screw through a 3/4-inch wood side member into a main member with 10 diameters of penetration. All are design values for dry service and normal load duration.
| Wood (specific gravity) | #8 withdrawal (lb per in. of thread) | #8 sideways Z (lb) | #10 withdrawal (lb per in.) | #10 sideways Z (lb) |
|---|---|---|---|---|
| Spruce-pine-fir (0.42) | 82 | 69 | 96 | 83 |
| Douglas fir-larch (0.50) | 117 | 88 | 135 | 107 |
| Southern pine (0.55) | 141 | 101 | 164 | 122 |
| Red oak (0.67) | 210 | 120 | 243 | 153 |
A myth worth correcting: screws are often called strong in shear and weak in pullout. By these numbers it is nearly the other way round. With 1.5 inches or more of thread in the receiving piece, a #8 screw’s withdrawal design value (124 lb in SPF) is well above its sideways value (69 lb), so sideways load is usually the limit. Pullout only governs when the thread is short, such as in thin material, or when a deep shelf levers the top screws outward.
Frequently Asked Questions
Screw Weight Limit
How much weight can a wood screw hold?
By NDS design values, a #8 wood screw through a 3/4-inch side member with about 1.6 inches of thread in the main piece carries 69 lb sideways in spruce-pine-fir, 88 lb in Douglas fir-larch and 101 lb in southern pine. In withdrawal it carries 82 to 141 lb per inch of thread. These are already design values with a safety margin, so the 3x factor is for ultimate or test numbers.
How many screws do I need for a shelf?
For a 50 lb static load, four #8 screws with 1.5 inches of thread in spruce-pine-fir studs have a combined NDS withdrawal value of about 4 × 82 × 1.5 = 494 lb, so four is plenty. Screws must hit studs, and a deep shelf levers the top screws outward, so check pullout, not just shear. Screws into drywall alone are not covered by NDS values.
Should I use manufacturer values?
Yes. Manufacturer technical data sheets give exact shear and pullout values per screw size and wood species, and are more accurate than generic estimates.
Does screw length affect weight limit?
Yes. The NDS withdrawal value is per inch of thread in the receiving piece, so a screw embedded 2 inches holds twice the pullout of one embedded 1 inch. Sideways values assume about 10 screw diameters of penetration and drop in proportion below that, down to a 6-diameter minimum.
Do screws in end grain hold less weight?
Yes, far less. The NDS does not allow a wood screw to be loaded in withdrawal from end grain at all (its end-grain factor is zero). Drive screws into face or edge grain, or use a different fastener or a glued joint when you must connect into an end.
What is the difference between shear and pullout strength?
Shear strength resists sideways forces (a shelf bracket under load). Pullout strength resists forces pulling the screw straight out. With 1.5 inches or more of thread in the receiving piece, NDS withdrawal values for a #8 screw exceed its sideways values, so shear is usually the limit; pullout governs with short thread engagement or when a deep shelf levers the screws outward.
How Accurate Are These Calculators?
This calculator is a planning-level estimate. It is not engineering advice and not code approval. Real capacity and cost depend on species, grade, moisture content, defects, fastening pattern, support condition, and regional pricing. For floors, decks, stairs, or any load-bearing or code-regulated work, check your local building code and have a qualified engineer or inspector confirm the design before you build or buy.
Sources and Methodology
These are original Woodworking Advisor calculator guides combining standard beam-bending, deflection, and lumber-measurement formulas with conservative planning assumptions and clear limitations.
- Screw length uses the standard 2/3 (hardwood) / 3/4 (softwood) penetration rule, capped at combined board thickness. Screw capacity values use NDS withdrawal equation 12.2-2 (W = 2850 G² D) and NDS Table 12L sideways values; gauge diameters and pilot sizes follow standard fastener references.
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