Screw Pull-Out Strength Calculator

Introduction: The Force That Yanks a Screw Straight Out

Screws resist two very different loads. Shear tries to slice the shank sideways, and withdrawal tries to pull it straight back out along its axis. Withdrawal is the one that surprises people: a screw that feels rock-solid to wiggle can still ease out of soft pine under a steady axial pull, because all that is holding it is the wood fiber wrapped between the threads. That grip is what fails when a heavy mirror tears off a wall or a ledger board sags away from a joist. This calculator estimates that axial holding force from three things you can measure at the bench — thread diameter, how much thread is buried in the wood, and how dense the wood is.

Density does most of the heavy lifting here, and the formula reflects that: it scales with the square of specific gravity, so a hardwood like maple holds several times more than a softwood like cedar for the identical screw. Embedment depth matters linearly — twice the buried thread roughly doubles the grip — and diameter helps because a fatter screw wraps more fiber per turn. Enter the three values and the tool returns two numbers in pounds and Newtons: the code-style reference design value and the far higher average ultimate at which threads actually tear out in testing.

The NDS Withdrawal Formula

The design-level holding force comes from the wood-screw withdrawal relationship in the National Design Specification for Wood Construction (NDS), fit to decades of pull tests across many species. Written per screw, the reference withdrawal design value is:

In this expression, is the specific gravity of the wood, is the nominal screw diameter in inches, and is the length of thread embedded in inches. The squared exponent on specific gravity is why density dominates the result. Because the design value already carries a large reduction from failure-level testing, the same handbook data also gives an average ultimate — where the joint actually lets go on the test bench:

One assumption is baked into both: the screw is driven into side grain, so the threads bite across the wood fibers. Drive it into end grain — straight into the cut ends of the fibers — and real withdrawal can drop by a third or more, which the formula does not capture.

The calculator converts your metric inputs to inches before applying the formulas, then converts pounds to Newtons by multiplying by . Treat the numbers as a ballpark: pilot-hole size, moisture, thread profile, and whether every buried turn is actually gripping all shift the real value.

Sources: the reference withdrawal design value W = 2850 G² D (lb per inch of thread penetration) is the wood-screw equation from the National Design Specification for Wood Construction (American Wood Council); the average ultimate p = 15,700 G² D lb/in comes from the USDA Forest Products Laboratory Wood Handbook, chapter on fastenings. Conversion: 1 lb = 4.44822 N; 1 in = 25.4 mm. Last reviewed July 2026.

Specific Gravity by Wood Species

The table lists approximate specific gravity values for common North American woods. Entering these numbers into the calculator lets you compare withdrawal strength across species. For instance, a 5 mm screw embedded 40 mm into cedar (G = 0.35) yields a far lower withdrawal force than the same screw in maple (G = 0.72). Because the formula squares G, that swap multiplies the density term by roughly 4.2 — more than quadruple the grip from wood choice alone. It is the reason a screw that anchors happily in fir can strip out of soft cedar under the same load. The species chart under the calculator draws this comparison automatically for whatever screw you enter.

Worked Example: A 4 mm Screw in Pine

Consider a screw with a 4 mm diameter embedded 30 mm into pine, which has a specific gravity of approximately 0.42. Converting to inches gives and . Plugging into the NDS design-value formula:

That works out to a reference design value of about 94 pounds (416 Newtons) — the code-style planning number the calculator returns for these inputs. The Wood Handbook average ultimate for the same screw is about 515 pounds (2,290 Newtons), roughly five and a half times higher, which is where the threads actually tear out on a test bench. The arithmetic also shows how sensitive both numbers are to wood choice: bump the pine up to oak (G = 0.67) and the design value climbs to about 238 pounds, while dropping to cedar roughly halves it.

From Design Value to a Safe Working Load

The 94-pound design value is the right starting point for planning, and the 515-pound ultimate explains where its margin comes from: the NDS reduction absorbs the messiness of a real installation — lumber that grades out weaker than the table, a screw driven slightly proud, moisture that softens the fibers, or a shelf that ends up carrying more than anyone planned. In actual code-checked design the reference value is then multiplied by adjustment factors for load duration, wet service, and temperature, so the working number can move in either direction. Never plan around the ultimate figure; it is an average failure point with no margin at all, and half the test joints let go below it.

Direction matters as much as the number. Threads driven into end grain — into the cut ends of the fibers rather than across them — hold far less than the side-grain case the formula assumes, so back up end-grain joints with a bracket or glue block rather than counting on the raw estimate. Wet-dry cycling gradually works screws loose as the wood swells and shrinks around each thread. A pilot hole sized to the screw and a controlled final torque keep the threads intact so they grip the full depth you measured; strip the hole while driving and the real withdrawal never reaches the calculated value.

Limitations: Where the Simple Formula Runs Out

The limitations follow from the assumptions: the equation was fit to plain straight-shank wood screws, so treat it as a floor rather than a spec for anything fancier. Structural and deck screws with aggressive deep threads, self-tapping tips, or high-low thread patterns often withdraw noticeably harder than the number suggests, and reputable brands publish their own tested pull-out values — use those when you have them. The formula also says nothing about geometry off the screw itself. Set a screw too close to an edge or an end and the wood splits along the grain before the threads ever reach their rated grip, which is why edge distance and spacing rules exist in real connection design.

Moisture is the other big blind spot. Green or damp wood grips softer because the fibers deform instead of pinching the thread, and a joint made wet can lose holding power as it later dries and shrinks away from the shank — or occasionally gain a little, at the risk of splitting. Both coefficients assume ordinary dry, in-service conditions. For anything outdoors, in a bathroom, or exposed to weather, size up, choose corrosion-resistant fasteners, and expect the true capacity to wander over the seasons.

How to Use It: Measuring Your Three Inputs

Two of the inputs you can read off a caliper. Screw diameter is the nominal shank diameter across the threads — the number you already know from the box, or measure at the widest point of the thread. Embedment depth is only the threaded length actually buried in the receiving piece: subtract any smooth shank near the head and anything poking out the far side, since only gripping threads count. Enter both in millimeters. For specific gravity, pull the wood species from the table above, or measure a dry offcut by dividing its mass by its volume and again by the density of water.

Press Calculate Strength and the tool converts your millimeters to inches (dividing by 25.4), runs both withdrawal formulas, and reports the NDS design value and the Wood Handbook ultimate in pounds and Newtons, along with a species comparison chart for the same screw. You can also pick a species from the dropdown to fill in its specific gravity automatically. Everything runs in your browser, so it works on a jobsite with no signal and never sends your numbers anywhere.

Sizing Screws for Shelves, Cabinets, and Brackets

The everyday use is a sanity check before you commit fasteners. Hanging a heavy shelf, a TV mount, or a coat rack? Estimate one screw’s withdrawal load, apply a safety factor, then divide the hanging weight (plus a generous allowance for someone leaning on it) by that allowable to see how many screws you actually need — and whether they should be longer or fatter to reach a deeper bite. In cabinet work the same logic sets how many screws tie a wall cabinet to the studs, or whether the short screws that came with a drawer slide will really hold a loaded drawer.

It is also a fast gut-check ahead of a proper structural calculation. A designer can rule out an obviously undersized fastener in seconds before opening the code book, and a contractor can hand a client a rough holding capacity to explain why a bracket needs three screws instead of one. Just remember the estimate stops where code compliance begins.

When Not to Rely on the Number

This is a planning estimate, not a guarantee, and a few situations push it well off. Off-axis or cyclic loads — a hinge that gets yanked at an angle, a fastener buzzed by vibration — loosen screws in ways a single static formula can’t see. Damaged or stripped threads, knots, checks, and low-grade lumber all undercut the assumed clean side-grain bite. And the formula only accounts for mechanical thread grip: if your joint leans on adhesive, friction, or clamping instead, the real capacity is a different problem entirely. For anything where failure is costly, pull-test a representative joint or bring in an engineer rather than trusting the printout.

Withdrawal questions woodworkers ask