Complete Guide to Screw Sizing and Selection

Understanding Screw Gauge Numbers

The American screw gauge system uses numbers from #0 to #14 (and sometimes higher) to indicate screw diameter. This numbering system originated in the 19th century and remains the standard for wood screws, sheet metal screws, and some machine screws in the United States. Understanding gauge numbers is essential for selecting appropriate screws and drilling proper pilot holes.

The gauge number corresponds to the diameter of the screw shank (the smooth portion below the head). Each increase in gauge number represents an increase in diameter of approximately 0.013 inches (though this isn't perfectly consistent across all sizes). A #6 screw measures 0.138 inches in diameter, while a #8 measures 0.164 inches, and a #10 measures 0.190 inches. These three sizes—#6, #8, and #10—account for the vast majority of screws used in home construction and woodworking projects.

For projects requiring greater holding power or working with thicker materials, #12 (0.216") and #14 (0.242") screws provide more strength. Smaller gauge screws like #2 and #4 are used for delicate applications like attaching hinges to cabinet doors, assembling small wooden boxes, or installing light switch plates. The smaller diameter reduces the risk of splitting thin materials.

Machine Screw Sizing: A Different System

Machine screws use a designation system that combines diameter with thread count. For smaller machine screws (under 1/4 inch), the system uses gauge numbers like wood screws, but adds the threads-per-inch count. A "#6-32" machine screw has a #6 diameter (0.138") with 32 threads per inch. A "#10-24" measures 0.190" diameter with 24 threads per inch.

Once machine screws reach 1/4 inch diameter and larger, the designation switches to fractional inches for diameter. A "1/4-20" machine screw is 0.250 inches in diameter with 20 threads per inch. A "3/8-16" measures 0.375 inches with 16 threads per inch. This dual system can initially confuse newcomers, but it becomes intuitive with use.

Thread count matters significantly for machine screws because they thread into tapped holes or nuts rather than self-tapping into wood. "Coarse" threads (fewer threads per inch) are designated UNC (Unified National Coarse) and provide faster assembly and better performance in softer materials like aluminum or plastic. "Fine" threads (more threads per inch), designated UNF (Unified National Fine), offer greater strength in harder materials and provide more precise adjustments, making them common in precision machinery and automotive applications.

Metric Screw Sizing

The metric system uses a straightforward designation: "M" followed by the nominal diameter in millimeters, then the thread pitch in millimeters. An "M6 x 1.0" screw has a 6mm diameter with threads spaced 1.0mm apart. An "M8 x 1.25" measures 8mm diameter with 1.25mm thread pitch. When only one number appears (like "M6"), it indicates coarse thread—the standard pitch for that diameter.

Common metric sizes include M3, M4, M5, M6, M8, M10, and M12. These correspond roughly (but not exactly) to imperial sizes, creating potential confusion. An M6 screw (6mm = 0.236") falls between a #12 (0.216") and #14 (0.242") gauge screw. An M8 (8mm = 0.315") sits between 1/4" (0.250") and 5/16" (0.3125"). This lack of perfect correlation means metric and imperial fasteners are rarely interchangeable—always replace a metric screw with metric, and imperial with imperial.

Modern imported furniture, electronics, and automotive components increasingly use metric fasteners, even in the United States. Maintaining both metric and imperial screw assortments and tools is becoming necessary for comprehensive home and automotive work.

Screw Length Selection

Choosing proper screw length is critical to joint strength and preventing damage. The fundamental rule for wood screws is that the threaded portion should penetrate at least two-thirds to three-quarters of the way into the base material (the piece you're screwing into, not through). This provides sufficient holding power while minimizing the risk of the screw point breaking through the far side.

For example, when attaching a 3/4-inch thick board to a 2x4 (actual thickness 1.5 inches), your screw must pass through the 3/4" board and penetrate at least 1 inch into the 2x4, requiring a minimum screw length of 1-3/4 inches. A 2-inch screw would provide better holding power with a margin for error.

Consider screw anatomy when calculating length. Flat-head (countersunk) screws have the head included in the overall length measurement—a 2-inch flat-head screw provides only about 1-7/8 inches of actual penetration after the head countersinks. Pan-head, round-head, and hex-head screws measure length from directly under the head, so the specified length is the actual penetration you'll achieve.

For drywall installation, standard 1-5/8 inch screws work for 1/2-inch drywall attached to wood studs. The screw penetrates about 1-1/8 inches into the stud, providing excellent holding power. For 5/8-inch drywall, use 2-inch screws. When screwing into metal studs, shorter screws (1-1/4 inch for 1/2-inch drywall) are appropriate since you're threading into metal, not wood.

Pilot Holes: Why They Matter

Drilling pilot holes before driving screws serves multiple critical functions. First, pilot holes prevent wood splitting, especially near board edges or in hardwoods. When you drive a screw without a pilot hole, the screw's threads act like a wedge, forcing wood fibers apart. If the outward pressure exceeds the wood's tensile strength, it splits. The pilot hole removes material where the screw will go, eliminating the wedging action.

Second, pilot holes reduce the driving torque required, making screw installation easier and reducing the risk of breaking screws or stripping drive recesses. This matters especially for small screws, brittle materials like MDF, or when using power drivers at high speed. Third, pilot holes improve screw placement accuracy—it's much easier to position a drill bit precisely than to start a screw point exactly where you want it.

Pilot hole diameter should match the screw's core diameter (the solid shaft inside the threads), not the major diameter (measured at the thread peaks). This allows the threads to bite into solid wood while preventing splitting. For softwoods like pine or cedar, pilot holes can be slightly smaller or sometimes omitted for larger gauge screws far from board edges. For hardwoods like oak or maple, always drill pilot holes sized properly for the specific screw gauge.

Many woodworkers drill stepped pilot holes using countersink bits that create both the pilot hole and the countersink recess for flat-head screws in one operation. These specialized bits save time and ensure the screw head will sit flush with or slightly below the wood surface.

Sheet Metal Screws: Special Considerations

Sheet metal screws feature sharp points and threads that extend to the tip, designed to cut their own threads in metal. Unlike wood screws that have a smooth shank, sheet metal screws are threaded along their entire length. They're designated by gauge number like wood screws, but the thread design differs significantly.

Two main types exist: Type A (coarse thread) and Type AB (finer thread with sharper point). Type A works best for thicker sheet metal (18 gauge and heavier), while Type AB handles thinner materials (22-24 gauge) where the finer threads provide better holding power. Self-drilling sheet metal screws, sometimes called "Tek screws," feature drill-bit-like points that eliminate the need for pre-drilling when joining thin sheet metal.

When joining sheet metal, pilot hole size depends on metal thickness and gauge of screw. For 22-24 gauge metal with a #8 screw, use a 1/8-inch pilot hole. For 18-20 gauge, increase to 9/64 inch. For 16 gauge or thicker, use 5/32 inch. These larger pilot holes in thicker material prevent the screw from breaking during installation and ensure the threads cut cleanly rather than deforming the metal.

Structural Screws and Lag Screws

Structural screws and lag screws handle heavy-duty applications like deck construction, attaching ledger boards, or securing posts. Lag screws (also called lag bolts) feature hex heads driven with wrenches or sockets, and come in larger diameters: 1/4", 5/16", 3/8", 1/2", and up. They're available in lengths from 1 inch to 12 inches or more.

Modern structural screws have largely replaced lag screws in many applications. They feature aggressive threads, hardened steel construction, and can often be driven without pilot holes in softwood. They're faster to install than lag screws (no pilot hole, no wrestling with a wrench in tight spaces) and provide comparable or superior holding power. Common brands include GRK, FastenMaster, and Simpson Strong-Tie, with sizes ranging from #8 to #14 gauge and lengths from 2-1/2 inches to 12 inches.

Both lag screws and structural screws require careful sizing. When attaching a deck ledger board to a house, building codes typically specify 1/2-inch diameter lag screws or equivalent structural screws, with specific spacing requirements. Always consult local building codes for structural applications—using undersized fasteners in structural applications isn't just risky, it's likely illegal.

Specialized Screw Types

Drywall screws are ubiquitous in modern construction. They feature sharp points, aggressive threads, and bugle-shaped heads that countersink cleanly without tearing the drywall paper facing. "W" type drywall screws (coarse thread) are for wood studs, while "S" type (finer thread) are for metal studs. Standard lengths are 1-1/4" (for 1/2" drywall), 1-5/8" (also for 1/2", more common), and 2" (for 5/8" drywall).

Deck screws are designed for exterior use with corrosion-resistant coatings or materials (stainless steel, coated steel). They feature aggressive threads for strong holding power in pressure-treated lumber and heads designed to countersink cleanly. Common sizes are #8 x 2-1/2" for decking boards to joists, and #10 x 3" or longer for joist hangers and structural connections.

Confirmat screws (also called Euro screws) are specialized fasteners for modern cabinet and furniture construction. They're larger diameter (typically 7mm) with coarse threads and require specific pilot holes drilled with Confirmat bits. They create incredibly strong joints in particleboard and MDF, making them industry standard for ready-to-assemble furniture.

Drive Types: Phillips, Square, Torx, and More

The drive type—the recess or shape that accepts your driver bit—significantly affects your ability to install screws efficiently without damage. Traditional slotted drives are largely obsolete except for decorative applications, as they cam out (slip) easily and provide poor torque transfer.

Phillips drives dominated for decades due to their self-centering design, but they intentionally cam out under high torque to prevent over-tightening. This "feature" becomes a frustration with modern power drivers. Square drives (Robertson), common in Canada but increasingly popular in the US, provide excellent torque transfer and resist camming out. They're a favorite among professional carpenters.

Torx (6-pointed star) drives offer even better torque transfer and virtually eliminate camming out. They're becoming increasingly common for construction screws, especially deck screws and structural screws. The main downside is the need for Torx bits, though these are now widely available. Combination drives (like Square-X, which accepts both square and Phillips bits) offer convenience at the cost of neither working perfectly.

Material and Coating Selection

Screw material and coating must match the application, especially for exterior use. Standard steel screws will rust quickly outdoors. Galvanized coatings provide basic corrosion resistance suitable for interior pressure-treated lumber or occasional exterior exposure. Hot-dip galvanized offers better protection than electroplated galvanized.

For deck construction with pressure-treated lumber, you need screws rated for that specific use—the copper compounds in treated lumber accelerate corrosion of standard screws. Many manufacturers now offer screws specifically rated for treated lumber with advanced coatings. For coastal environments or wherever maximum corrosion resistance is needed, stainless steel screws (304 or 316 grade) are the gold standard, though significantly more expensive.

Brass and bronze screws offer corrosion resistance and aesthetic appeal for marine applications, decorative hardware, and antique restoration. However, brass is soft and breaks easily—always drill proper pilot holes and consider driving a steel screw first to cut threads, then replacing it with the brass screw.

Making Your Selection

When selecting screws for your project, consider these factors in order: First, determine the required length based on materials you're joining. Second, choose the appropriate gauge for the load and material thickness. Third, select the screw type (wood, machine, sheet metal, structural) based on materials and application. Fourth, choose drive type based on your tools and the installation conditions. Finally, select material and coating based on environmental exposure.

For common projects, here are quick recommendations: General woodworking: #8 x 1-1/4" to 2" wood screws, square or Torx drive. Drywall: #6 x 1-5/8" drywall screws for 1/2" board to wood studs. Deck construction: #10 x 2-1/2" coated deck screws for decking, structural screws for ledgers and posts. Cabinet assembly: Confirmat screws or #8 wood screws depending on construction style. Electronics: #4-40 or #6-32 machine screws. Furniture assembly: #8 x 1-1/4" to 2" wood screws or confirmat screws for engineered wood products.

Don't skimp on fasteners—using the wrong size or type can lead to joint failure, splitting, or corrosion. Quality screws cost marginally more but provide significantly better performance and reliability. Your local hardware store or home center can help match the right fastener to your specific project needs.