Screw Size Chart: Every Type, Gauge & Pilot Hole (Wood, Drywall, Metal, Machine, Lag)

A "#8 x 1-1/2 inch screw" means nothing until you know what the numbers mean — and picking the wrong gauge, length, or pilot hole is why screws strip, wood splits, or panels pull loose under load.

This guide covers sizing for every screw type you'll actually use — wood, drywall, sheet metal, machine, lag, and metric — with the pilot hole, clearance hole, and drive type for each.

The Golden Rule: Sizing Depends on the Screw Type and Material

Unlike concrete anchors, where sizing rules are dictated by mechanical expansion, screw sizing is dictated by two things: what the screw is threading into, and how that material responds to being split apart. A pilot hole that works perfectly in pine will split oak. A machine screw doesn't use a "pilot hole" at all — it needs a tapped hole cut to a specific thread-engagement percentage. Get the category wrong before you even look at a number, and no chart will save you.

Why Screw Sizing Rules Differ

Wood fibers compress and tear; a pilot hole exists to remove just enough material so the threads can bite without splitting the surrounding wood. Metal doesn't compress the same way — machine screws thread into a hole that's been cut (tapped) to an exact percentage of full thread engagement, usually 75%, because 100% engagement makes the screw nearly impossible to drive and doesn't meaningfully add strength. Drywall screws need no pilot hole at all in most cases — their coarse W-type thread is specifically designed to self-pierce the paper face and bite into wood stud without splitting either material, since drywall itself isn't structural enough to split the way solid lumber does.

Screw Size Chart by Type

Wood Screws

Wood screw sizing uses a gauge number for diameter (higher gauge = thicker screw) and a separate length in inches. For most general woodworking, #8 is the most versatile size — light enough for trim, strong enough for furniture. The screw should penetrate at least two-thirds of its total length into the receiving piece.

Full breakdown, countersink sizes, and hardwood-splitting tips: Pilot Hole Size Chart for Every Screw Gauge and Wood Screw Size Chart.

Drywall Screws

Use 1-5/8" coarse-thread W-type screws for standard 1/2" drywall on wood studs — the vast majority of residential work. Switch to S-type fine-thread for metal studs. No pilot hole needed on either type; the self-piercing tip is the entire point of the design.

Full chart by thickness and spacing: Drywall Screw Size Chart.

Machine Screws (UNC / UNF)

Machine screws thread into a tapped hole, not a wood-style pilot hole. US sizes run #0 through #12, then by fractional inch. The first number is diameter, the second is threads per inch (TPI) — a "6-32" screw is a #6 diameter with 32 threads per inch. UNC (coarse) is standard for general use; UNF (fine) is preferred in thin material and slightly stronger.

Source: standard tap drill tables per ASME B1.1, sized for approximately 75% thread engagement — the industry-standard balance of strength and driveability.

Sheet Metal Screws

Self-tapping sheet metal screws still need a pilot hole — "self-tapping" refers to cutting threads, not drilling through steel. Size the pilot to the screw's root (minor) diameter, and go slightly larger in thicker or harder stock so the screw doesn't snap. Self-drilling (Tek) screws with a drill-point tip skip the pilot entirely in steel up to their rated thickness.

Lag Screws

Lag screws need two holes: a clearance hole through the piece being fastened, sized to the shank diameter, and a pilot hole in the receiving wood, sized smaller — roughly two-thirds of the shank diameter in softwood, closer to the thread's root diameter in hardwood.

Metric Screws (M-Series)

Metric screw sizing uses "M" followed by the nominal diameter in millimeters — an M6 screw has a 6mm major diameter. Tap drill size for metric machine screws follows a simple formula: major diameter minus thread pitch, rounded to the nearest standard bit. For an M8x1.25 screw: 8.0 − 1.25 = 6.75mm, rounded up to 6.8mm.

Clearance hole values per ASME B18.2.8 (normal fit). Tap drill values calculated per the major-diameter-minus-pitch formula, standard for 75% thread engagement.

Drive Types and Head Styles

The drive type determines how much torque you can apply before the bit slips (cams out) and strips the screw head. Here's a real, non-obvious fact: Phillips was deliberately engineered to cam out. When it was developed in the 1930s, the angled flanks were designed to let the driver slip at a set torque specifically to protect the lightweight power tools of that era from over-tightening. It's not a design flaw — it's doing exactly what it was built to do, which is also why it strips so easily under a modern high-torque impact driver.

• Phillips — the most common drive in North America. Sizes #0 through #3, with #2 covering most general-purpose screws. Prone to cam-out by design.
• Torx (star) — the best cam-out resistance of any common drive. Sizes T10 through T30 cover most construction screws. Increasingly standard on premium deck and structural screws.
• Robertson (square) — a slight taper lets the screw self-center and stay on the bit for genuine one-handed driving. Dominant in Canada, less common in the US.
• Hex / Allen — strong torque transfer in a simple, low-cost shape. Standard for flat-pack furniture and machine hardware.

Pilot Hole vs. Clearance Hole vs. Countersink — What Each Does

These three holes serve completely different jobs, and mixing them up is the single most common sizing mistake.

A pilot hole is drilled in the piece receiving the threads — it's smaller than the screw so the threads have something to bite into. A clearance hole goes through the top piece being fastened, sized to the screw's shank, so the screw passes through freely and clamps the two pieces together rather than just threading into both. A countersink recesses the head flush with (or just below) the surface. Using a pilot-hole-sized bit where you need a clearance hole is a common mistake — it makes the screw thread into both pieces instead of clamping them, and the boards never pull tight.

Common Mistakes That Strip Threads or Split Wood

Choosing the Right Screw

Quick-Reference Summary

Frequently Asked Questions

Related Guides

• Pilot Hole Size Chart for Every Screw Gauge — Full breakdown, every gauge and material
• Wood Screw Size Chart — Complete guide to types, gauges, and lengths
• Drywall Screw Size Chart — Coarse vs fine thread, W-type vs S-type
• Structural Screws vs Lag Bolts — Strength, code, and cost compared
• Drill Bit Selector — Instant bit recommendation for any material