Screw sizes explained: how to measure & identify a screw

Last updated: July 12, 2026

A wood screw's size is written as a gauge number and a length (for example, a #8 × 1½" screw). The gauge is a diameter code, and it trips people up because it works the opposite way to drill bits. Here's what it means, how to measure a screw you already have, and how to turn that into every hole size you need.

The short version: a wood screw is sized by gauge (its diameter, #4 to #14 for most work) and length (in inches). A higher gauge number means a thicker screw, the reverse of drill-bit numbers. Measure the outer thread diameter, match it to a gauge, and you can look up everything else.

What the gauge number means

The gauge is a code for the screw's diameter, specifically the outer (major) diameter across the threads. Unlike drill-bit numbers, which get smaller as the number climbs, screw gauges run the intuitive way: a #6 is thinner than a #8, which is thinner than a #10. Most wood screws you'll meet are #4 through #14, with #8 the everyday workhorse.

There's a simple formula behind it: diameter (inches) = gauge × 0.013 + 0.060. So a #8 is 8 × 0.013 + 0.060 = 0.164", and a #10 is 0.190". You never have to do the math (the wood screw size chart lists every gauge's diameter in inches and millimetres), but it's why the numbers step up evenly.

Gauge → diameter quick reference

Match a caliper reading to a gauge with this. It's the shank (outer thread) diameter for every gauge, #2 to #14 — the odd sizes (#3, #5, #7, #9) sit between the common evens:

• #2 — 0.086" (2.2 mm)
• #3 — 0.099" (2.5 mm)
• #4 — 0.112" (2.8 mm)
• #5 — 0.125" (3.2 mm)
• #6 — 0.138" (3.5 mm)
• #7 — 0.151" (3.8 mm)
• #8 — 0.164" (4.2 mm)
• #9 — 0.177" (4.5 mm)
• #10 — 0.190" (4.8 mm)
• #12 — 0.216" (5.5 mm)
• #14 — 0.242" (6.1 mm)

How to measure and identify a screw

Say you've got a screw in hand and no packet. Three quick measurements identify it:

1. Diameter (the gauge), with calipers. Close the jaws of a caliper across the outer edges of the threads, near the middle of the screw. Read it in inches (or mm) and match it to the table above: 0.164" is a #8, 0.190" is a #10. This is the single measurement that gives you the gauge.
2. Diameter, with a ruler. No caliper? Lay the screw on a ruler and measure the thread width as best you can, or lay several thread crests against the scale. It's less precise, but it'll get you to the right gauge — and a reading that falls between two even gauges usually means an odd gauge (#3, #5, #7, #9), not a bad measurement.
3. Length, measured separately. Length is not part of the gauge; it's stated in inches. Measure a flat/countersunk screw from the top of the head to the tip (it sits flush, so the whole thing counts). Measure a pan or round head from under the head to the tip. So a #8 × 1½" screw means a #8-gauge screw, 1½" long.

While you're at it, note the <b>head type</b> — flat (countersunk, sits flush), pan or round (sits on the surface), or bugle (drywall/deck). The head type decides whether you countersink, and a flat head's width is what sets the countersink size.

Now get every hole size that goes with it

Once you know the gauge, sizing the holes that go with the screw is just a lookup. There are three you might drill:

• Pilot hole — the hole the screw threads into. Pick your gauge and material in the pilot-hole size finder and it gives you the exact bit.
• Countersink — the bevel so a flat head sits flush; it matches the head diameter. See the countersink size chart.
• Clearance hole — the shank hole in the top board so the joint pulls tight. The three-holes guide covers when you need it.

Metric vs. gauge screws

Gauge is the US/imperial system. Much of the world (and most flat-pack and imported furniture) uses metric screws instead, sized by M-number, where the M is simply the diameter in millimetres: an M4 is 4 mm across, an M5 is 5 mm. Because a gauge is also just a diameter code, the two line up by size: a #8 ≈ M4, a #10 ≈ M5, a #6 ≈ M3.5. The wood screw size chart lists the nearest metric size for every gauge.

They're close, but not always exactly interchangeable. The exact diameter and the thread pitch (how far apart the threads sit) can differ between a gauge screw and its nearest metric size. For general DIY and matching furniture hardware, the nearest-size equivalence is fine; for a precise thread match (driving into an existing metric-threaded insert, say), check the specific thread spec.

Machine screws are measured differently

One last distinction: <b>machine screws</b> (uniform, blunt-tipped threads for a nut or a tapped hole) use the same #-gauge numbers but are specified by diameter <em>and</em> threads-per-inch (like "#8-32"), not by gauge alone. That's a different system from wood screws, and identifying a random fastener off a specific appliance or vehicle is its own rabbit hole. This guide is about identifying a standard wood screw.

The bottom line

A wood screw is a gauge (diameter) plus a length. Measure the outer thread diameter with calipers (or a ruler in a pinch), match it to a gauge, and note the head type. From there the pilot, countersink and clearance sizes all follow. Identify the screw once, and every hole size is a lookup away.