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 = 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 each common gauge:

• #4 — 0.112" (2.8 mm)
• #6 — 0.138" (3.5 mm)
• #8 — 0.164" (4.2 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 if you're between two, the chart's millimetre column helps you decide.
3. Length — 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

This is the payoff: once you know the gauge, you know everything else about the holes. Here's the on-ramp:

• 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.