Wood screw size chart (free printable PDF)
How big is a #8 screw — and what's it in metric? This chart gives the screw's own dimensions by gauge: shank (thread) diameter in inches and millimetres, the nearest fraction, the flat-head width, and the nearest metric M-size. It's the screw's size, not the pilot-hole size — download the free printable PDF or print it from the page.
Wood screw size chart
| Gauge | Shank diameter | mm | Nearest fraction | Flat-head diameter | Nearest metric |
|---|---|---|---|---|---|
| #4 | 0.112" | 2.8 mm | 7/64" | 7/32"5.6 mm | M2.5–M3 |
| #6 | 0.138" | 3.5 mm | 9/64" | 9/32"7.1 mm | M3.5 |
| #8 | 0.164" | 4.2 mm | 5/32" | 21/64"8.3 mm | M4 |
| #10 | 0.190" | 4.8 mm | 3/16" | 25/64"9.9 mm | M5 |
| #12 | 0.216" | 5.5 mm | 7/32" | 7/16"11.1 mm | M5.5–M6 |
| #14 | 0.242" | 6.1 mm | 1/4" | 1/2"12.7 mm | M6 |
Shank diameter = the outer thread diameter — the screw’s thickness, which is what the gauge number encodes (a higher gauge = a thicker screw). Flat-head diameter is the head width; it’s what a countersink is sized to match. #8 is highlighted — the everyday gauge. Diameters follow the standard gauge formula (gauge × 0.013 + 0.060"); mm = inch × 25.4.
Nearest metric is the closest M-size by diameter (M = diameter in mm) — handy for flat-pack and imported hardware. Metric and gauge screws are close but not always exactly interchangeable: the exact diameter and thread pitch can differ, so these are nearest-size equivalences for general use, not precise thread specs. For an exact thread match, check the specific spec.
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Screw size vs. pilot-hole size
This chart is the screw’s own size — how thick the screw is (its shank diameter, i.e. its gauge) and how wide the head is. That’s different from the pilot hole, which is smaller than the screw, and the countersink, which matches the head. Identify the gauge here, then look up the holes.
Not sure of your screw’s gauge? See how to measure & identify a screw — measure the thread diameter and match it to a row above.
Know your gauge? Get the pilot-hole bit and material sizing in one tap.
Open the pilot-hole finderCommon questions
What size is a #8 screw?
A #8 wood screw has a shank (outer thread) diameter of 0.164" — about 4.2 mm, or the nearest fraction 5/32". Its flat head is about 21/64" (8.3 mm) across. That's the screw's own size; the gauge (#8) is the diameter code, and length is given separately in inches.
What is a #8 screw in mm?
A #8 screw is 4.2 mm in shank diameter (0.164"). It's the most common wood-screw gauge. Note this is the screw's thickness — the pilot hole you drill for it is smaller, and the countersink for its head is larger.
What is the head diameter of a #10 screw?
A standard flat (countersunk) #10 wood screw has a head about 25/64" across — roughly 9.9 mm. That head width is why a #10 flat head is countersunk with a matching bit; see the countersink size chart for the exact countersink diameter per gauge.
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Where these numbers come from
Shank diameters follow the standard wood-screw gauge formula (diameter = gauge × 0.013 + 0.060 inches), matching Bolt Depot and other references; millimetres are computed from the inch value (× 25.4). Flat-head diameters are the standard flat-head wood-screw head sizes. These are conventional reference values — a specific screw can vary a little by manufacturer and head style.
Screw dimensions are conventional standard values and can vary slightly by manufacturer and screw style (flat, pan, bugle). For a critical fit, measure your actual screw.