Screw Size Chart (Inch & Metric) + Photo Identifier

Number screw sizes (#0 to #14)

The diameter follows a simple rule: 0.060 in + 0.013 in x the gauge number. Wood screws use the same gauge diameters (ASME B18.6.1) but have their own coarse threads. #14 is mostly a wood screw size; machine screws jump from #12 to 1/4 in.

Fractional screw sizes (1/4 in and up)

Larger sizes, with wrench sizes, are on the bolt size chart.

Metric screw sizes (M2 to M12)

If a metric screw is sold without a pitch (just "M6"), it's the coarse pitch. Fine pitches for M2–M6 exist but are rare in hardware stores, so we list fine pitches from M8 up.

Wood screw pilot hole sizes

These match Bolt Depot's and Sawinery's charts. Pilot hole charts vary: Engineers Edge, for example, lists smaller holes (3/32 in for a #8 in hardwood). Test in scrap. If the wood splits, go up 1/64 in; if the screw strips or won't hold, go down. Tapered countersink bits use different sizes, so follow the bit maker's chart.

How to measure a screw

1. Diameter: measure across the threads with calipers. Compare it to the tables above. Expect a reading a hair under nominal.
2. Thread pitch: use a thread gauge. Without one, count threads along 1/2 in and double it for threads per inch (inch screws), or measure across 10 threads and divide by 10 for the pitch in mm (metric).
3. Length:
   • Pan, round, truss and hex heads: from under the head to the tip.
   • Flat (countersunk) heads: overall, from the top of the head to the tip.
4. Head and drive: note the head shape (flat, pan, hex, button) and the drive (Phillips, slotted, Torx, hex socket).

Common mistakes

• Mixing up #10 and M5. #10 is 4.83 mm, M5 is 5.0 mm, and 10-32 and M5 x 0.8 have almost the same pitch. They don't interchange.
• Buying the wrong thread for the size. #10 comes in 10-24 and 10-32, and 1/4 in comes in 1/4-20 and 1/4-28. Count the threads.
• Measuring a flat-head screw from under the head. You'll buy one too short.
• Using a machine screw in wood (or a wood screw in a tapped hole). Wood screws have coarse threads and a pointed tip; machine screws need a matching nut or tapped hole.
• Skipping the pilot hole in hardwood. That's how screws snap and boards split.

Don't want to measure? Snap it

Snap the old screw next to a ruler and Mechy tells you the size, thread and length. If the thread has to be exact, confirm it with a thread gauge or a nut that fits. Identify a screw from a photo.