Countersink sizes: wood screws, machine screws, metric

A countersink is the cone-shaped recess that lets a flat-head screw sit flush with the surface, and its size is just two numbers: a diameter matched to the screw head, and an angle matched to the screw standard ^{[3] [5]}. This page prints three charts: flat-head wood screws by gauge, flat-head machine screws from #2 to 1/2 inch, and metric M3 to M10. Every row cites two or more named sources, and where publishers disagree, the row says so.

Chart 1: flat-head wood screws (82 degrees)

Woodworking countersinks are matched to flat-head wood-screw heads. The diameters below are the practical woodworking sizes, and W.L. Fuller, the taper-bit maker whose chart is the woodworking reference, publishes the underlying head bores ^{[1] [2]}:

Sources vary slightly at #12 (7/16 to 1/2 in); the woodworking chart errs to 1/2 in so the head is sure to seat ^[1]. Fuller's own head bores run a touch smaller than the woodworking diameters; the difference is the clearance that guarantees flush. Drill the pilot before the countersink, and pair every gauge with the right pilot from our pilot hole chart.

Chart 2: flat-head machine screws, #2 to 1/2 inch

Machine-screw countersinks are an ANSI affair, published as body drill (clearance for the shank), countersink diameter, and pilot hole. Two independent machining references print identical tables ^{[3] [4]}:

A third machining publisher's 82 degree table differs on some rows (it prints body diameters about 0.06 in larger and fraction-granularity drills); we follow the two tables that agree row for row and note the variance rather than average it ^[5].

Chart 3: metric, M3 to M10 (90 degrees)

Metric flat-head screws are countersunk at 90 degrees, and pilot holes come in close, normal, and loose fits (H12/H13/H14). Two engineering references publish these charts with minor differences, so the table shows the spread ^{[6] [7]}:

The spread is real: one reference derives the theoretical sharp diameter, the other a nominal head diameter. For a flush finish in wood or plastic, start at the smaller number and test on scrap; in metal, follow your drawing's fit class. Metric threads also need their own tap drills, on our tap drill chart page.

Countersink vs counterbore

The two recesses solve different problems, and the confusion costs money in the wrong bit ^{[1] [3]}. A countersink is a cone for a flat-head screw to sit flush. A counterbore is a straight-walled, flat-bottomed pocket used to sink a head below the surface for a wood plug, or to seat a socket-head cap screw or a washer. Fuller's chart is built around exactly this pairing: for #8, an 11/32 in head bore with a 3/8 in counterbore for the plug ^[1]. If you are drilling plugs to cap the screws, cut the counterbore with a Forstner bit; the sizes for that are on our Forstner bit sizes page.

How to actually cut one

Order of operations, agreed across the machining references ^{[3] [1]}: drill the pilot hole first, then the body (clearance) hole, then cut the countersink to depth, checking against the actual screw on scrap before running the workpiece. In wood, a combination pilot-and-countersink bit (Fuller's C-numbered taper drills, or any countersink-pilot combo) does both in one pass and keeps the cone concentric with the pilot. Depth rule: countersink until the head sits flush; slightly deeper if you want it below flush for filling. Chatter in a hand drill is cured by lower speed and starting the cone before the pilot is at full depth.

Speeds for the bits themselves follow the same material logic as ordinary twist drilling; the numbers are on our drill speed chart, and the driver settings that avoid cam-out are on drill torque settings.

Related pages

• Pilot hole chart: the pilot size for every screw, before any countersinking
• Forstner bit sizes: counterbore and plug cutting in wood
• Tap drill chart: metric and SAE tap drills
• Screwdriver bit types: PH, PZ, Torx, hex drivers
• Self tapping vs self drilling screws: the sheet-metal screws that pair with countersunk heads in metal work
• Counterbore vs countersink: the geometry and drawing-symbol reference behind every table on this page

Sources

1. W.L. Fuller, Wood Screw Chart (actual head sizes, minimum counterbore diameters, recommended counterbore sizes, straight pilot drills by gauge, C-series countersink numbers): wlfuller.com
2. WoodWiki, Wood screw sizes guide (gauge diameters, #8 as the cabinet default, application table by gauge): woodwiki.org
3. RivCut, Countersink Size and Angle Chart (82/90 degree flat-head machine-screw table, #2 to 1/2 in, body drills and pilot holes, angle-by-standard guidance): rivcut.com
4. Xtmade, Countersink Size Chart (identical 82/90 degree machine-screw tables, independent corroboration of RivCut row by row): xtmade.com
5. CNCJY, Size Charts for Typical Countersink Dimensions (divergent 82/90/100 degree tables and angle-by-use-case matrix; variance noted, not averaged): cncjy.com
6. EngineerLaboratory, Countersunk Hole Size Chart for Socket Flat Head Machine Screws, ANSI Metric (pilot fits H12/H13/H14, countersink diameters M2-M10 at 90 degrees): engineerlaboratory.com
7. EngineersBible, Countersunk Hole Size for Socket Flat Head, ANSI Metric (independent M3-M24 chart; M6 13.71mm example, spread vs EngineerLaboratory disclosed in the table): engineersbible.com
8. WhatDrillBit, Wood Screw Countersink Size Chart (woodworking diameters by gauge #4-#14, plug/bung counterbore column, 82 degree wood-screw standard): whatdrillbit.com