Forstner bit sizes: the charts, inch and metric

A Forstner bit drills a clean, flat-bottomed hole guided by a center point and a rim that shears the outer edge, which is why cabinetmakers reach for one at hinge cups, dowel joints, and pocket holes ^{[3] [1]}. This page prints the actual size charts: the inch set contents from CMT and Irwin Marples, the metric sets from 10 to 55mm, and the maximum RPM by diameter. Every number cites two or more named sources; nothing is converted between unit systems.

What a Forstner bit is for

The design brief is clean, precise, flat-bottomed (blind) holes and pocket holes in wood, which is exactly how Irwin positions its Marples line: tough carbon steel, ultra-sharp edges, for hard and soft woods ^{[3] [2]}. The rim-guided cut means you can drill overlapping holes, angled holes, and holes on the edge of a board that a twist drill would wander on. What it is not for is metal, concrete, or speed: the large contact area heats fast, and the honest operating mode is slow RPM, steady feed, and frequent chip clearing. For holes in other materials see our drill bits by material page, and for thin sheet metal a step bit is the right tool instead.

The inch size ladder, and what sets actually contain

Inch Forstner bits are made from 1/4 to 3 inches, with 1/16 in increments available as individual bits ^[1]. But nobody buys 45 individual bits, so the practical question is set contents. Three widely stocked sets, verbatim:

Two consequences of the skip pattern. First, hole sizes between the steps (11/16, 13/16, and so on) mean buying an individual bit, not rounding down, because a Forstner hole is the finished hole. Second, 1-3/8 in appears in inch sets precisely because it is the imperial stand-in for the 35mm hinge cup ^{[1] [6]}.

Bits at the top of the range exist past 2-1/8 in: 2-1/2 and 3 in bits are sold individually and in carbide-tipped sets, and the largest common carbide-tipped sets reach 55mm ^{[1] [6]}. Past about 2 in, though, a hole saw is usually the better tool for through-holes; the Forstner advantage is the flat bottom, which a hole saw cannot give.

The metric chart

Metric Forstner bits typically run 6 to 50mm in individual sizes, with 10 to 50mm the common set span and 35mm near-universal for hinge work ^{[1] [6]}. A representative 10 piece carbide-tipped (TCT) set sold for hardwood hinge and cable work contains:

Note the unit-system trap: a 1-3/8 in bit is 34.9mm, not 35mm. For a European hinge cup use the true 35mm bit; 34.9mm will bind. This is the same no-conversion convention we keep on the masonry sizes page.

Speed: maximum RPM by diameter

Forstner bits must slow down as they get bigger, or the rim burns. Two publishers print the same ladder; CMT also publishes per-bit maxima that agree at the small end and run more conservative in hardwood ^{[1] [4]}:

Handheld drills are workable at the small end (CMT lists its smaller bits as suitable for electric and cordless drills in softwood ^[4]); from about 1-1/4 in, clamp the workpiece and use a drill press, clearing chips on every retract. For material-specific speeds on ordinary twist bits, see the drill speed chart.

Shanks and chucks

The Irwin Marples set uses a reduced shank so the bits fit 3/8 in and larger chucks, which is the practical floor for cordless drills ^{[2] [3]}. Larger bits in the 16 piece set and big singles still need that 3/8 in minimum; if your drill is a 10mm (3/8 in) chuck machine you are at the edge, and a 13mm chuck gives headroom. The trade-offs between chuck sizes are on our drill chuck sizes page.

Heat is the enemy

CMT's usage notes are blunt: repeated shallow cuts with two or three seconds of cooling between passes extend bit life, because chip buildup and constant contact remove the temper from the cutting edges ^[4]. Excessive force overheats the bit the same way. The practical drill: plunge a second or two, retract to clear chips, plunge again, and if the shank is too hot to hold, the RPM or the feed is wrong. Dull bits can be brought back; the angles and jigs for that are on our drill bit sharpening page.

Related pages

• Drill bits by material: which bit family for wood, metal, concrete, tile
• Step drill bit sizes: thin-material alternative with one bit, many sizes
• Hole saw sizes: big through-holes where a Forstner is too slow
• Pilot hole chart: screw pilot sizes before any countersinking
• Counterbore vs countersink: symbols, hole geometry, and which screw head needs which

Sources

1. WorkshopCalc, Forstner bit size chart reference (inch range 1/4 to 3 in at 1/16 increments, metric 6-50mm, 35mm hinge note, RPM band table, set size norms): workshopcalc.com
2. JB Tools, Irwin 14-piece Marples Forstner set 1966893 listing (verbatim size list, carbon steel, reduced shank, 3/8 in and larger chucks): jbtools.com
3. Irwin Tools, Marples Forstner drill bits product page (carbon steel, pocket and flat-bottomed holes, reduced shank, hard/soft wood positioning): irwintools.com
4. McFeelys, CMT Forstner bit sets page (7-piece and 16-piece verbatim contents, per-bit RPM table softwood/hardwood, overall length 3-35/64 in, cooling and overheating guidance): mcfeelys.com
5. Lowe's, IRWIN Marples 14-piece woodboring Forstner set listing (same SKU 1966893 and contents as JB Tools, independent retailer corroboration): lowes.com
6. CherryPicks, Forstner drill bits roundup (Vouyar 5-piece 5/8, 3/4, 1, 1-1/4, 1-3/8 in set and BeQell 10-piece 16-55mm TCT set listings with lengths): cherrypicksreviews.com
7. Grove Park, Forstner bits guide (10-piece TCT set 16-55mm listing, 85mm total length, hinge-cutter positioning): groveparkgolf.com