How to measure a drill bit size: calipers, gauge plates, and worn markings
The size of a drill bit is one number: the diameter of the hole it drills, set by the widest part of its cutting flutes. Everything else on the bit, the shank, the length, the stamp, is either a convenience or a trap. This page covers the four ways to recover that number when the stamp has worn off or never existed: calipers, a drill gauge plate, a string-and-ruler fallback, and decoding whatever ghost of the marking survives [1] [3]. It then works the measurement the other direction, the situations people actually measure bits for: choosing a bit for a wood screw (pilot or clearance), for an anchor or wall plug, for masonry and concrete, and reading a size in millimeters versus inches. Full size tables live elsewhere on this site; the small cross-reference below is orientation only.
The one-minute answer
Read the shank first: most bits carry their size stamped or laser-etched there, as a fraction, a number or letter gauge, or millimeters [4] [5]. If the stamp is gone, close digital caliper jaws across the widest point of the flutes near the tip, on the raised margins at the flute edges, and read to 0.001 in or 0.01 mm [1] [2]. Never measure the smooth shank: on reduced shank bits it is smaller than the hole the bit drills [1] [3]. Take two or three measurements with the bit rotated between them and average, because bits are not perfectly round and wear makes them less so [1] [16].
Pick the method by situation
| Situation | Method | Watch out for | Sources |
|---|---|---|---|
| Stamp on the shank is legible | Read it. Fractions are inches, a bare number or letter is a wire gauge size, a number like 6.0 is millimeters | Number gauges run backwards: #80 is 0.0135 in, #1 is 0.2280 in | [1] [5] |
| Stamp worn off, calipers on hand | Jaws across the widest part of the flutes, near the tip | Not the shank; not the flute valleys, which read small | [1] [3] |
| Reduced shank bit, big hole, small chuck | Measure the cutting flutes only. On Silver and Deming bits the shank is 1/2 in while the flutes run 9/16 to 1 1/2 in | The stamped size is the cutting diameter, never the shank | [5] [17] |
| Reading seems oval or the bit is old | Measure 2 to 3 times, rotating the bit between jaw closings, and average | A single caliper reading can flatter a worn, out-of-round bit | [1] [16] |
| A shoebox of unmarked bits to sort | Drill gauge plate: find the smallest hole the bit enters with a snug, wiggle-free fit | Gauge plates are go/no-go, not precision; calipers beat them for close work | [2] [3] |
| No calipers and no gauge | Wrap string around the flutes, mark the overlap, measure against a ruler, divide by pi (3.14) | Approximate only: fine for a 10 mm plug, useless for telling 5/32 from 11/64 | [20] [21] |
| Tiny number-gauge bit | Gauge plate preferred: number and letter sizes are hard to caliper accurately at that scale | A #53 and a #54 differ by 0.0045 in, thinner than most caliper technique | [2] [6] |
| Bit for a wood screw | Measure the screw's root diameter, then pick a pilot bit just under it, or a clearance bit just over the shank | Pilot and clearance are different holes; charts exist because eyeballing fails | [7] [8] |
| Bit for a wall plug or anchor | Match the anchor body's outer diameter, ignoring ribs; plug and bit are the same number | Metal anchors want an exact hole; plastic in soft walls may want slightly small | [9] [11] |
| Masonry bit for a concrete anchor | Measure the carbide tip's cutting diameter; match it to the anchor's required hole | Worn masonry bits drill undersized holes and the anchor will not set | [10] [15] |
Calipers: where the jaws go, and why
Digital calipers are the reference method because they read the cutting diameter directly, to 0.001 in or 0.01 mm, without the parallax guessing a round object against a flat ruler invites [3] [4]. Placement is the whole game. A twist bit's widest surface is the margin, the narrow raised band running along the outer edge of each flute; the jaws must sit on the margins at the widest point, near the tip [3] [2]. Dropped into the flute valleys, the jaws read noticeably small; set on the shank they read wrong whenever the shank is reduced [3] [1]. Hold the bit clean, close the jaws firm but without squeezing, and take the reading at the widest point you can find while rotating slightly [2] [1].
One reading is one opinion. Bits come off the grinder not perfectly round, and twin-flute twists can even drill slightly lobed holes in thin material, so practitioners take two or three measurements at different rotations and average them [1] [16]. If your three readings of a nominal 10 mm bit give 9.8, 9.85, 9.82, that agreement tells you more than any single number.
Reduced shank and Silver and Deming: the shank will lie to you
Large-diameter bits are routinely sold with the shank turned down to 1/2 in so they fit a standard drill chuck: the Silver and Deming pattern carries cutting diameters from 9/16 up to 1 1/2 in on a 1/2 in shank, in a fixed 6 in overall length [17] [18]. A 3/4 in Silver and Deming bit therefore measures 0.500 in on the shank and 0.750 in across the flutes, and the stamped size, when present, is the cutting diameter, the hole size, not the shank [5] [17]. The same logic applies in miniature: some makers undersize shanks slightly so the bit does not bind in the hole it is drilling [4]. The rule that survives all of this: the flutes define the hole; measure the flutes.
The drill gauge plate: fast, and honest about its limits
A drill gauge (bit gauge, index plate) is a steel or plastic plate drilled with a graduated row of holes, each labeled with its size. You insert the bit and find the smallest hole it enters with a snug, wiggle-free fit; that label is the size [2] [4]. For sorting a inherited box of bits, nothing is faster, and for number and letter gauge sizes, the sub-1/4 in wires where caliper technique struggles, several references prefer the plate outright [2] [6].
Two cautions. First, sources diverge on which end to insert: one guide says start with the shank end, another insists on inserting the flutes because they are the widest part and define the hole [3] [6]. We disclose the divergence rather than average it. On a standard jobber bit, where shank and flutes share a diameter, both give the same answer; on a reduced shank bit they do not, and the flute reading is the one that matches the hole the bit drills. Second, a gauge plate is a go/no-go test with the plate's own tolerance inside it; when a job needs a real diameter, for tapping or a press fit, calipers are the tool [2] [4].
String and ruler: the fallback, with its limits stated
With no calipers and no gauge, wrap a length of thread or thin string snugly around the widest part of the flutes, mark where it overlaps itself, lay it against a ruler, and divide that circumference by pi (3.1416) to get the diameter. A 25 mm wrap is a 7.96 mm bit, effectively 5/16 in. Method guides accept the ruler route for getting a general idea on larger bits while flagging that precise sizing needs the proper tools [20] [21], and we flag it too: string stretches, sits in the flute valleys, and reads small. Treat the result as plus or minus half a millimeter, fine for matching a colour-coded wall plug, not fine for choosing between adjacent fractional sizes 0.4 mm apart.
Reading what survives of the marking
Before measuring anything, give the stamp a last chance. Size markings live on the shank, usually near the chuck end where drilling abrasion cannot reach them, often beside a material code like HSS [5] [4]. Repeated chucking polishes them away, but a 5x magnifier often shows a ghost of the indent, and two documented tricks recover faint stamps: a gentle rub with 0000 steel wool to clear rust and debris, and colouring the area with permanent marker then wiping it off, which leaves ink trapped in the indent [6].
Once legible, decode the system. Four are in circulation [1] [5]:
- Fractional inch, the North American default: 1/16, 5/64, 3/32 and so on in 1/64 steps. Decimal value is numerator over denominator: 7/32 is 0.2188 in [1] [5].
- Number gauge, #1 through #80, filling the range below 1/4 in. Runs backwards: #1 is 0.2280 in, #60 is 0.0400 in, #80 is 0.0135 in [1] [5].
- Letter gauge, A (0.2340 in) through Z (0.4130 in), sitting between the big number sizes and 1/2 in fractional [1] [19].
- Metric, the rest of the world: the stamped number is the diameter in millimeters, so a 6.0 bit drills a 6 mm hole [1] [5].
Every one of these converts through the same bridge: 1 inch is exactly 25.4 mm [5] [19]. The complete number, letter, and fractional tables with every decimal and millimeter equivalent are on our jobber drill bit sizes page; the short orientation table later on this page is not a substitute for it.
Measuring for a screw: pilot and clearance are different holes
When the goal is a bit for a screw, measure the screw, and first decide which of two holes you are drilling [8]. A pilot hole goes in the receiving piece and is slightly smaller than the screw's root diameter, so the threads still bite; too small and the wood splits, too large and the screw strips [8] [7]. A clearance hole goes through the top piece and is larger than the shank so the screw slides freely and its head clamps the joint shut [8]. A #8 wood screw illustrates the spread: a 7/64 in pilot in softwood, 1/8 in in hardwood, and an 11/64 in clearance hole through the top board [8], agreeing with Bolt Depot's straight-bit columns for the same gauge [7].
Published pilot charts disagree with each other by a drill size here and there, usually over how tight the pilot should be in softwood [8]; the full gauge-by-gauge tables, hardwood and softwood, tapered and straight, are on our pilot hole chart page, and tap drilling for machine threads has its own tap drill chart. What measuring adds is the cases the charts cannot see: a screw of unknown gauge gets its root diameter measured with the same calipers, then matched to the nearest chart row.
Measuring for a wall plug or anchor: match the body
Anchors are the reason most people measure bits at all, and the rule is short: the hole matches the anchor. For expansion anchors in masonry, wedge and sleeve anchors included, the hole diameter equals the anchor's outer diameter; a 1/2 in wedge anchor demands a 1/2 in hole, with no fudge in either direction [10] [13]. For plastic wall plugs, the bit matches the plug's nominal diameter: the packaging says so, and when the packaging is gone, measure the plug body with calipers and pick the same-number bit [9] [11]. Measure the body, not the ribs or flanges; ribs are meant to bite into an interference fit [12].
The one sanctioned exception: plastic plugs in soft or crumbly substrate, where the wall itself may enlarge under a full-size bit. Some guidance then drills slightly small, a 5.5 mm hole for a 6 mm plug, or 1/16 in under in imperial walls, so the plug grips and does not spin [12] [13]. UK colour-coded plugs make the matching visual: yellow 5 mm, red 6 mm, brown 7 mm, each with a same-number drill bit, per Rawlplug's own product data [11] [22]. Blue is where published tables diverge, 8 mm in some charts and 10 mm in others, and the colour code is a convention, not a standard, so the packaging wins [22]. Hole depth matters as much as diameter: drill at least plug length plus 5 to 10 mm so debris has somewhere to go [9] [22]. The full anchor-to-bit tables, plastic anchors and mollies and toggles for drywall on one page and concrete anchors on another, are our wall anchor sizes and concrete anchor bit size guides.
Masonry and concrete bits: measure the carbide, respect the wear
A masonry bit's cutting diameter is set by its carbide tip, which is usually a touch wider than the steel body behind it, so the calipers belong on the tip's widest edges, not the shank and not the body flutes. The anchor hole a rotary-hammer bit drills only matches its nominal diameter while the carbide is within spec: makers grind a visible wear mark into premium SDS bits, and once the cutting edges wear down to that mark the bit can drill an undersized, out-of-round hole and the anchor will no longer set to its rated values [15] [10]. A worn masonry bit that measures 9.8 mm across the carbide is not a 10 mm bit for anchor work. Measuring is also the honest check on bits sold loose: hold the tip against the anchor body, flutes to body, and the two diameters should align [12] [10]. The inch and mm size runs by shank type are cataloged in our masonry bit size chart and SDS bit size chart.
Millimeters or inches: reading the same bit both ways
Which system you read depends on the plug, screw, or chart in your other hand, and the conversion is one multiplication either direction: inches times 25.4 gives millimeters, millimeters divided by 25.4 gives inches [5] [19]. The same reading expressed in mm or in inches describes the same bit, which is why measuring drill bits in mm is the same procedure as measuring them in inches; only the display differs. Most digital calipers have a unit button, which removes the arithmetic entirely and is the safer route, because 6 mm and 1/4 in sit 0.35 mm apart and a slip lands you on the wrong plug. Two anchors of the system worth knowing: metric sets step in 0.5 mm or 1 mm increments from about 1 to 13 mm [5], and fractional sets step in 1/64 in, which is 0.397 mm, so near the middle of the range most metric sizes have no exact fractional twin and every match is an approximation to state, not a coincidence to claim.
Quick cross-reference: fraction, decimal, millimeters
This table is orientation for the sizes that come up in plug, screw, and anchor work, not a reference: between almost any two entries below sit number, letter, and finer fractional sizes. The complete tables are on the jobber drill bit sizes page [19] [23].
| Fraction | Decimal (in) | Millimeters | Closest metric bit | Sources |
|---|---|---|---|---|
| 1/16 | 0.0625 | 1.59 | 1.5 or 2 mm | [19] [23] |
| 3/32 | 0.0938 | 2.38 | 2.5 mm | [19] [23] |
| 1/8 | 0.1250 | 3.18 | 3 mm | [19] [23] |
| 5/32 | 0.1563 | 3.97 | 4 mm | [19] [23] |
| 3/16 | 0.1875 | 4.76 | 5 mm | [19] [23] |
| 13/64 | 0.2031 | 5.16 | 5 mm | [19] [23] |
| 1/4 | 0.2500 | 6.35 | 6 mm (0.35 mm smaller) | [19] [23] |
| 5/16 | 0.3125 | 7.94 | 8 mm | [19] [23] |
| 3/8 | 0.3750 | 9.53 | 10 mm (0.47 mm larger) | [19] [23] |
| 1/2 | 0.5000 | 12.70 | 13 mm | [19] [23] |
Three mistakes that give the wrong number
Measuring the shank. The single most common error, and the most expensive on reduced shank stock: the smooth end reads 1/2 in on a bit that drills 3/4 in holes [1] [3]. On ordinary jobber bits shank and flutes agree, which trains the habit that the Silver and Deming bit then punishes.
Trusting a worn bit's nominal size. A bit that measures at its stamp can still drill undersized: as the peripheral corners of a drill wear, the hole it produces comes out smaller than nominal, and worn masonry bits are retired by their wear mark for exactly this reason [14] [15]. The converse also circulates, the claim that drills always cut a hair oversize; machinists' threads push back with 0.5 mm undersize anecdotes on the same pages, which is the tolerance conversation, not a law [16]. For anchor holes and press fits, measure the hole the bit actually drilled, not the number on the bit. When the fix is sharpening rather than retirement, that is our drill bit sharpening page's territory.
One reading, treated as gospel. Ovality from manufacture, wear, and regrind means a single caliper measurement is a sample of one [1] [16]. Rotate and repeat, and if the spread between readings exceeds a couple of hundredths of a millimeter, the bit is out of round and the average, not any single reading, is your best estimate of the hole it will drill.
Related
For the complete fractional, number, and letter tables with decimal and mm equivalents: jobber drill bit sizes. For bit choice per screw gauge: pilot hole chart, and for machine threads: tap drill chart. For anchors: wall anchor sizes covers drywall and hollow wall, concrete anchor bit sizes covers masonry. For the masonry and SDS size runs: masonry bit sizes and SDS bit sizes. For worn but salvageable bits: drill bit sharpening. For what the chuck on the other end can hold: drill chuck sizes. Fiberglass wears size stamps off fast and eats small bits; the drilling technique that limits both is in how to drill into fiberglass. Aluminum is gentler on markings but harder on technique; see how to drill into aluminum.
Sources
- Wryno Global, How to Read a Drill Bit Size Chart: metric, fractional, and wire gauge systems with #1 at 0.2280 in and #80 at 0.0135 in; caliper method spelled out as measure the cutting diameter not the shank, across the flutes at the widest point, 2 to 3 measurements averaged; fraction-to-decimal by dividing numerator by denominator (wrynoglobal.com/how-to-read-drill-bit-size-chart)
- Engineer Fix, How to Tell What Size a Drill Bit Is: caliper jaws perpendicular to the flutes capturing the full diameter across the cutting lips, cutting diameter rather than shank, firm contact without deforming; drill gauge or index plate usage, smallest snug hole, and its advantage for number and letter sizes that are difficult to caliper (engineerfix.com/how-to-tell-what-size-a-drill-bit-is)
- HomeCorner, How to Measure a Drill Bit: the shank fallacy on reduced shank bits; margins as the widest point and flute valleys reading small; gauge plate as binary go/no-go; digital calipers reading to 0.001 in (homecorner.blog/how-to-measure-drill-bit-sizing)
- HomeCorner, How to Figure Out the Size of a Drill Bit: shank markings in fractional, metric, or letter stamps; gauge plate snug-fit rule; calipers for high-tolerance work where gauge plates fall short; shanks sometimes made slightly smaller than cutting diameter to prevent binding (homecorner.blog/how-to-figure-out-drill-bit-size)
- ScienceInsights, How to Read Drill Bit Sizes in Inches and Metric: stamped shank sizes with HSS codes and their wearing away; number gauge #80 smallest 0.0135 in to #1 largest 0.228 in; metric stamps as direct millimeter diameters, common sets 1 to 13 mm; 1 inch equals 25.4 mm; Silver and Deming stamped size referring to cutting diameter not shank (scienceinsights.org/how-to-read-drill-bit-sizes-in-inches-and-metric)
- GoalGuide, How to Tell What Size a Drill Bit Is: magnifier ghost of worn stamps, 0000 steel wool cleaning, permanent-marker wipe to reveal indents; gauge plate snug fit with flutes inserted not just the shank; calipers across the flutes at the tip (goalguide.blog/how-to-tell-drill-bit-size)
- Bolt Depot, Pilot Hole Sizes for Wood Screws: official per-gauge table, hardwood and softwood, tapered and straight bits, with countersink sizes; #8 straight 1/8 in hardwood and 7/64 in softwood (boltdepot.com/Fastener-Information/Wood-Screws/Wood-Screw-Pilot-Hole-Size)
- BuildToolHQ, Pilot Hole Size Chart: pilot vs clearance vs countersink definitions; #8 at 7/64 in softwood, 1/8 in hardwood, 11/64 in clearance; charts differing by a drill size over pilot tightness; clearance hole letting the head clamp the joint (buildtoolhq.com/post/screw-pilot-hole-size-chart)
- Quora, How do I choose the right size drill bit for a wall plug: packaging as primary reference, 6 mm plug takes 6 mm drill; match diameter not length; measure plug with calipers when packaging missing; hole depth plug length plus 5 to 10 mm; oversized hole failure mode (quora.com/How-do-I-choose-the-right-size-drill-bit-for-a-wall-plug)
- Quora, How do I choose a drill bit for an anchor: expansion and wedge anchors need a hole equal to outside diameter; concrete screws match core diameter; plastic plugs match outer diameter; visual bit-to-anchor shank comparison (quora.com/How-do-I-choose-a-drill-bit-for-an-anchor)
- MP Moran, Rawlplug UNO Universal Wall Plug product data: plug diameter and drill bit diameter both listed as 5 mm yellow, 6 mm red, 7 mm brown, with matching screw ranges and plug lengths (mpmoran.co.uk, product 109131413)
- Engineer Fix, What Size Drill Bit for a Plastic Anchor: match outer diameter of the body excluding ribs and flanges; interference fit with light hammer tap; 5.5 mm bit for a 6 mm plug in soft wall; flutes aligned against the anchor shaft for visual check; masonry needing exact match, not interference (engineerfix.com/what-size-drill-bit-for-a-plastic-anchor)
- HomeCorner, How to Choose the Right Drill Bit Size for an Anchor: metal expansion and wedge anchors exact match, 1/2 in wedge anchor takes a 1/2 in hole; plastic plugs in soft drywall or plaster drilled 1/16 in smaller for grip; toggle bolts larger holes in hollow wall (homecorner.blog/drill-bit-size-for-anchors-guide)
- Quality Magazine, Get a Fine Finish for All Your Holes: Guhring engineering source; as the peripheral corners of the drill wear, the hole's diameter comes out slightly smaller, with margins scraping on retraction (qualitymag.com/articles/84326-get-a-fine-finish-for-all-your-holes)
- Zhonghuan Tools, How to Read an SDS Drill Bit Spec Sheet: the wear mark as a retirement indicator; worn or eccentric bits drilling undersize, oversize, or out-of-round holes void the assumptions behind published anchor load values (zhonghuantools.com/en/resources/how-to-read-sds-drill-bit-spec-sheet)
- Engineering StackExchange, Why are drill bits always slightly undersized: practitioner thread; manufacturing tolerance answer rejecting a universal 0.2 mm undersize; shank wear and difficulty measuring across flutes as measurement confounders; twin-flute drills producing slightly lobed holes in thin material (engineering.stackexchange.com/questions/8672)
- DrillBitsWorld, Silver and Deming Drills: reduced shank twist drills with 1/2 in shank and cutting diameters 9/16 to 1 1/2 in for standard 1/2 in chucks; shank machined to 1/2 in regardless of cutting diameter (drillbitsworld.com/product-category/silver-deming-drills)
- Stewart Supply, IRWIN Silver and Deming HSS Fractional 1/2 in Reduced Shank Drill Bit product pages: all Silver and Deming bits 6 in long with a 1/2 in reduced shank fitting all 1/2 in drills, three-flatted shank for chuck grip (stewartsupply.com)
- RivCut, Drill Size Chart Decimal Equivalents: full number, letter, fractional, and metric decimal table; A at 0.2340 in through Z at 0.4130 in; exact 25.4 mm basis (rivcut.com/resources/drill-decimal-chart)
- DailyHandTools, How To Measure Drill Bit Size Correctly: ruler usable for a general idea especially on larger bits, specialized tools recommended for precision (dailyhandtools.com/how-to-measure-drill-bit-size)
- ToolsAdvisers, How to Measure Drill Bit Diameter: measurement options including ruler or caliper, micrometer, and drill bit gauge; caliper accuracy cited around 0.01 mm (toolsadvisers.com/how-to-measure-drill-bit-diameter)
- WhatDrillBit, Wall Plug Sizes: UK colour code yellow 5 mm, red 6 mm, brown 7 mm, blue commonly 10 mm with a minority of sources saying 8 mm, grey 10 to 12 mm; colour coding a convention not a standard, packaging the final word; bit matches plug, screw one to two sizes smaller (whatdrillbit.com/wall-plugs)
- Turn2 Engineering, Drill Bit Size Chart: independent fractional-to-decimal-to-mm table agreeing with RivCut value for value across the 1/16 to 1 in range (turn2engineering.com/charts-and-tables/drill-bit-size-chart)