How to read drill bit sizes: number, letter, fractional and metric

01What does a drill bit size chart actually tell you?

A drill bit size chart tells you the cutting diameter behind each label, nothing more. It converts a label, whether a number, a letter, an inch fraction or a millimetre figure, into a decimal inch figure and a millimetre figure so you can compare bits from different systems. It does not tell you which bit a job needs, whether the bit fits your chuck, or what type of bit suits the material. Those answers come from other charts and from the tool's specs.

Twist drills are sold in four sizing systems: number sizes from #80 to #1, letter sizes from A to Z, fractional inch sizes in 1/64 in steps, and metric sizes in millimetres. Number and letter sizes fill the gaps between neighbouring fractions, which is why thread-tapping charts rely on them. The full tables for all four systems are on our drill size conversion chart; this page explains how to use them and the charts that sit beside them.

02How do you tell which system a bit is labelled in?

Look at the format of the marking on the shank, the index case or the packet. Each system has a distinct format, and once you know which one you have, you know which table to open.

• Number sizes carry a number, usually with a # sign or the word No. They run from #80, the smallest at 0.0135 in, to #1, the largest.
• Letter sizes carry a single capital letter. A is the smallest at 0.234 in, just above #1, and Z the largest at 0.413 in.
• Fractional sizes carry an inch fraction, the common format on US bits and sets.
• Metric sizes carry a millimetre figure, the format on bits sold in Europe. Metric jobber drills are made to DIN 338, according to Starrett's catalogue.

The direction of each series is the trap. In the number series a higher number is a smaller drill, while letters and fractions grow as you would expect. Reading the number series the wrong way can put you dozens of sizes from the bit you meant.

03How do you read a drill size chart, step by step?

The same five steps work for a wall chart, a drill index lid or the tables on our size chart page.

1. Identify the system. Read the marking and decide whether it is a number, letter, fractional or metric size.
2. Open the matching table. Find the label in the column for that system, not in a neighbouring system's column.
3. Read the decimal or millimetre value. Read the decimal inch value from left to right, digit by digit, or read the millimetre column if you work in metric.
4. Compare on the common scale. To find an equivalent in another system, look for the nearest decimal in that system's table and note whether it is larger or smaller.
5. Check the job chart. If the hole will be tapped, use a tap drill chart; if it will take a wood screw, use a pilot hole chart; if it passes a pipe or conduit, use the fitting's outside size.

04What do real lookups look like?

Real lookups start from a job, not from a bit. The table below follows six common jobs through to the size, its system and the chart that gives it. Notice how often the answer is a number or letter size that a fractional set does not contain: a 1/4-20 thread needs a #7 drill, and a 5/16-18 thread needs letter F. Notice too that letter E and a quarter-inch fractional bit come out identical at 0.250 in, one of the few exact matches across systems.

05How do you read a tap drill chart row?

Read a tap drill chart row from left to right: thread name, then the drill for the thread engagement you want. An inch thread such as 1/4-20 gives the nominal diameter and the threads per inch; a metric thread such as M10 x 1.5 gives the diameter and the pitch in millimetres. The drill column then gives the size in whichever system lands closest to the ideal hole, which is why inch charts mix number, letter and fractional sizes in one column.

Many charts show two drill columns. Little Machine Shop lists drills for 75% / 50% thread, with the higher engagement suggested for aluminium, brass and plastics and the lower one for steel, stainless and iron, because the slightly larger hole is easier to tap. That guidance comes from one source. The inch values themselves agree across the two charts consulted (21 of 21 match for the threads both list). Metric charts disagree in places: for M12 and M14 the University of Pennsylvania and Engineering ToolBox give slightly different drills, so check which chart your tap maker follows. The full table is on our tap drill size chart.

A common belief is that the strongest thread comes from a drill at the screw's minor diameter. Charts deliberately use partial engagement instead, and list larger drills for hard metals because they are easier to tap without breaking the tap.

06How do you read a pilot hole chart for wood screws?

Find the screw gauge in the first column, then read the pilot size for your wood (hardwood or softwood) and, if the screw passes through a top board, the shank or clearance hole. The screw gauge is not a drill size: a #8 screw and a #8 drill are unrelated, because the screw number belongs to a separate gauge system. For a #8 traditional wood screw, WOOD Magazine's chart gives a 7/64 in pilot in hardwood and 3/32 in in softwood.

Two charts can give different answers for the same screw. MISUMI's chart lists pilot sizes by tapered or straight bit, and several of its figures differ from WOOD Magazine's, so this site displays WOOD Magazine's values and notes MISUMI's as an alternative. Hardwood always takes the larger pilot in both charts. Metric screws are often specified by the screw maker instead: Rotho Blaas, for example, gives a 4.0 mm pre-drill for one of its HBS softwood screws. The full gauge-by-gauge table is on our screw pilot hole chart.

07Why do conduit, pipe and hole saw sizes not match the label?

Because conduit and pipe are named by a nominal trade size, while the hole has to fit the actual outside diameter of the conduit fitting or pipe. Klein lists the knockout for the smallest conduit size in its table as 0.875 in, well above the nominal figure printed on the conduit. Steel pipe works the same way: its outside diameter is set by the nominal pipe size and stays the same across schedules, so you size the hole from the outside diameter plus some clearance.

Hole saws add a second label to read, the arbor. Small hole saws from 9/16 to 1-3/16 in use a small-thread arbor with a 1/2–20 thread, and larger ones need the large-thread arbor, so a hole saw size also tells you which arbor to buy. The size tables are on our page of hole saw sizes for pipe and conduit, and current sets are compared in the hole saw kits guide. For a spread of sizes in thin sheet, unibit-style step bits cover many holes with one cone; their brand numbers, such as #1 or #4, are family labels and not sizes.

08Which numbers on a bit or packet are not the cutting size?

Shank size, length series and system names are printed alongside the cutting size and are easy to misread as it. Check which number you are looking at before comparing.

• Shank and chuck capacity. Chuck size names the largest shank a chuck accepts, not the largest hole it can drill. A 1/2 in chuck takes shanks up to that size, and reduced-shank bits cut larger holes on a shank that still fits. The guide to drill chuck capacity covers keyed, keyless and hex holders.
• Hex shank size. Impact-rated bits and driver bits share a 1/4 in hex shank, whatever the cutting diameter.
• SDS shank size. SDS-plus shanks are 10 mm and SDS-max shanks 18 mm. These are shank sizes for rotary hammers and say nothing about the hole.
• Length series. Words such as stub, jobber and taper describe length, not diameter. Jobber is the most common length, stub bits are shorter and stiffer, and taper or long-series bits reach deeper.

09When does reading the chart give the wrong answer?

Reading the chart gives the wrong answer when the chart is rounded, when a near size is substituted for an exact one, or when the label belongs to a different system from the one you assumed. Each mistake has a quick check.

• A rounded chart. Little Machine Shop rounds six number sizes to three decimals (#69, #28, #25, #18, #10 and #5), while AmesWeb and Wikipedia carry four. If two charts disagree in the last digit for one of those sizes, neither is wrong; one is rounded.
• A near metric substitute for a tap drill. A metric bit that is close to a #7 or letter F is still a different hole and changes thread engagement. Use the size the tap chart names.
• Screw gauge mistaken for drill number. A #10 screw does not want a #10 drill. Use the pilot hole chart.
• Trade size mistaken for hole size. Conduit and pipe sizes are nominal, so the hole is always larger than the label suggests.
• Sizes above the chart's range. The fractional table on our chart page stops at one inch because the sources stop there; larger holes belong to hole saws and step bits.

For the types of bit each size comes in, from brad-point to cobalt, the drill bits by material hub groups the buyer guides, and the technique guides section holds the other how-to pages. If you are unsure which bits your drill or driver accepts, you can look up what fits your tool by model.

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