Tap drill size chart: which drill each UNC, UNF and metric thread needs

01What is a tap drill, and why is it smaller than the thread?

A tap drill is the bit that makes the hole before a tap cuts the thread, and it is always smaller than the thread's nominal size because the tap needs metal left in the wall to form the thread. The nominal size of a thread is its major diameter, measured across the crests; the tap drill sits close to the minor diameter at the roots, leaving enough material for the tap to cut most of the full thread profile.

Most charts, including the inch and metric tables on this page, give two drills per thread, one for each of 75% / 50% thread. The first figure is the usual default. The second is a slightly larger drill that leaves less metal for the tap to remove, which makes tapping easier in tough material at the cost of a shallower thread. Thread percentage here means how much of the full theoretical thread depth the tap actually cuts, not a share of strength.

02Tap drill chart for UNC and UNF threads

The inch table covers machine-screw sizes from #0 up to one inch, in both coarse (UNC) and fine (UNF) pitches. Each row gives the thread, its major diameter, threads per inch (TPI), and two drills, one for each of the 75% / 50% thread columns. Every drill is shown in its own system, number, letter or fraction, followed by the decimal inch and millimetre values, so you can find it in any drill index. Little Machine Shop and the University of Pennsylvania's design chart agree on the standard drill for every thread they both list, from #0-80 to 1/2-20 (cross-check result: 21 of 21 match). Rows from 9/16 in to one inch come from Little Machine Shop alone.

03Tap drill chart for metric coarse threads

Metric threads are named by major diameter and pitch: in M8 x 1.25 the first figure is the diameter in millimetres and the second the distance between thread crests, also in millimetres, and the tap drill is given in millimetres too. For the three most common sizes the sources agree: 5.0 mm for M6 x 1.0, 6.8 mm for M8 x 1.25 and 8.5 mm for M10 x 1.5. The table adds the inch conversion for readers working with an imperial drill set.

Two sources were compared and they do not always match. The University of Pennsylvania chart gives values for both thread percentages up to M14, while Engineering ToolBox, working from the ISO coarse series, gives one value per thread without stating the percentage. For M2.5, M12 and M14 the two disagree by a small fraction of a millimetre; the table shows the UPenn figure and marks the row. M3.5 and the sizes from M16 upward appear only in Engineering ToolBox. M18 is missing from both of those charts, so it is not in the table, but it is now confirmed elsewhere: the tap drill for M18 x 2.5 coarse is 15.5 mm, a figure AmesWeb's metric chart and Hoffmann Group's GARANT M18 hand tap set agree on.

04How do you work out a metric tap drill without a chart?

Subtract the pitch from the thread diameter. AmesWeb's metric chart puts the rule as tap drill roughly equals major diameter minus pitch, and the confirmed sizes follow it closely: M18 x 2.5 gives 15.5 mm, and the fine M8 x 1 gives 7.0 mm.

The rule is a quick check rather than a replacement for the table. It gives a single figure without a thread percentage, so in hard steel you may still prefer the slightly larger drill the coarse table lists as its second value, and where the table has a row, use it.

05What tap drill do fine-pitch metric threads need?

A fine-pitch metric thread needs a larger tap drill than the coarse thread of the same diameter, because its teeth are shallower. The three fine pitches people meet most often are M8 x 1, which takes 7.0 mm; M10 x 1.25, which takes 8.8 mm; and M12 x 1.5, which takes 10.5 mm. Compare M8 x 1 with the coarse M8 x 1.25 at 6.8 mm: same diameter, different drill, which is why the pitch in the callout matters.

The table below covers fine pitches from M8 to M20. AmesWeb and Engineering ToolBox agree on every row, though neither states the thread percentage behind its figures, and no tap maker's fine-pitch chart was reached, so check the maker's data for critical work.

06What drill bit size do you need for a 1/4-20 tap?

Use a #7 drill, which is 0.201 in across. It is the most-searched tap drill size because 1/4-20 is the standard coarse quarter-inch thread. A #7 is a number drill, so it is often missing from fractional sets; it is worth buying singly if you tap this thread more than once.

The table also gives a lower-percentage alternative for this thread, a fractional size a little larger than the #7, for steel or stainless where a full-depth thread makes the tap hard to turn. The fine-thread version, 1/4-28 UNF, has more threads per inch and therefore shallower teeth, so its tap drill is a larger number drill; read its own row rather than reusing the coarse-thread drill.

07What drill size do 5/16 and 3/8 taps need?

A 5/16-18 tap needs a letter F drill, which measures 0.257 in, and a 3/8-16 tap needs a 5/16 in drill. Notice that the 3/8-16 tap drill has the same nominal fraction as the 5/16 thread: that is a coincidence of the numbers, not a sign that the two can share a drill. Using a 5/16 in bit for a 5/16-18 tap would leave a hole as large as the thread itself, with nothing for the tap to cut.

Both threads also come in fine pitches, 5/16-24 and 3/8-24 UNF, and each calls for a letter drill listed in its own row. Going up the coarse series, the half-inch thread 1/2-13 UNC takes a 27/64 in drill.

08Should you drill for 75% or 50% thread?

Use the deeper-thread drill for soft materials and the larger, shallower-thread drill for hard ones. Little Machine Shop labels its two columns (75% / 50% thread) by material: the first for aluminium, brass and plastics, the second for steel, stainless and iron. The larger drill leaves less metal for the tap to remove, so the tap turns more easily in a tough material.

A common misconception is that the strongest thread comes from drilling to the screw's minor diameter so the tap cuts a full profile. Both charts used here stop short of a full-depth thread, and Little Machine Shop lists the larger drills for the hardest metals because they are easier to tap. The University of Pennsylvania chart takes the same approach, listing two thread percentages rather than a single minor-diameter drill.

09How do you read a thread callout and find its drill?

Split the callout into size and pitch, find the matching row, and read the drill from the column for your material. An inch callout such as #10-32 UNF names a #10 machine screw (its major diameter is in the second column) followed, after the dash, by its threads per inch. Its row gives a #21 drill in the standard column. The coarse #10-24 UNC thread shares the same major diameter but has fewer, deeper threads, so its row calls for a smaller #25 drill. Reading the TPI column is what stops you mixing up two threads with the same screw number.

A metric callout works the same way. M8 x 1.25 is the coarse eight-millimetre thread, so the metric table gives 6.8 mm. When a drawing writes only M8 with no pitch, it means the coarse series. If the callout lists a smaller pitch, such as M8 x 1, use the fine-pitch table instead, which gives 7.0 mm for that thread.

Once you have the drill name, the decimal and millimetre values in the same cell let you find it in a drill index, or confirm a metric bit is close enough for a soft material. For the full list of number, letter and fractional decimals, the decimal drill size table covers every size up to one inch.

10Which drill bit should you use for a tap hole?

A high-speed steel twist drill handles tap holes in most metals, and Wikipedia describes cobalt high-speed steel as the choice for stainless steel and other hard materials. Starrett's catalogue describes split-point drills ground to 135 degrees as self-centering, which helps the hole start exactly on the punch mark. Starrett recommends lubricant when drilling metal: it cuts friction, keeps the edge sharp and lowers the risk of the bit breaking.

Cobalt and carbide bits are more brittle than plain high-speed steel, so keep the drill square to the work and avoid side loads. If the bit wanders before it bites, the hole and the thread both end up off position; the page on stopping a drill bit from wandering goes through the fixes. For choosing a bit for steel, aluminium or stainless, the buyer guide to drill bits for metal compares current options.

11When doesn't a tap drill chart give the right hole?

The chart gives a hole for cutting a thread, so it is the wrong reference whenever the hole is not going to be tapped with a standard cutting tap. The main exceptions are:

• Clearance holes. A hole the screw passes through is larger than the thread, not smaller. Drilling a clearance hole with the tap drill leaves the screw jammed in the wrong part.
• Wood screws. Wood is not tapped; screws cut their own thread in a pilot hole sized by gauge and wood type, which the screw pilot hole chart covers.
• Pipe threads. Tapered pipe threads are outside these tables, and fine metric pitches beyond the sizes in the fine-pitch table are not covered.
• Large holes. Above one inch the inch table stops. Large pass-through holes for pipe and conduit are a hole saw job rather than a tapped hole; the guide to matching hole saws to fittings gives outside diameters and knockout sizes.
• Disputed sizes. For M2.5, M12 and M14 the two sources differ; check the tap maker's own data for those.

12Where do number and letter drills fit into tap drilling?

Most inch tap drills are number or letter sizes because the ideal hole falls between two fractions. A #7, a letter F or a #25 sits between neighbouring sixty-fourths, so a plain fractional set cannot supply the right hole for many threads. The guide to number and letter drill sizes explains how those series are labelled and why the number series runs backwards. The drill bits by material hub lists bit families for metal, wood and masonry.

For holes that do not need a thread, a stepped cone drills many sizes in sheet metal and enclosures; the unibit-style step bits guide compares the common ones, and the hole saw kit picks cover larger holes. The reference charts section collects every size table on this site, and you can look up what fits your tool to check which shanks your drill accepts. For a copy to pin up beside the drill press, the inch and metric tables are part of the free printable drill and tap charts.

13Questions readers ask