Tap Drill Chart: Metric, UNC, UNF and Pipe Thread Sizes Table (Free PDF)

Rows show the Thread size. Columns show the Tap drill (mm) to drill before running the tap in.

This is the standard ISO metric thread formula: major diameter minus pitch = tap drill diameter, and the values follow this normal rule all the way up through M64 and down through M1. So, for instance, use a 10 minus 1.5 = 8.5 mm drill on an M10 x 1.5 thread, a 5 mm on an M6 x 1, and the pattern continues without stopping all the way up and down that scale.

In cases where the actual result isn't a stocked drill bit, the chart shows the closest standard drill bit. For instance, the M8 produces 6.75 mm, so the chart lists 6.8 mm. The M12 produce 10.25 mm, so the chart lists 10.2 mm. Tap and drill makers publishes these results in their DIN 336 selection tables.

The pitch (mm) is the coarse pitch, as specified in the ISO 724 standard for every diameter. You use this to recognize the thread on an unfamiliar part with a pitch gauge, and you subtract this value. It’s also what you’re looking for when picking a suitable drill.

This drill decimal column is in inches. The conversion to inches allows shops that work off a fractional drill chart or a drill index to select the nearest size drill, whether it is a letter, number, or fraction. Our drill bit size chart illustrates the size systems themselves.

Thread engagement (%) The percentage is how much of the total theoretical thread height the listed drill produces. For most drills, that’s between 74 and 79 percent, which is exactly what the major minus pitch rule aim to achieve.

Drilling for 100 percent thread is the most common mistake. According to tests reproduced in Machinery's Handbook, increasing your threading from 75 to 100 percent engagement increases tapping torque by about three times, while adding just a few percent to strip-out strength. This is the quickest way to crack a small tap.

In most steel joints, the bolt fails before the internal thread strips above 70 percent engagement or so. For more on the numbers here, see our thread engagement chart for a detailed rundown of percent engagement vs. joint strength.

Make adjustments based off the material. Drill down or slightly less (down 0.05 mm) if soft, gummy, or thin wall, such as plastics, brass, aluminium.

Go up one drill size if hard/tough, such as alloy steel above ~30 HRC; that’s what many machinists do. E.g., 8.5mm M10 becomes 8.6. You get better tap life, plus less torque on installation. Accepting, say, 65 percent thread is worth it. Same deal for blind holes, which pack chips into their flutes and increase torque even more.

Mistakes here fail, keep your eyes on the near misses.

92 percent thread engagement, eats taps; an M8 x 1.25 hole drilled 6.5 mm

This one's a popular grab from the index as it is adjacent to 6.8. It gives 92 percent engagement and will eat taps.

A 1/4 in (6.35 mm) drill is even worse. It'll eat taps

Using a 7 mm drill in an M8 hole leaves 62 percent thread engagement; fine in steel but marginal in soft aluminium.

This table refers specifically to cutting taps only. For forming (roll) taps a bigger hole is required and this is addressed in the forming tap table below. Fine pitches/spark plug combinations goes into the metric fine table.

The 3.7 mm drill (stocked) is marked as M4.5 x 0.75. It engages 82 percent. Some charts show 3.75 mm. This is correct for this purpose and is what shows up in the formula value.

Special orders in most markets are M60 and M64 taps. Instead, holes of this size is typically turned on a mill or lathe to thread them. So the table covers all the ISO 724 coarse threads anyway, and still includes the drill sizes.

Unlike the coarse table, the fine pitch table spans the ISO fine pitch series. In the Thread size column, look for the full designation, pitch, and diameter to drill out. From there, you can calculates the size using the major minus pitch formula as before: M10 x 1.25 = 10 - 1.25 = 8.75 mm. The stock tap drill here would be 8.8 mm.

Note the fine will always have a bigger drill than its same-diameter coarse counterpart because of the small pitch. For example, M12 coarse taps with a 10.2 mm drill bit, M12 x 1.5 taps with a 10.5 mm drill bit, and M12 x 1 taps with an 11 mm drill bit.

Most sizes comes in multiple pitches, so the Pitch (mm) column is key. For example, M10 is available at 0.75, 1, and 1.25; M12 are available at 1, 1.25, and 1.5.

Before committing to your drill bit, use a thread gauge to check the pitch of the mating bolt. A 9 mm hole for M10 x 1 is oversize for an M10 x 1.25 tap, you’ll have a weak 58 percent thread.

The Drill decimal (in) and Thread engagement (%) columns are just like those from the coarse table.

Fine threads are used when coarse threads fail. They are used for spark plugs like M10 x 1, M12 x 1.25 or M14 x 1.25; thin-wall tube and hydraulic fittings, adjustment screws, etc., and in general any material that is harder which will take less torque due to the shallower thread cut.

Because it is shallow, fine pitch loses a higher percentage of engagement for every 0.1 mm of oversize drilled than coarse pitch do. So accurate drilling is even more critical. Spot drill first and then drill. If possible, you should of use a stub-length drill.

For example, never confuse the pitch groups listed in the chart's table. Even though M6 has a coarse (1 mm) pitch, an M8 by 1 is a fine thread that take a 7 mm drill.

Also, don’t mix M14 coarse with M14 x 1.25; one is 2 mm and the other is the spark plug thread. Sometimes, both sizes has the same diameter but different pitches, and when the bolt goes into the tapped hole, it will engage for a turn or two and then bind. Backing out prevents stripping it.

The table ends at the point where the ISO 724 fine series is typically tapped. To be complete, we add in M64 x 4. We use the formula again, but this time for a 60 mm drill, just like with the largest of the coarser threads.

A tap of this size is hard to find; typically this size is single-point cut. If you don’t know the pitch, look at our chart of thread pitches to see which fine pitch goes with which diameter. Once you know that pitch, then use this table to know what to drill.

The Thread size column can be read as diameter-TPI. So #10-24 means a number 10 machine screw with 24 threads per inch. Similary, 1/2-13 reads as a half-inch bolt with 13 TPI.

In addition, you will find ASME B1.1 fixes the coarse pitch count on the TPI column. For each thread there are two drill provided in the chart. Tap drill (75% thread) is the classic value for soft materials. Tap drill (50% thread) is the easier choice for cast iron, stainless, and steel.

Next to each drill size there is its corresponding decimal equivalent. When you don’t have that drill size in your index, you simply use the decimal to sub in the nearest metric drill.

Combination Drill Designations: Three drill designating systems combine to make up the chart. Fractions. There are letter drills. There are also number drills. They are mixed for a reason. Standard values sort that way.

It’s the accurate reference column: the decimals. Whatever it says on the shank, you know #7 are always 0.2010 inches. See our drill bit size chart for the complete system of number, letter, and fractional drills all in one place.

It uses the same physics as metric threads. To find the tap drill, subtract 1/TPI from the major diameter. This value always falls near the 75 percent column (inch rule of thumb). Case in point: 1/4 minus 1/20 = .200, while the chart show #7 at .2010.

The reason for the 50 percent column is because increased tapping torque increases rapidy with engagement. A 50 to 55 percent thread will out-pull the bolt in most joints, even in difficult materials. It also reduce tap breakage drasticly. Strength vs. engagement is quantified on our thread engagement chart.

Some pairs look interchangeable, but they are not. For example, 13/64 (0.2031) and #7 (0.2010) drill into 1/4-20 threads. But 7/32 (0.2188) is 50 percent. It is not some sloppy 75 percent.

Letter U drill (0.368) is right for 7/16-14 threads. It is also good enough, at roughly 82 percent of the chart’s tolerance, using a 3/8 drill (0.375). At 50 percent engagement, use a 25/64 drill (0.3906).

Some charts published online moves up one size on smaller screws. So they show a #39 when it should of been a #38 for 5-40; a #17 instead of a #16 for 12-24. These each make usable threads too. It’s just a handful of percent of engagement between them.

For sizes 1 1/8-7 up to 2-4 1/2, you get just one number in the 72 to 78 percent range, and that’s it; you’re out of the two-column system. That’s what the 75 percent column carries, which is the value that references publish in the 72 to 78 percent band. The 50 percent cells are empty, they lack any numbers to back them up.

Typically, taps this large is run on a machine with a high oil cut. They’re often single-pointed or even thread milled instead.

As you can see, I didn’t include #0-80 here because there isn’t an equivalent #0 UNC thread. It goes in the UNF table below.

The UNC table is formatted the same way. The table contain the thread size in diameter-TPI. The table shows the TPI number according to ASME B1.1. The table also shows 50 percent drill size for hard material and 75 percent drill size for softer materials. The decimal inch equivalent are shown for each entry.

Fine pitch threads is shallower than coarse threads. That’s why all of the UNF drills is bigger than their matching UNC drill. A 3/8-24 thread is drilled with a letter Q drill (0.3320), whereas a 3/8-16 thread are drilled with a 5/16 (0.3125). A 1/2-20 thread is drilled with a 29/64, where a 1/2-13 thread is drilled with a 27/64.

We find all that AN hardware in the instrument, aircraft, and automotive sectors. They uses UNF threads by default. In these environments vibration resistance is an issue. Thin section sizes and the need for fine adjustments is also issues. That’s why you’ll see 0-80 listed here (the smallest) which is available only in the fine version. That’s why it opens up this table rather than the UNC one.

The larger than 1-inch UNF threads is in the constant pitch UN 12 series (12 TPI). You will find them listed as UNF in the fine-thread charts. The published drill values is standard.

Over-sized holes are not as forgiving with fine threads as they are with coarse threads. If you enlarge a 28 TPI by 0.005 inch, it will cost you some 11 percent in engagement. With a 20 TPI, it’s only about 7 percent.

Use a spot drill, then. Maintain rigidity in the drill, keep it short. For precision work, check your hole first. A hand drill will typically cut between 0.002-0.005 inch larger. That’s why charts target 75 percent instead of 85 percent.

When you are choosing drills, don't get confused about what size. A #10-32 thread requires a 0.1590 drill. An M5 x 0.8 thread take a 4.2 mm drill or 0.1654 inch. Those are close enough to start a tap and ruin a hole.

The same happens with an M6 x 0.75 and a 1/4-28. You will have the bolt fit loose for a couple turns, and then lock. That's no tap drill issue. In fact, it’s nearly always an inch/metric mismatch.

Don't bother trying to use this table for UNEF threads either. UNEF means extra fine. 1/4-32 is one of those UNEF threads for instance. Their pitches are even thinner than that. They requires their own drill sizes.

Compare thread dimensions and strengths in our UNC vs UNF thread chart to choose between coarse and fine.

The chart above shows the drill sizes for UNF threads. The chart assumes that you have the tap in hand. You simply need the proper sized hole.

The Pipe size column contain the name of the size of pipe, but it doesn't correspond to a measured diameter at all. For example, a 1/2 NPT thread measures 0.840 in across the pipe, which means that its actualy measured diameter is larger then the pipe size indicates.

That's where the Pipe OD (in) column come into play. To figure out what type of thread an unfamiliar pipe might be, just measure the OD and count the number of threads per inch (TPI) and then read across the table.

The NPT tap drill column show the standard tap drill that is used to make a direct hole for tapping. It has both a decimal value as well as one of those funky sizes that seems to have no corresponding decimal. Note: No taper reamer required for this approach. ASME B1.20.1 reference tables, Engineers Edge, and Engineering ToolBox all agree.

NPT has a 1-in-16 (3/4 in per ft) taper. A straight drilled hole will approximate a good taper but not be quite right. The tap form full threads where it engages, and shallower and shallower threads as you near the mouth.

Fastener charts don't vary more than a single drill up or down from each other like published tap and drill charts do. Some tap charts calls for the next drill down so that they gets a little bit more thread depth at the top of the hole. So for instance, they might say use 45/64 rather than 23/32 for 1/2 NPT. That decision mean increased tapping torque.

Either way works, but this chart lists the easy-to-tap standard value, the bigger hole. If it's an important joint, use the smaller hole size that requires a reamer to get the correct form. Then you'll need to cut the taper out with a 1-in-16 pipe reamer prior to tapping.

Dryseal pipe threads (ASME B1.20.3) include NPTF tap drill and NPTF with reamer columns. In hydraulic and fuel applications, these threads goes where threads with sealant would typically be used.

NPTF has a modified root and crest height. It is identical to NPT in taper and pitch, but the parts will crush into each other to create an interference seal. This seal require no sealant or PTFE tape. As a result, the drill size runs one size smaller, like Q for 1/8 rather than R.

While you can thread an NPT tap into an NPTF hole or vice versa, it won't create a dry-seal. A leak on a dry-sealed joint points to something worth checking: What standard did the actual tap conform to?

Always use a sealant or PTFE tape on plain NPT joints. Do not drive them any further than 2-3 turns past hand tight. Each additional turn expand the female portion due to the taper, which is usually why the casting crack.

Final fit also depends on how deep you tap. For a through hole, you should of run it out so the little end extends maybe one thread past the far side. On the first hole of a batch, gauge with a fitting instead of by feel because they will vary slightly.

It's only for tapered American pipe threads. For straight NPS threads you need a bigger size bit (e.g., R for NPT vs. Use S for 1/8-27 NPS).

There are also British G threads that use an entirely different 55 degree form, which we cover with our BSPP table below. Though there are sizes above 4 in in the standard, they're seldom tapped. Weld or thread them instead.

G stands for British Standard Pipe Parallel threads.

The same logic apply for their sizes using the nominal bore. So G 1/2 is half inch of pipe bore. Its thread is 20.95 mm across. And that number is listed under Major dia (mm).

To be sure, go and measure it with a gauge and count the TPI. It's either 28, 19, 14, or 11. Then drill a hole that matches the Tapping drill (mm) size. If you only have an imperial index then look at the Drill decimal (in) column.

The thread have a Whitworth profile of 55 degrees and a G form.

55 degree Whitworth profile with round roots and round crests

It follows ISO 228 standards.

It is not the 60 degree American form.

The parallel thread do not wedge tightly.

The gasket face, O-ring, or bonded washer under the fitting shoulder seal against the joint.

Only clamping force is provided by the thread. Drilling for taps is near the minor diameter inside. Joint never relies on thread interference. There is no percent-thread adjustment.

Do not use two similar-looking systems. Do not mix them. G 1/2 is 14 TPI, 20.95 mm. 1/2 NPT is 14 TPI, 21.3 mm OD. They will start to connect and then leak or jam.

It has the same number of threads per inch (TPI). It has a different taper and angle. In Britain, its tapered version is called BSPT. Seals via the threads as NPT does. Internal threads is cut by a taper tap. Smaller by 0.3 to 0.5 mm different than the G value. An 8.4 mm rather than 8.8 mm for a 1/8 size.

You tap for G if it's going into a fitting that depends on a washer. If it depends on some kind of compound or tape on the taper, it's BSPT. Then this column isn't your drill size.

In Asia and Europe, pneumatic and hydraulic ports often has G threads. Instrument gauges everywhere have them.

When tapping, chamfer the hole mouth to the major diameter. A parallel tap begins square. The chamfer holds the washer in place.

Check thin-wall bosses to make sure the wall is thick enough for the listed drill. Note that the 11 TPI pitch is deep at 2.309 mm. The large G taps cut a lot of metal. Above G 1 1/2, thread-mill or tap in steps.

Beyond that, we have a little-mentioned size called G 1/16, which doesn’t appear on the table because there is no verified drill data.

As noted above, G 2 through G 4 are still provided, though you could of assumed they will be thread milled or single-pointed in practice.

Threads are formed rather than cut when a forming tap are used. The metal in the groove that will become the threads is pushed aside. This pushed-out metal rise and becomes the crests.

Since there must be somewhere for the pushed-out metal to go, forming tap holes is larger than those drilled for cutting taps. For example: drilling a forming tap hole for M6 using the cutting-tap diameter of 5 mm will cause the tap to bust through or seize the workpiece.

This table show the thread size, drill decimal, thread percentage, and the manufacturer's recommended drill.

The 65 percent column is general purpose recommendation from the manufacturers. This is where the stock drill mentioned in the table will hit.

Use 75 percent if you want the most thread in softer materials. Use 55 percent if you are having problems with work hardening alloys or tough materials and need an easier tap in place.

A quick rule of thumb for hole size is: Hole = major diameter minus half the pitch. For cutting taps, full pitch is used. Example: an M6 x 1 would form around 5.5 mm. A 1/4-20 would form around 0.227 in. These agree with the table.

Roll tap hole sizes is proprietary because every maker grinds a slightly different lobe form. Balax, Guhring, and Emuge charts will vary by a few ten-thousandths.

The table uses Balax Thredfloer data for 6H (metric) and 2B/3B (unified) classes. Consider this table representative and refer to the one packed with your actualy tap.

Forming taps requires hole size discipline; that’s the whole game. From 75 to 55 percent thread, the listed value vary by 0.002 to 0.004 in. If you wander off on a drill 0.003 in oversize, that costs you ten points of engagement without even knowing it.

An undersize hole spikes enough torque to break the tap. It has no flute so the tap can’t easily be backed out in pieces. Measure the actual hole, not the drill; use a stub drill or ream for anything critical.

Ductile = forms tap

The materials include austenitic stainless, low carbon and free machining steel, copper alloys, zinc die castings, and aluminum. Has decent elongation and is roughly below 32 HRC.

Anything else, like hard steel, cast iron or brittle plastic won’t work. It’ll break rather than flow. In the proper metal, you don't have to clean out chips from a blind hole.

You get grain flow without interruption, so your tapped thread is stronger and work-hardened. The taps will also last many times longer than the cutters used in production.

You can see there is a tiny cup-like cleft or form in the crest of the thread, where the metal joined together. That’s harmless, and normal; it simply appears unfinished. The cleft is beyond the contact area for the mating bolt.

Thread strength equals or exceeds cut threads equally engaged. Thus the percent columns is similar to those found above under cut threads. You should of considered how metal flow works.