Tap Drill Size Chart: How to Pick the Right Drill Bit (UNC/UNF/Metric)

Open a published chart for 1/4-20 and the answer is letter F, 0.2570 in. Choose one letter too large and thread engagement climbs past 90 percent. Tap torque spikes, and in stainless the square shears by the third hole. Choose one too small and engagement drops below 60 percent. The thread strips on the first real load. The right drill size is a standard number, not a guess.

This guide reads the chart with you, pins down the 75 percent default, and shows exactly where the shortcut formula stops matching the standard.

The Snapshot

• 1/4-20 UNC: letter F drill, 0.2570 in, 6.53 mm, about 75 percent thread per the published ANSI B1.1 chart.
• The default: pick the chart drill that lands near 75 percent thread. It balances tap life against thread strength for most ductile materials.
• M8x1.25: 6.8 mm, the published ISO chart answer. The shortcut d minus P gives 6.75 mm and points at the same zone.
• The full table: every size, from 0-80 through 1-1/2 in and M1 through M24, sits in the interactive chart at /tools/tap-drill-size-chart/.

Start at the Tool: How to Read a Tap Drill Chart

A tap drill chart is a lookup, not a calculation. You enter with the thread, and the row hands back a drill. The interactive tap drill chart covers UNC, UNF and metric in one search box and shows the decimal size behind every letter and number. That matters, because the columns are where beginners misread the answer.

A standard row runs left to right: the thread callout, the tap drill as a letter or number, the same size as a decimal, and the resulting percent of thread engagement. Some charts add a nearest fractional drill column, and that column causes the most trouble.

The confusion is naming, not sizing. Letter and numbered drills do not line up with fractions in any obvious way. Letter F, the 1/4-20 tap drill, is 0.2570 in, while a 1/4 in drill is only 0.2500 in. Someone who rounds 0.2570 to "about 1/4" undersizes the hole and buys a broken tap. Convert every letter or number to its decimal before you walk to the rack, and confirm the drill is stamped with the size.

One last check: confirm the chart's standard. ANSI B1.1 listings and ISO tables are published gauges, but shop charts occasionally differ by a letter on a few rows. The decimal and engagement columns resolve the disagreement.

The 75 Percent Thread Rule

Percent thread engagement measures how deep the tap cuts relative to the full thread height. A 100 percent thread means internal and external threads meet sharp corner to sharp corner at crest and root. Nobody wants that. The crest is the weakest, least supported part of the thread form, and forcing engagement into it multiplies torque without adding useful strength.

The strength curve is nonlinear. Going from 50 to 75 percent adds a large share of the available shear area, because the load-carrying flanks grow quickly in the lower half of the thread height. Going from 75 to 100 percent adds little strength while driving torque up steeply. Published thread-strength calculations put roughly 85 percent of the load capacity inside the first 75 percent of engagement (treat 85% as engineering shorthand: run the ANSI B1.1 shear-area formulas for a load-critical joint). That is why 75 percent became the shop default: most of the strength, normal torque, predictable tap life.

Two directions move off the default deliberately. Go up toward 80 to 85 percent when the female thread is the weak link or the load is shear-heavy: thin-wall assemblies where pull-out is the failure mode, and high-vibration joints where a stripped thread is a field failure. The cost is torque, and it lands hardest in stainless.

Go down toward 60 to 70 percent when tap life is the constraint: soft, gummy metals that pack the flutes, deep blind holes with poor evacuation, and production work where a regrind schedule matters more than the last 10 percent of strength. Start at the chart, and only leave the band for one of these named reasons.

Formula vs Chart: The Simple Math and Where It Lies

The metric shortcut fits on a napkin. Take the nominal diameter, subtract the pitch, and the result is the tap drill. M8x1.25 gives 8 minus 1.25, so 6.75 mm. The published ISO chart says 6.8 mm. The formula landed within 0.05 mm and pointed at the same drill you would buy anyway. That agreement is why the rule survives: d minus P lands in the 65 to 75 percent engagement ballpark for most coarse metric threads, and the chart rounds the result to a stock size.

Imperial does not cooperate. Shops quote the same idea as major diameter minus one divided by threads per inch, but run it on 1/4-20 and you get 0.200 in, while the ANSI chart answer is letter F at 0.2570 in. Inch thread heights do not scale the same way from size to size, and letter and number drills are standardized steps, not continuous outputs. On UNC and UNF rows, the drill comes from the published chart. Use the formula as a sanity check that you grabbed the right row, never as the source.

Even the chart answer sits inside a tolerance band, and four real-world effects push your hole away from the printed decimal. A worn or deflected drill cuts oversize, sometimes enough to move a letter. A coated drill's published diameter is usually the uncoated size, and the film adds a few thousandths. Swarf packed in a blind hole lifts the tap and makes it cut off-center. And soft alloys spring back and flow, so measured engagement can differ from the static chart number. Treat the decimal as a target and keep drills sharp enough that the hole matches the stamp.

UNC, UNF and Metric: Same Method, Different Columns

The method never changes across the three families: find the row, read the drill, land near 75 percent. What changes is where the answer comes from and how far apart the steps are.

UNC is the coarse inch series, and its chart rows use the letter and number drill ladders. UNF is the fine series at the same diameter. A finer pitch means a shorter thread, so the tap drill sits closer to the major diameter. The 5/16 family shows it: coarse 5/16-18 gives letter I at 0.3190 in, and finer 5/16-24 calls for a larger decimal still, straight from the published ANSI B1.1 chart. Grabbing the coarse-thread drill for a fine tap snaps small taps. Read the threads-per-inch before you read the drill.

Metric coarse threads run on millimeter drills, and the published ISO rows are short enough to memorize a few. M3 wants 2.5 mm. M4 wants 3.3. M5 wants 4.2. M6 wants 5.0. M8 wants 6.8. M10 wants 8.5. M12 wants 10.2. These track d minus P within a few hundredths and drift slightly above it as the thread grows, because the chart rounds toward stock sizes. Fine pitches follow the same rule with a smaller subtraction: M8x1 takes 7 mm, not 6.8.

Mixed hardware is where these worlds collide. If you stock inch drills only and ask what the m8 tap drill size in inches is, 17/64 in, at 0.2656 in, is the common inch-side answer, a hair under the 6.8 mm chart size. Close enough for shop work, not close enough for a gauge-controlled hole.

Adjust for the Material

The chart row assumes generic ductile behavior. Three common shop materials break that assumption, and one of them changes what "the answer" even means.

Gray cast iron. The flake graphite fractures the chip into dust and the hole measures honestly, so most shops tap cast iron straight at the chart. A hair under the chart size buys extra engagement cheaply, because brittle iron does not load the tap the way steel does. What matters more is evacuation: blow the dust out before the tap enters, or the abrasive slurry polishes the cutting cones and holes drift oversize mid-run.

304 stainless. Stay on the chart, exactly. Do not undersize "for strength". Stainless work-hardens, and an undersize hole raises flank pressure, which raises heat, which hardens the very surface the next tooth is cutting. The result is a galling, torqued-up tap and a torn thread, not a stronger joint. What earns its keep is a sharp, polished-flute drill running proper speed and feed, with cutting fluid reaching the cut. The tooling details are on the stainless machining page.

Aluminum and soft non-ferrous. The adjustment depends on how the thread is made. A cut tap removes material, and the chart size works. If anything, drop toward 65 to 70 percent engagement to lighten the load on flutes that pack with swarf. A form tap, or roll tap, moves metal aside instead of cutting it. Nothing leaves the hole as a chip, so published forming charts give a pilot slightly larger than the cut-tap drill. The offsets run 0.1 to 0.2 mm over on small metric sizes, because less displaced volume means far lower forming torque (form-tap pilots are series-specific; use the vendor's number). The flow-and-swell behavior is on the aluminum machining page.

Plug, Taper and Bottoming Taps Change the Answer

They do not. The drill size is set by the thread and the target engagement, not by the tap's lead. A plug, taper and bottoming tap all cut the same 1/4-20 thread from the same letter F hole. The chamfer changes reach and starting behavior, and that is where the decisions live.

The chamfer is the ground-off lead at the end of the tap, measured in thread pitches of full form behind it. A plug tap runs about 7 to 8 threads, a taper tap about 3 to 5, and a bottoming tap about 1.5 to 2, per the published style ranges. More chamfer spreads the cut over more teeth, so the tap pulls itself in with less torque. Less chamfer grabs sooner and asks for a rigid holding setup. In a through hole, start with the most chamfer you own; save the bottoming tap for holes that stop before the surface.

In a blind hole the chamfer governs thread depth. The thread stops one chamfer length above where the tap stops, and the hole needs relief below for the remaining flutes. Drill depth equals required thread depth, plus the chamfer in pitches, plus 2 to 3 pitches of relief, plus room for the drill point. A bottoming tap with a 1.5 pitch chamfer cutting 10 mm of thread wants a hole near 14 mm deep. If it still cannot reach, run a second operation with a longer-chamfer tap instead of a smaller drill. Matched taps and dies should list style and chamfer so you can run the depth math before you order.

Worked Examples: 1/4-20, 5/16-18, M8x1.25

1/4-20 UNC

Start with the thread: 1/4 in major, 20 threads per inch, coarse series. The ANSI B1.1 row answers letter F, 0.2570 in, 6.53 mm, about 75 percent engagement. Whether you search the 1/4-20 tap drill size or type it loosely as 1 4 20 tap drill size, every published chart lands on letter F, and the drill bit to tap 1/4 20 in is an F, not a quarter-inch.

5/16-18 UNC

Same chain, one row down. The coarse 5/16-18 row gives letter I, 0.3190 in, 8.10 mm, about 75 percent. Note that letter I is larger than a 5/16 in drill, 0.3125 in. Anyone looking up the 5/16-18 tap drill size and grabbing a plain 5/16 drill has undersized the hole and is running near the 90 percent band where broken squares live.

M8x1.25

The metric chain runs in millimeters end to end. M8x1.25: nominal 8 mm, pitch 1.25 mm. The published ISO chart row says 6.8 mm, and that is the drill to order. Sanity-check with the formula: d minus P, 8 minus 1.25, equals 6.75 mm, within 0.05 mm of the chart row. For the m8 1.25 tap drill size, order the 6.8 mm; in inch-only stock, 17/64 in at 0.2656 in sits a hair under it. In production stainless, drill at the chart size with a sharp carbide drill rather than undersizing. You can open a hole, but you cannot un-cut one.

Frequently Asked Questions

What is the drill size for a 1/4-20 tap?

Letter F, 0.2570 in, 6.53 mm, per the published ANSI B1.1 chart, about 75 percent thread. A 1/4 in drill, at 0.2500 in, is undersized and pushes engagement toward the 90 percent band where taps break.

What is the M8x1.25 tap drill size?

6.8 mm, straight from the published ISO metric chart. The d minus P formula returns 6.75 mm, which confirms the row rather than replacing it. In inch units, 17/64 in is the common shop answer when metric drills are not in the rack.

What is the M10 tap drill size?

8.5 mm for coarse M10x1.5, about 75 percent engagement on the standard chart. The formula gives 10 minus 1.5, so 8.5 mm exactly, which is why M10 is the row people quote when they claim the shortcut always works. M10x1.0 fine is different: 9 mm.

What is the drill size for a 1/2 inch tap?

For 1/2-13 UNC, most common shop charts list 31/64 in, at 0.4844 in, about 75 percent engagement (published charts split here: letter Q at 0.4970 in or a 1/2 drill for 1/2-13, depending on target engagement, so check the standard sheet your shop follows). At this size the difference between rows is real metal, so check which engagement your chart intends before you drill.

What is the drill size for a 6-32 tap?

Number 29 drill, 0.1360 in, 3.45 mm, per the published ANSI chart, about 75 percent thread. Small numbered drills drift oversize as they wear, and a 6-32 thread has little margin at that scale. Check the drill with a pin gauge before you trust the hole.

What is the drill size for a 10-32 tap?

Number 21 drill, 0.1590 in, 4.04 mm, for 10-32 UNC on the published chart. The 10-32 UNF fine thread uses a larger number drill, so read the threads-per-inch, not just the 10.

The Bottom Line

Pick the drill from the published chart row, convert letter or number to decimal, and land near 75 percent thread unless the material or the tap style gives you a named reason to move. The d minus P shortcut is a metric sanity check, never the imperial source. Form taps, blind holes and stainless each bend the rules in one direction only, and a disagreement between two printed charts is a question of engagement target, not opinion. When the thread is load-critical, compute the shear area instead of trusting the rule of thumb. The lookup is already done for you at /tools/tap-drill-size-chart/, size by size, UNC, UNF and metric in one place.

InsertCore supplies carbide drills and HSS taps gauged to the published ANSI and ISO sizes, so the F you order measures 0.2570 in on day one. Tell us the threads, the material and the hole type, and the quote comes back with drill sizes matched to your chart.