This page helps CNC process engineers, programmers, and tooling buyers choose a tap drill size for metric, UNC, and UNF threads: pick the thread system and pitch first, read cut-tap drill charts with a named percent-of-thread basis, keep form-tap prep holes on a separate larger-hole path, then lock tolerance, material ductility, and effective thread height before RFQ. Sister OWN stays on the live form-tap versus cut-tap compare. This page is the hub tap-drill chart OWN; it is not a dedicated 1/4-20 page, 5/16 page, 10-32 page, bolt clearance-hole chart, NPT tap-drill book, form-versus-cut encyclopedia, insert or rivnut OWN, or a full tap-type blind-hole catalog, and it does not invent an unconditional ranking, price, life claim, or unsourced size.
Start from the live form tap vs cut tap guide when method (form versus cut) is still open. Browse live constructions on taps and carbide drill bits. When a thread mill may replace tapping, use the live thread milling vs tapping aluminum guide and browse thread mills. Hole-process framing sits on the live precision holemaking selection guide. Dedicated size pages for 1/4-20, 5/16, and 10-32, plus a bolt clearance-hole chart and an NPT tap-drill guide, will be linked after they publish (soft-note until live). End with RFQ when the package is ready: custom cutting tool RFQ.
Quick answer
- Lock system and pitch before any drill letter: name ISO metric coarse/fine (for example M6x1.0 versus M6x0.75), UNC, or UNF, and write TPI or pitch on the traveler. A #10-24 and a #10-32 are different threads; a 6-32 UNC machine screw is not a metric M size.
- Use cut-tap charts at a stated percent-of-thread basis (commonly about 75% on published shop charts): common cut-tap starters include #6-32 to a #36 (0.1065 in), #10-24 to a #25 (0.1495 in), #10-32 to a #21 (0.1590 in), 1/4-20 to a #7 (0.2010 in), and M6x1.0 to 5.0 mm. Charts can disagree by one stock size; verify against the drawing class and the tap maker chart before production.
- Keep form taps separate: a forming (roll) tap needs a larger prep hole than a cut tap for the same nominal thread. Do not reuse the cut-tap drill. Confirm ductility, lubrication, and the tap maker form-drill window; method detail stays on the live form-versus-cut guide.
- Next step: name material, hole type (blind/through), percent-of-thread or class intent, cut versus form, and drill size with units, then open RFQ with tap and drill intent open if needed. Dedicated 1/4-20, 5/16, 10-32, clearance-hole, and NPT pages remain soft-noted until live.
Selection path (stop when a field is missing):
| Step | What you lock | If missing |
|---|---|---|
| 1 System and pitch | Metric / UNC / UNF, pitch or TPI, coarse vs fine | Stop; recover the print callout |
| 2 Method | Cut tap, form tap, or open; ductility note | Do not pick a drill from habit alone |
| 3 Percent of thread / class | Target engagement or thread class and how it will be checked | Keep drill open; do not invent a % |
| 4 Prep-hole size | Drill designation + decimal/mm, source chart family | Do not tap from a clearance hole |
| 5 Material and hole type | Grade/condition, blind vs through, depth, interruptions | Escalate before production |
| 6 Trial and RFQ | First-off gauge result + matched RFQ package | Open RFQ with incomplete SKU if needed |
Jump: System and pitch · Cut-tap charts · Form-tap prep · Worked examples · Tolerance and material · Dedicated entries · RFQ · Confirm · FAQ
Pick the thread system and pitch first
A tap drill size is meaningless until the thread callout is locked. Shop slang that says "a 10" or "a quarter" without TPI or pitch is how the wrong chart row gets used.
- ISO metric: write major diameter and pitch in millimetres (M8x1.25 coarse versus M8x1 fine). Unmarked metric fasteners are usually coarse, but the print wins. Fine pitch leaves less material to remove and needs a larger drill than the matching coarse size at the same major diameter.
- UNC (Unified National Coarse): inch major size with a coarse TPI, such as #6-32, #8-32, #10-24, or 1/4-20. Unified inch thread geometry is framed in the ASME B1.1 family; use public standards literacy such as this codes and standards portal for naming only, not as a pasted B1.1 substitute.
- UNF (Unified National Fine): the same nominal size with a finer TPI, such as #10-32 or 1/4-28. A #10-24 UNC and a #10-32 UNF share a nominal major diameter near 0.190 in but need different tap drills.
- Not this page's OWN: NPT and other pipe tapers belong on a dedicated soft-noted NPT guide. Bolt clearance (through) holes are not tap drills; they stay soft-noted on the clearance-hole chart until live. Thread milling versus tapping stays on its live sibling.
Public screw tables such as this laboratory UNC/UNF screw table and fastener literacy such as this tap and drill size chart help teams name size, TPI, and common cut-tap drills. Metrology literacy such as this public metrology laboratory entry and this weights and measures overview supports keeping inch and millimetre units honest on the traveler.
Cut-tap drill charts (metric, UNC, UNF)
The tables below list widely published cut-tap drill sizes aimed at roughly 75% of full thread for general mechanical work. That percent-of-thread basis is a shop convention used on many public charts; it is not a universal strength guarantee and not an HC measured result for every grade. Source boundary for these rows: ISO metric coarse/fine practice using the common rule of thumb (drill ≈ major diameter − pitch, rounded to a stock drill) as reflected in public metric tap-drill references, and ASME B1.1-style Unified inch chart practice as reflected in public UNC/UNF tap-drill references. Engineering applicability: cut taps only; ductile or free-cutting materials at commercial thread targets; blind or through holes when chip exit is planned. Verify against the drawing thread class and the tap maker chart before production. Charts sometimes disagree by one letter or millimetre size (for example some #10-24 rows list #25 while others list #26).
CNC threading literacy such as this CNC threading and tapping overview and this tap drill chart sizes overview frames percent-of-thread talk without turning this page into a feeds library. A consolidated public chart such as this UNC UNF and metric tap drill chart and this tap drill reference table supports cross-checking common sizes. Threading education such as this threading knowledge overview and this tapping knowledge overview adds process naming only.
Metric cut-tap starters (ISO coarse / selected fine)
| Thread | Pitch (mm) | Typical cut-tap drill (mm) | Approx. basis | Confirm when… |
|---|---|---|---|---|
| M3 | 0.5 | 2.5 | ~75% cut-tap chart | Fine pitch M3x0.35 or form tap |
| M4 | 0.7 | 3.3 | ~75% cut-tap chart | Hard material needing lower % |
| M5 | 0.8 | 4.2 | ~75% cut-tap chart | Print class or maker chart differs |
| M6 | 1.0 | 5.0 | ~75% cut-tap chart | M6x0.75 fine (often ~5.25 mm) |
| M8 | 1.25 | 6.8 | ~75% cut-tap chart | M8x1 fine (often ~7.0 mm) |
| M10 | 1.5 | 8.5 | ~75% cut-tap chart | M10x1.25 / M10x1 fine rows |
| M12 | 1.75 | 10.2 | ~75% cut-tap chart | M12x1.5 / M12x1.25 fine rows |
Source boundary: ISO metric coarse (and named fine) cut-tap practice; public chart consensus for ~75% engagement; units millimetres. Omit sizes you cannot match to a drawing pitch. National standards literacy such as this national standards overview supports naming ISO/DIN thread families only.
UNC / UNF cut-tap starters (selected common sizes)
| Thread | Series | TPI | Typical cut-tap drill | Decimal (in) | Confirm when… |
|---|---|---|---|---|---|
| #6-32 | UNC | 32 | #36 | 0.1065 | Form tap or thin wall |
| #8-32 | UNC | 32 | #29 | 0.1360 | Maker chart or lower % target |
| #10-24 | UNC | 24 | #25 | 0.1495 | Some charts list #26; verify |
| #10-32 | UNF | 32 | #21 | 0.1590 | Do not swap with #10-24 |
| 1/4-20 | UNC | 20 | #7 | 0.2010 | Deep dive soft-noted on dedicated page |
| 1/4-28 | UNF | 28 | #3 | 0.2130 | Deep dive soft-noted with 1/4-20 |
| 5/16-18 | UNC | 18 | F | 0.2570 | Deep dive soft-noted on 5/16 page |
Source boundary: ASME B1.1-style Unified inch cut-tap chart practice; public UNC/UNF tap-drill references at ~75% engagement; units inch letter/number drills with decimal. Dedicated long-form pages for 1/4-20 / 1/4-28, 5/16, and 10-32 remain soft-noted until live so this hub does not steal their OWN. Public distributor help such as this tap drill sizes help page is literacy only, not an HC trial.
Schematic illustration: not a catalogue ranking or cutting-data table. Confirm drill diameter, percent-of-thread target, and cut versus form on your drawing and tap maker chart.
Form-tap prep holes (separate from cut-tap charts)
Forming (roll / cold-form) taps displace material instead of cutting chips at the flanks. Because material must flow into the thread form, the prep hole is larger than the cut-tap drill for the same nominal thread. Copying a cut-tap chart onto a form tap is a common root cause of swell, incomplete crests, or excess torque.
- Method first: if form versus cut is still open, finish that decision on the live form tap vs cut tap guide before hunting a drill letter.
- Ductility gate: industry tendency favors form taps on ductile workpieces (many aluminum alloys, low-carbon steels, some stainless grades) and cut taps on brittle or hard-to-form materials (many cast irons, some hardened conditions). Grade certificates beat nicknames.
- Percent-of-thread framing: form-tap maker charts often aim at a different engagement window than the ~75% cut-tap convention (industry discussion often cites mid-range targets such as about 55% to 75% depending on material and maker). This page does not invent a permanent form-tap size matrix as an HC measured guarantee. Use the tap maker form-drill chart for the SKU you will run.
- Lubrication: forming usually needs a robust lubricating film; cut tapping often prioritizes chip wash. Keep fluid and method on the same traveler line.
- What we omit: exact form-drill letters for every metric and Unified size are not pasted here when they would pretend to be universal. Prefer "confirm with tap maker / drawing" over an unsourced cell.
| Axis | Cut-tap tendency | Form-tap tendency | Keep open when… |
|---|---|---|---|
| Prep-hole size | Smaller core; chips removed | Larger prep hole; material displaces | Maker form chart missing |
| % thread talk | Often ~75% on public cut charts | Often a maker-specific mid band | Acceptance method unnamed |
| Material | Broader, including brittle grades | Needs ductility to flow | Certificate grade unknown |
| Blind hole chips | Flute exit plan required | Chip-free flanks; still need lobe clearance | Bottom depth unknown |
| RFQ note | State cut method | State form / roll method | Method still undecided |
This compare table is conditional. It is not an unconditional ranking of form over cut or of any brand. Tapping education portals cited above support naming only.
Worked examples (searchable intents, hub depth only)
These examples answer common queries without replacing dedicated sister pages. Each size still needs drawing class and maker confirmation before production.
| Intent example | Callout to lock | Common cut-tap starter | Hub note |
|---|---|---|---|
| Bit size for 6-32 tap | #6-32 UNC | #36 (0.1065 in) | Machine-screw UNC; not metric |
| 10-24 drill bit size | #10-24 UNC | #25 (0.1495 in) | Do not use #10-32 drill |
| Drill bit for 10-32 tap | #10-32 UNF | #21 (0.1590 in) | Deep dive soft-noted later |
| 1/4-20 tap drill | 1/4-20 UNC | #7 (0.2010 in) | Dedicated page soft-noted |
| M6 cut-tap drill | M6x1.0 | 5.0 mm | Confirm fine pitch if marked |
| Form tap on any of the above | Same callout + form method | Larger than cut-tap row | Use maker form chart |
Source boundary: same cut-tap chart families as the tables above; examples are teaching aids, not shop results. Unit conversion literacy such as this NIST SP 811 units guidance and this NIST SP 811 publication PDF supports inch/mm discipline on travelers. Engineering calculator literacy such as this engineering calculators portal helps name engagement math without inventing HC life data.
Tolerance, material, percent of thread, and effective height
Drill size, thread class, material, and usable engagement interact. Treat them as a set of gates, not as a single magic letter drill.
- Percent of thread: a higher engagement target means a smaller prep hole and higher tapping torque. Many commercial cut-tap charts sit near 75%; hard materials or deep blind holes often move shops toward a lower percent to protect the tap. Strength gains above roughly the mid-70s are often small relative to the torque rise in industry discussion. Do not invent a life or strength percentage as an HC result.
- Thread class / tolerance: the print may call a class (for example Unified 2B-style acceptance or a metric class). Class and percent of thread are related but not interchangeable labels. Gauge the finished thread to the print method.
- Material ductility and hardness: stainless and tough alloys often prefer a slightly larger drill (lower %) on cut taps to reduce break risk. Soft aluminum may hold a smaller drill, but form tapping may still want its own larger window. Record grade and condition beside the drill.
- Effective thread height / depth: usable full threads matter more than a single diameter number. Blind holes need unused depth for the tap chamfer (and for form lobes). Interrupted holes and thin walls change both cut and form risk.
- Tap drill versus clearance hole: a tap drill leaves material for the thread. A bolt clearance hole is intentionally larger so the bolt shank passes. Do not tap a clearance hole and call it a tap drill. Clearance-hole sizing stays soft-noted until that chart is live.
| Boundary | Lock on the sheet | Fail signal | Do not invent |
|---|---|---|---|
| % thread / class | Target and check method | Gauge fail or tap break | Fake strength % as HC data |
| Material | Grade, condition, ductility note | Torn crests, swell, break | Life % from grade slang |
| Hole type / depth | Blind/through, unused depth | Chip pack or incomplete form | Price or ROI from depth |
| Drill vs clearance | Named as tap drill or clearance | Loose bolt or missing thread | Unsourced "universal" hole |
Schematic illustration: boundaries only, not a catalogue ranking. Confirm tap-drill versus clearance intent on the drawing before picking a drill.
Toolholding literacy such as this tool holder technology overview and cutting-data literacy such as this cutting data notes portal support naming sync tapping and holder stiffness beside drill choice. Research society literacy such as this CIRP research portal frames process research boundaries only. Fastener mechanics literacy such as this bolt science portal supports naming engagement talk without inventing case counts.
Dedicated chart entries and compare posture (soft until live)
This hub answers the cluster of metric/UNC/UNF lookups and the cut-versus-form drill split. Sister pages will own deeper single-size or special-thread work. Until those URLs are live, treat the following as soft notes in prose only (no hard 200 claim):
- 1/4-20 and 1/4-28 deep dive (planned path
/resources/1-4-20-tap-drill-size) - 5/16-18 and 5/16-24 deep dive (planned path
/resources/5-16-tap-drill-size) - 10-32 deep dive (planned path
/resources/10-32-tap-drill-size) - Bolt clearance hole chart (planned path
/resources/bolt-clearance-hole-chart) - NPT tap drill sizes (planned path
/resources/npt-tap-drill-sizes)
Compare tables on this page stay conditional: match the bottleneck (method, pitch, material, percent of thread). An unconditional "best tap drill" ranking is not a process. Competitor brand promotion is out of scope in the visible body.
Procurement checklist (RFQ inputs)
Send what you know. Leave life, price, and unverified cycle-time fields blank. Do not invent an unconditional tap-drill ranking.
- Drawing revision and thread callout (metric / UNC / UNF, pitch or TPI, class if known).
- Cut tap, form tap, or open method; ductility note if known.
- Percent-of-thread or engagement target and how it will be checked.
- Proposed tap-drill size with units (letter/number drill and/or mm) and chart family used.
- Material grade, condition, and coolant or forming-oil intent.
- Blind versus through, thread depth, unused depth, interruptions, thin walls.
- Machine sync / rigid tapping notes, holder type, and stick-out method if known.
- Acceptance gauges or inspection method; first-off notes.
- Identifiers needed for repeat orders.
Browse live constructions here: taps and carbide drill bits. When a thread mill may be the better path, browse thread mills. Open the request form when the package is ready: custom cutting tool RFQ.
Confirm the specification with engineering support
Before production ramp, confirm thread callout, cut versus form, prep-hole size with units, percent-of-thread or class intent, material condition, hole type and depth, and acceptance method with engineering. Hand the package to a product and RFQ path for a matched tap and drill recommendation, not a catalogue guess. HENTRY / HC supports drawing-based tap and carbide drill proposals. No price, MOQ, or unverified lead time is stated on this page.
Specification fields to confirm
- Proposed tap model or drawing (cut or form) and matching drill designation.
- Thread callout: system, pitch/TPI, class if known.
- Percent-of-thread or engagement target and check method.
- Material grade / condition and lubricant mode.
- Blind / through, depth, unused clearance, interruptions.
- Machine sync / holder / stick-out notes if known.
- Inspection and sample acceptance expectations.
- Identifiers needed for repeat orders.
Trial plan (minimum)
- Same drawing revision and acceptance criteria as the checklist.
- First-off record of torque feel, chip or form signal, gauge result, and hole size before compensation edits.
- Change one variable at a time when comparing drill size, percent of thread, or cut versus form under the same material and acceptance intent.
- Stop when torque grows abnormally, crests look incomplete, gauges fail, or the tap breaks.
- Do not claim a drill size "won" without those controls.
Review listed constructions here: explore taps · explore carbide drill bits · explore thread mills. Research and practice portals such as this CIRP research portal (also cited above) and tooling resource literacy such as this tooling resources overview support industry literacy only.
Request a tap drill / tap recommendation
Suggested enquiry subject: tap drill - [thread callout] - [cut/form] - [material] - [blind/through] - [drill size]
Tap Drill Size Chart FAQ
What size drill bit for a 6-32 tap?
For a cut tap on #6-32 UNC, published shop charts commonly list a #36 drill (0.1065 inch) at about 75 percent of full thread. Confirm the print class, material, and whether the method is cut or form. A form tap needs a larger prep hole from the tap maker chart. This hub answer does not invent a life or strength percentage.
What size drill bit for a 10-24 tap?
For a cut tap on #10-24 UNC, many public charts list a #25 drill (0.1495 inch) near the 75 percent basis. Some charts list #26 instead; verify against the tap maker chart and drawing. Do not use the #10-32 drill. Form taps use a separate larger prep hole.
What size drill bit for a 10-32 tap?
For a cut tap on #10-32 UNF, published charts commonly list a #21 drill (0.1590 inch) near the 75 percent basis. Keep #10-24 and #10-32 on different rows. A dedicated 10-32 page will expand this after it publishes. Form taps still need the maker form-drill window.
How do metric cut-tap drills relate to pitch?
For ISO metric coarse threads, a common cut-tap rule of thumb is drill diameter approximately equal to major diameter minus pitch, rounded to a stock drill, which targets about 75 percent engagement on many charts. Examples include M6x1.0 to 5.0 mm and M8x1.25 to 6.8 mm. Fine-pitch callouts need different drills. Always confirm pitch on the print.
Why must form-tap drills be larger than cut-tap drills?
Form taps displace material into the thread form instead of cutting chips at the flanks, so the starting hole must leave room for that flow. Reusing a cut-tap drill on a form tap often causes swell, incomplete crests, or excess torque. Choose method first, then use the form-tap maker chart for the size window.
What is the difference between a tap drill and a bolt clearance hole?
A tap drill is sized to leave material so the tap can form or cut the internal thread. A bolt clearance hole is intentionally larger so the bolt shank passes freely without engaging threads in that plate. Do not treat clearance sizes as tap drills. A dedicated clearance-hole chart will cover fit classes after it publishes.
Which inputs should I send in an RFQ for tap drill and tap selection?
Send drawing revision, thread callout with pitch or TPI, cut versus form or open, percent-of-thread or class intent, proposed drill size with units, material and lubricant intent, blind versus through and depth notes, acceptance method, and trial notes. Leave unverified life and price fields blank.
When does this page not apply?
It does not apply when you need invented life percentages, prices, ROI, or unverified case counts; when you need an unconditional best-drill ranking; when you need a dedicated 1/4-20, 5/16, 10-32, clearance-hole, or NPT encyclopedia; when you need the full form-versus-cut book instead of prep-hole rules; or when system, pitch, method, and acceptance notes are missing. Fix the inputs first.
Sources and related guides
Outbound references support Unified and metric thread naming, public tap-drill chart literacy, tapping and threading education, metrology and units portals, toolholding and cutting-data literacy, and manufacturing research portals cited in the body. Manufacturer-specific tap drill charts remain tied to their own products. The decision walkthrough is an HC editorial teaching aid, not a measured customer result. Sources checked 24 October 2026 (Asia/Shanghai).
- Codes and standards portal (public standards entry; linked in body)
- Laboratory UNC/UNF screw table (public lab table; linked in body)
- Tap and drill size chart (third-party fastener education; linked in body)
- UNC UNF and metric tap drill chart (third-party reference; linked in body)
- Tap drill chart sizes overview (third-party education; linked in body)
- CNC threading and tapping overview (third-party education; linked in body)
- Tap drill reference table (third-party reference; linked in body)
- Threading knowledge overview (third-party education; linked in body)
- Tapping knowledge overview (third-party education; linked in body)
- Tap drill sizes help page (third-party distributor education; linked in body)
- Public metrology laboratory entry (public metrology entry; linked in body)
- Weights and measures overview (public metrology entry; linked in body)
- NIST SP 811 units guidance (public metrology entry; linked in body)
- NIST SP 811 publication PDF (public metrology entry; linked in body)
- National standards overview (public standards entry; linked in body)
- Engineering calculators portal (third-party tooling education; linked in body)
- Tool holder technology overview (third-party education; linked in body)
- Cutting data notes portal (third-party tooling education; linked in body)
- CIRP research portal (third-party research society; linked in body)
- Bolt science portal (third-party fastener mechanics literacy; linked in body)
- Tooling resources overview (third-party tooling literacy; linked in body)
Related live guides: form tap vs cut tap · thread milling vs tapping aluminum · precision holemaking selection guide · taps · carbide drill bits · thread mills · Knowledge hub · custom cutting tool RFQ. Dedicated 1/4-20, 5/16, and 10-32 tap-drill pages, a bolt clearance-hole chart, and an NPT tap-drill guide will be linked after they publish (soft-note until live).