Thread and hole
- Thread series, nominal diameter, pitch or TPI, and class.
- Internal-thread depth and blind or through-hole condition.
- Hole count, location and available edge distance.
- Cutting tap, forming tap or another specified process.
Tap Drill Chart: Metric, UNC and UNF Hole Sizes
Use this tap drill chart to find a cutting-tap starting drill size, compare metric and inch callouts, and include the necessary details in a custom tapped-part drawing. The tables below stay visible when JavaScript is unavailable.
Chart scope:
These are cutting-tap reference sizes for common threads. A forming tap, blind hole, material-specific process, or finished-thread requirement needs its own review.
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From drawings or samples
Non-standard metal components made to client inputs.
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Machining processes
CNC turning, milling, wire EDM, grinding and boring.
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Material families
Steels, stainless steels, nickel-based alloys and aluminum alloys.
Metric Tap Drill Chart
Start with the thread’s nominal diameter and pitch, then read the drill column as a cutting-tap reference for metric threads. The drill and tap chart filters both the metric and the Unified tables below.
Find a thread or drill
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Series
- All series
- Metric
- UNC
- UNF
Showing 18 chart rows.
| Thread | Pitch | Cutting-tap drill | Decimal in | Series |
|---|---|---|---|---|
| M3 × 0.5 | 0.5 mm | 2.5 mm | 0.0984 | Coarse |
| M4 × 0.7 | 0.7 mm | 3.3 mm | 0.1299 | Coarse |
| M5 × 0.8 | 0.8 mm | 4.2 mm | 0.1654 | Coarse |
| M6 × 1 | 1.0 mm | 5.00 mm | 0.1969 | Coarse |
| M6 × 0.75 | 0.75 mm | 5.25 mm | 0.2067 | Fine |
| M8 × 1.25 | 1.25 mm | 6.75 mm / 17/64 in | 0.2657 | Coarse |
| M8 × 1 | 1.0 mm | 7.00 mm | 0.2756 | Fine |
| M10 × 1.5 | 1.5 mm | 8.50 mm | 0.3346 | Coarse |
| M10 × 1.25 | 1.25 mm | 8.75 mm | 0.3445 | Fine |
Source note
The values and inch equivalents are transcribed from the 6G Tools metric chart. In a machine shop, a wrong M8 pitch can change the production pilot from 6.75 mm at M8 × 1.25 to 7.00 mm at M8 × 1; these drill sizes are not interchangeable, so match the exact row to the available drill bit.
UNC and UNF Tap Drill Chart
Unified threads are threads per inch (TPI), and the drill can be a numbered, lettered or fractional bit. This drill size chart connects screw size and TPI to the pilot diameter.
| Thread Series | Cutting-tap drill | Decimal in | Approx. mm |
|---|---|---|---|
| 1/4-20 UNC | #7 | 0.2010 | 5.11 |
| 5/16-18 UNC | F | 0.2570 | 6.53 |
| 5/16-24 UNF | I | 0.2720 | 6.91 |
| 3/8-16 UNC | 5/16 in | 0.3125 | 7.94 |
| 3/8-24 UNF | Q | 0.3320 | 8.43 |
| 7/16-14 UNC | U | 0.3680 | 9.35 |
| 7/16-20 UNF | 25/64 in | 0.3906 | 9.92 |
| 1/2-13 UNC | 27/64 in | 0.4219 | 10.72 |
| 1/2-20 UNF | 29/64 in | 0.4531 | 11.51 |
Source note: Drill designations and decimal values follow the 6G Tools inch chart. In a machine shop, a wrong TPI risks a mismatched hole: 3/8-16 UNC uses 0.3125 in while 3/8-24 UNF uses 0.3320 in; this row is not a substitute for the required thread class and acceptance criteria.
Tap Drill and Clearance Hole Diagram
A tap drill prepares the hole that will receive internal threads; a clearance hole lets a fastener pass through another part. The two hole types serve different functions in the assembly.
- Tapped part: internal thread
- Upper part: clearance hole
Before tapping
The pilot diameter is chosen for the tap type and planned engagement of thread. A wrong choice risks damage to internal threads.
Before assembly
The passing part needs a clearance fit; it does not use the tapped-hole row automatically.
Specifications / Notes
Conceptual cross-section, not to scale. Select clearance-hole fit separately; ISO 273 distinguishes fine, medium and coarse clearance-hole series.
Cutting Tap Selection Notes
A published chart usually supplies one starting drill value. Guhring’s calculator note describes that single value as an approximate 75% thread and discusses the torque trade-off when lower target percentages are suitable.
The trade-off is practical: a 6.75 mm M8 reference can be the wrong production call if the tap type changes, because forming taps displace metal. Reusing that cutting-tap row blindly would be the wrong choice; confirm against the selected tap, material and part acceptance requirements.
Tap type
Cutting and forming taps displace metal differently. Use a process-specific drill recommendation for a forming tap.
Hole geometry
Through and blind holes change chip evacuation and usable thread depth. Include the hole condition in the part drawing.
Finished thread
Nominal drill size in and of itself does not establish an accepted thread. Identify the class and inspection criteria in the drawing or the specification that is referenced.
Custom Tapped Metal Parts We Machine at Zhenling
Shanghai Zhenling Hardware Co., Ltd. was established in 2006, with our factory in Jiashan County, Zhejiang. Our input route is simple: We machine non-standard parts from client drawings or samples.
- Turning We use CNC lathes for turned components.
- Milling We use CNC milling machines for custom components.
- Wire EDM We use CNC wire-cut EDM machines.
- Finishing cuts We use CNC grinding and boring machines.
- Multiaxis work We operate 3-axis, 4-axis, and imported 5-axis machining centers.
- Structural steels We machine carbon, alloy, and bearing steels.
- Stainless grades We work with 303, 304, 304L, 316, 316L, and 321 stainless steels.
- Special materials We process nickel-based alloys, C276, 904L, and aluminum alloys.
- Part families We make assemblies, flanges, rollers, valve bodies, plugs, and other custom parts.
For a custom threaded component, price follows material, geometry, thread specification, quantity and finishing.
Send the part drawing for a project-specific reviewDrawing Review Checklist and Inspection Criteria
A clear thread callout changes a reference drill size into a reviewable request. Include any external specification that provides acceptance criteria, even if those criteria are incorporated by reference.
Part and acceptance
- Material grade, condition and relevant hardness requirement.
- Drawing revision, quantity and mating-part context.
- Required inspection or gaging criteria.
- Finish or corrosion requirements where they affect the thread.
Worked Examples for M6, M8 and 1/4-20
M6 × 1 metric
Read the 1.0 mm pitch row: the listed cutting-tap drill is 5.00 mm (0.1969 in). Review material, thread class and hole depth before specifying the production process.
M8 × 1.25 metric
The cited row lists 6.75 mm (17/64 in; 0.2657 in decimal). The chart value is a starting reference for a cutting tap, not a universal forming-tap diameter.
1/4-20 UNC
Find 20 TPI in the UNC table: the listed drill is #7, 0.2010 in (5.11 mm). Confirm the Unified class and acceptance criteria separately.
Process Notice
A wrong pitch, tap type or inspection assumption can invalidate an otherwise correct drill-chart lookup. Compare the example against the part drawing before production. Source rows: metric chart and UNC chart.
When a Chart Row Is Only a Starting Point
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01 /
Distinguish tap-drill holes from bolt clearance holes; the wrong hole type risks assembly mismatch.
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02 /
State a cutting-tap assumption beside any chart row; a forming tap requires its own pilot-hole selection.
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03 /
Preserve exact M8 values when comparing a 6.75 mm source row with another drill series.
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04 /
Explain that a nominal drill value does not set finished-thread acceptance or inspection.
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05 /
Give a drawing checklist for blind/through geometry, engagement, material and acceptance method.
Note
A chart row is not always the right production drill for a deep blind hole. Cutting Tool Engineering’s deep-hole guide explains why chip evacuation and tool choice become important as thread depth grows.
Tap Drill Engineering Tools & Calculators
Drill Diameter Unit Converter
Convert drill bit dimensions between metric and imperial standard units instantly. Ensure accurate tooling specifications for international engineering drawings.
Open Drill Diameter Unit ConverterMetric Cutting-Tap Pilot Estimate
Calculate precise pilot hole sizes by subtracting the thread pitch from the nominal metric diameter. A reliable reference for CNC pre-machining preparation.
Open Pilot Estimate ToolThread Callout Checklist
Verify standard engineering thread callout formats to avoid manufacturing discrepancies. Ensure your CNC machining requests meet standard drafting specifications.
Open Thread Callout ChecklistFAQ: Frequently Asked Questions
The cited metric chart lists 5.00 mm for the cutting-tap starting hole.
The source row lists 6.75 mm, also shown as 17/64 in.
The cited inch chart is #7, which is approximately 0.2010 in or 5.11 mm.
No. One prepares an internally threaded part; the other allows the fastener to pass through a mating part.
Forming taps require a different method of pilot-hole selection. Check the tap manufacturer’s recommendation for the material and target thread.
No. Guhring states that a chart’s single-size value is approximate and gives a reason to explain why the engagement can change with application conditions.
Reference tables can have different preferred bit series, rounding, and engagement assumptions. Follow the cited row and examine the actual tap and the part needs.
Indicate thread designation, class or tolerance, location, depth, material, and applicable inspection criteria or referenced specification.
Send the drawing or sample details through the part-review form, along with the thread and material requirements.
