The Fix / Size guides
Tap Drill Size Chart: Metric, UNC, UNF & BSW
Find a suggested drill diameter before cutting an internal thread. This Simfix chart lists metric coarse M3–M24 and selected UNC, UNF and BSW threads, with nominal diameters, pitch or threads per inch, and links to matching fastener examples.
Quick answers: M6 × 1 needs a suggested 5 mm drill; M8 × 1.25 needs 6.8 mm; M10 × 1.5 needs 8.5 mm. Confirm the thread marking before choosing a drill.
Simfix tap drill size chart. Four A4 pages covering all 62 thread sizes.
Showing 62 of 62 threads.
No threads match that search. Try a diameter such as M8, an imperial size such as 1/4, or a family such as UNC.
Metric coarse tap drill chart: M3–M24
Metric sizes show nominal diameter × pitch in millimetres. These rows cover coarse pitch only.
| Thread | Diameter (mm) | Pitch (mm) | Suggested tap drill (mm) | Matching fastener example |
|---|---|---|---|---|
| M3 × 0.5 | 3 | 0.5 | 2.5 | View M3 × 0.5 fastener |
| M3.5 × 0.6 | 3.5 | 0.6 | 2.9 | View M3.5 × 0.6 fastener |
| M4 × 0.7 | 4 | 0.7 | 3.3 | View M4 × 0.7 fastener |
| M4.5 × 0.75 | 4.5 | 0.75 | 3.7 | No example listed |
| M5 × 0.8 | 5 | 0.8 | 4.2 | View M5 × 0.8 fastener |
| M6 × 1 | 6 | 1 | 5 | View M6 × 1 fastener |
| M8 × 1.25 | 8 | 1.25 | 6.8 | View M8 × 1.25 fastener |
| M10 × 1.5 | 10 | 1.5 | 8.5 | View M10 × 1.5 fastener |
| M12 × 1.75 | 12 | 1.75 | 10.2 | View M12 × 1.75 fastener |
| M14 × 2 | 14 | 2 | 12 | View M14 × 2 fastener |
| M16 × 2 | 16 | 2 | 14 | View M16 × 2 fastener |
| M18 × 2.5 | 18 | 2.5 | 15.5 | View M18 × 2.5 fastener |
| M20 × 2.5 | 20 | 2.5 | 17.5 | View M20 × 2.5 fastener |
| M22 × 2.5 | 22 | 2.5 | 19.5 | View M22 × 2.5 fastener |
| M24 × 3 | 24 | 3 | 21 | View M24 × 3 fastener |
UNC tap drill chart
Unified National Coarse threads use a 60° form. TPI means threads per inch.
| Thread | Diameter (mm) | TPI | Suggested tap drill (mm) | Matching fastener example |
|---|---|---|---|---|
| #2-56 UNC | 2.1844 | 56 | 1.85 | View #2-56 UNC fastener |
| #3-48 UNC | 2.5146 | 48 | 2.1 | View #3-48 UNC fastener |
| #4-40 UNC | 2.8448 | 40 | 2.3 | View #4-40 UNC fastener |
| #5-40 UNC | 3.175 | 40 | 2.6 | View #5-40 UNC fastener |
| #6-32 UNC | 3.5052 | 32 | 2.8 | View #6-32 UNC fastener |
| #8-32 UNC | 4.1656 | 32 | 3.4 | View #8-32 UNC fastener |
| #10-24 UNC | 4.826 | 24 | 3.8 | View #10-24 UNC fastener |
| #12-24 UNC | 5.4864 | 24 | 4.5 | No example listed |
| 1/4-20 UNC | 6.35 | 20 | 5.1 | View 1/4-20 UNC fastener |
| 5/16-18 UNC | 7.9375 | 18 | 6.6 | View 5/16-18 UNC fastener |
| 3/8-16 UNC | 9.525 | 16 | 8 | View 3/8-16 UNC fastener |
| 7/16-14 UNC | 11.1125 | 14 | 9.4 | View 7/16-14 UNC fastener |
| 1/2-13 UNC | 12.7 | 13 | 10.8 | View 1/2-13 UNC fastener |
| 9/16-12 UNC | 14.2875 | 12 | 12.2 | View 9/16-12 UNC fastener |
| 5/8-11 UNC | 15.875 | 11 | 13.5 | View 5/8-11 UNC fastener |
| 3/4-10 UNC | 19.05 | 10 | 16.5 | View 3/4-10 UNC fastener |
| 7/8-9 UNC | 22.225 | 9 | 19.5 | View 7/8-9 UNC fastener |
| 1-8 UNC | 25.4 | 8 | 22.2 | View 1-8 UNC fastener |
UNF tap drill chart
Unified National Fine threads use a 60° form. The listed 1-inch size is 1-12 UNF; 1-14 UNS is excluded.
| Thread | Diameter (mm) | TPI | Suggested tap drill (mm) | Matching fastener example |
|---|---|---|---|---|
| #3-56 UNF | 2.5146 | 56 | 2.1 | No example listed |
| #4-48 UNF | 2.8448 | 48 | 2.35 | No example listed |
| #5-44 UNF | 3.175 | 44 | 2.65 | No example listed |
| #6-40 UNF | 3.5052 | 40 | 2.9 | No example listed |
| #8-36 UNF | 4.1656 | 36 | 3.5 | View #8-36 UNF fastener |
| #10-32 UNF | 4.826 | 32 | 4.1 | View #10-32 UNF fastener |
| #12-28 UNF | 5.4864 | 28 | 4.6 | No example listed |
| 1/4-28 UNF | 6.35 | 28 | 5.5 | View 1/4-28 UNF fastener |
| 5/16-24 UNF | 7.9375 | 24 | 6.9 | View 5/16-24 UNF fastener |
| 3/8-24 UNF | 9.525 | 24 | 8.5 | View 3/8-24 UNF fastener |
| 7/16-20 UNF | 11.1125 | 20 | 9.8 | View 7/16-20 UNF fastener |
| 1/2-20 UNF | 12.7 | 20 | 11.5 | View 1/2-20 UNF fastener |
| 9/16-18 UNF | 14.2875 | 18 | 12.8 | View 9/16-18 UNF fastener |
| 5/8-18 UNF | 15.875 | 18 | 14.5 | View 5/8-18 UNF fastener |
| 3/4-16 UNF | 19.05 | 16 | 17.5 | View 3/4-16 UNF fastener |
| 7/8-14 UNF | 22.225 | 14 | 20.5 | View 7/8-14 UNF fastener |
| 1-12 UNF | 25.4 | 12 | 23.5 | No example listed |
BSW tap drill chart
British Standard Whitworth threads use a 55° form. These are the preferred BSW sizes in this chart; other Whitworth-form sizes are excluded.
| Thread | Diameter (mm) | TPI | Suggested tap drill (mm) | Matching fastener example |
|---|---|---|---|---|
| 1/8-40 BSW | 3.175 | 40 | 2.55 | View 1/8-40 BSW fastener |
| 3/16-24 BSW | 4.7625 | 24 | 3.7 | View 3/16-24 BSW fastener |
| 1/4-20 BSW | 6.35 | 20 | 5.1 | View 1/4-20 BSW fastener |
| 5/16-18 BSW | 7.9375 | 18 | 6.5 | View 5/16-18 BSW fastener |
| 3/8-16 BSW | 9.525 | 16 | 7.9 | View 3/8-16 BSW fastener |
| 7/16-14 BSW | 11.1125 | 14 | 9.3 | View 7/16-14 BSW fastener |
| 1/2-12 BSW | 12.7 | 12 | 10.5 | View 1/2-12 BSW fastener |
| 9/16-12 BSW | 14.2875 | 12 | 12.1 | No example listed |
| 5/8-11 BSW | 15.875 | 11 | 13.5 | View 5/8-11 BSW fastener |
| 3/4-10 BSW | 19.05 | 10 | 16.25 | View 3/4-10 BSW fastener |
| 7/8-9 BSW | 22.225 | 9 | 19.25 | View 7/8-9 BSW fastener |
| 1-8 BSW | 25.4 | 8 | 22 | View 1-8 BSW fastener |
How to choose your tap drill
- Read the complete thread marking. M8 × 1.25 specifies diameter and pitch. For imperial sizes, use both the diameter and TPI, then check whether the thread is UNC, UNF or BSW.
- Choose the row for that exact thread. The suggested drill is smaller than the thread’s nominal diameter, leaving material for the cutting tap.
- Check the job and tap instructions. Confirm the recommendation for your workpiece material, required engagement and hole tolerance. A forming tap needs a different starting hole.
For metric cutting taps, diameter minus pitch is a useful rough estimate: M10 × 1.5 gives 10 − 1.5 = 8.5 mm. A calculated estimate does not replace the manufacturer’s specified drill and tolerance.
Tap drill, bolt diameter and clearance hole
These are three different dimensions. For M8 coarse, the nominal thread diameter is 8 mm and this chart suggests a 6.8 mm cutting-tap drill. A clearance hole is larger than the bolt and has no thread. Use the fastener size guide for measuring and clearance-hole guidance.
Need inches or a diameter comparison?
Use the Metric / Imperial Fastener Converter for mm-to-inch conversions and the nearest diameter in another thread family. A near diameter does not make a replacement thread compatible.
Buying a fastener for the thread
The table links are fastener examples with the named thread, not drills or taps. Check the product’s pitch, length, head, stainless grade and strength before buying. For more options, browse imperial fixings or browse Simfix fasteners.
Tap drill questions
What drill size do I need for an M8 tap?
For a cutting tap marked M8 × 1.25, the suggested starting drill is 6.8 mm. An M8 fine thread has a different pitch and needs a different drill. Check the tap maker’s advice for your material and thread engagement.
What is the difference between a tap drill and a clearance hole?
A tap drill leaves material for a tap to make an internal thread. A clearance hole lets the bolt pass through without threading into that part. The sizes in this chart are for cutting tap holes, not clearance holes.
How do I read an imperial thread size?
In 1/4-20 UNC, 1/4 inch is the nominal diameter, 20 is the number of threads per inch, and UNC is the thread family. To express its pitch in millimetres, divide 25.4 by 20: 1.27 mm.
Can I use the same drill chart for a forming tap?
No. A forming tap displaces material rather than cutting it and needs a different starting hole. Use a chart or recommendation specifically supplied for your forming tap and workpiece material.
Why is the suggested M12 coarse tap drill 10.2 mm?
The listed recommendation for M12 × 1.75 is 10.2 mm. Subtracting pitch from diameter gives 10.25 mm, which is a rough guide rather than the chart’s suggested drill. Confirm the required hole tolerance with your tap manufacturer.
Are similar metric and imperial threads interchangeable?
No. Match the specified thread family, diameter and pitch. UNC and UNF use a 60-degree thread form; BSW uses 55 degrees. A similar diameter or a shared threads-per-inch value does not establish compatibility.
Data and scope
Thread dimensions and suggested cutting-tap drills checked against the Sutton Tools Application Guide, printed page 214, on 5 October 2026. Imperial nominal diameters are converted using exactly 25.4 mm per inch.
Coverage: 15 metric coarse rows, 18 UNC, 17 UNF and 12 preferred BSW. Metric fine, BSF, BA, UNS, pipe threads and forming-tap drills are outside this chart. Do not substitute a coarse-thread row for a fine thread.