Tap Drill Size Calculator
Find the perfect drill bit size for your tapping needs. Select thread type and tap size to get instant results.
Select a tap size
Choose thread type and tap size from the dropdown below
Tap & Drill Size Chart
| Tap Size | Drill Size (Fraction) | Drill Size (Decimal) | Drill Size (mm) |
|---|---|---|---|
| Metric | |||
| M3×0.5 | 7/64" | 0.110" | 2.5mm |
| M4×0.7 | 5/32" | 0.130" | 3.3mm |
| M5×0.8 | 13/64" | 0.165" | 4.2mm |
| M6×1.0 | 13/64" | 0.197" | 5.0mm |
| M8×1.25 | 17/64" | 0.268" | 6.8mm |
| M10×1.5 | 21/64" | 0.335" | 8.5mm |
| M12×1.75 | 25/64" | 0.402" | 10.2mm |
| M16×2.0 | 35/64" | 0.551" | 14.0mm |
| Unified (UNC/UNF) | |||
| 4-40 | 3/32" | 0.089" | 2.26mm |
| 6-32 | 7/64" | 0.106" | 2.69mm |
| 8-32 | 9/64" | 0.136" | 3.45mm |
| 10-24 | 5/32" | 0.149" | 3.78mm |
| 10-32 | 5/32" | 0.159" | 4.04mm |
| 1/4-20 | 13/64" | 0.201" | 5.11mm |
| 5/16-18 | 17/64" | 0.257" | 6.53mm |
| 3/8-16 | 5/16" | 0.312" | 7.92mm |
| 7/16-14 | 23/64" | 0.368" | 9.35mm |
| 1/2-13 | 27/64" | 0.421" | 10.69mm |
| BSW (Whitworth) | |||
| 1/8 BSW | 7/64" | 0.104" | 2.65mm |
| 3/16 BSW | 5/32" | 0.161" | 4.10mm |
| 1/4 BSW | 13/64" | 0.209" | 5.30mm |
| 5/16 BSW | 17/64" | 0.268" | 6.80mm |
| 3/8 BSW | 21/64" | 0.327" | 8.30mm |
| 7/16 BSW | 25/64" | 0.382" | 9.70mm |
| 1/2 BSW | 27/64" | 0.425" | 10.80mm |
| NPT (Pipe Thread) | |||
| 1/8 NPT | 11/32" | 0.339" | 8.61mm |
| 1/4 NPT | 7/16" | 0.438" | 11.13mm |
| 3/8 NPT | 37/64" | 0.578" | 14.68mm |
| 1/2 NPT | 47/64" | 0.734" | 18.64mm |
Note: For critical applications, always verify sizes against manufacturer specifications.
How to Calculate Tap Drill Size?
The tap drill size is calculated based on the thread type and tap size. For metric threads, the formula is:
Formula:
Drill Size = Tap Diameter - Pitch
Example:
For M8×1.25 tap:
Drill Size = 8 - 1.25 = 6.75mm ≈ 6.8mm
Tap Drill Size Reference
| Thread Type | Common Tap Sizes | Recommended Drill Size |
|---|---|---|
| Metric | M3, M4, M5, M6, M8, M10, M12, M16 | 2.5mm, 3.3mm, 4.2mm, 5.0mm, 6.8mm, 8.5mm, 10.2mm, 14.0mm |
| Unified | 4-40, 6-32, 8-32, 10-24, 1/4-20, 5/16-18, 3/8-16 | #43, #36, #29, #25, #7, F, 5/16" |
| BSW | 1/8, 3/16, 1/4, 5/16, 3/8, 7/16, 1/2 | 7/64", 5/32", 13/64", 17/64", 21/64", 25/64", 27/64" |
| NPT | 1/8, 1/4, 3/8, 1/2, 3/4, 1, 1-1/4, 1-1/2 | 11/32", 7/16", 37/64", 47/64", 15/16", 1-3/16", 1-17/32", 1-25/32" |
FAQs
What is a tap drill size?
A tap drill size is the diameter of the hole that needs to be drilled before tapping (cutting) threads. It's slightly smaller than the tap's major diameter to ensure proper thread engagement.
Why is the correct tap drill size important?
Using the correct tap drill size ensures proper thread depth and strength. Too small a hole will break the tap, while too large a hole will result in weak threads that can strip easily.
How accurate is this calculator?
This calculator uses standard industry formulas and tables for tap drill sizes. All results are accurate to industry standards. However, for critical applications, always verify with manufacturer specifications.
What's the difference between UNC and UNF?
UNC (Unified Coarse) has fewer threads per inch and is used for general purpose applications. UNF (Unified Fine) has more threads per inch and is used where vibration resistance or fine adjustment is needed.
Related Calculators
