NPT threads are tapered
The thread tightens as the tapered male and female threads engage. Drill size matters, but tap depth and sealing practice matter too.
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NPT tap drill chart
NPT tap drill sizes are for tapered pipe threads, not straight machine screw threads. Use the drill chart as a starting point and check depth, sealing method, and fitting requirements.
1/8 NPT
ACTIVE- Tap size:
- 1/8 NPT
- Recommended tap drill:
- Q
- Decimal inch:
- 0.3320 in
- Metric mm:
- 8.433 mm
- Nearest fractional drill:
- 21/64
- Thread standard:
- NPT
Common NPT references
A 1/8 NPT tap commonly uses a Q drill. A 1/4 NPT tap commonly uses a 7/16 drill. Pressure work should be verified against the applicable standard and fitting data.
2 results
Scope: NPT Sort by: Tap size Filters
| Tap size | Thread pitch / TPI | Recommended tap drill | Decimal inch | Metric mm | Clearance drill | Thread type |
|---|---|---|---|---|---|---|
| 1/8 NPT | 27 TPI | Q | 0.3320 | 8.433 | n/a | NPT |
| 1/4 NPT | 18 TPI | 7/16 | 0.4375 | 11.113 | n/a | NPT |
1/8 NPT
NPT27 TPI
Q
0.3320 in
8.433 mm
1/4 NPT
NPT18 TPI
7/16
0.4375 in
11.113 mm
TPI = Threads Per Inch. Data is for reference only. Always verify with current tooling standards.
References
Popular tap drill lookups
View allTap drill guides
Full chartTap drill chart Use the tap drill chart to find drill sizes for UNC, UNF, metric, metric fine, and NPT threads with decimal inch and metric mm values. Open guide UNC tap drill chart Find UNC tap drill sizes for coarse inch threads, including 10-24, 1/4-20, 5/16-18, 3/8-16, and 1/2-13. Open guide UNF tap drill chart Find UNF tap drill sizes for fine inch threads, including 10-32, 1/4-28, and 3/8-24 with decimal and metric values. Open guide Metric tap drill chart Find metric tap drill sizes for common coarse metric threads, including M6 x 1.0, M8 x 1.25, and M10 x 1.5. Open guide Metric fine tap drill chart Find metric fine tap drill sizes for fine pitch metric threads, including M8 x 1.0 and M10 x 1.25. Open guide Tap drill calculator Calculate tap drill sizes for inch and metric threads, then compare the result with common shop drill sizes. Open guide Drill and tap chart Match common tap sizes with drill sizes for UNC, UNF, metric, metric fine, and NPT threads. Open guide Drill bit size chart Compare drill bit sizes used for tapping, including fractional, number, letter, decimal inch, and metric values. Open guide Clearance hole chart Find clearance hole guidance for screws and bolts, with close, normal, and loose fit context for tapped assemblies. Open guide Roll tap drill size Understand why roll taps and form taps use larger drill sizes than cut taps, and when to check manufacturer data. Open guide Form tap drill size Find practical guidance for form tap drill sizes and why thread forming taps need a different starting hole than cutting taps. Open guide Pipe tap drill size Find pipe tap drill size guidance for NPT threads, including 1/8 NPT and 1/4 NPT reference values. Open guide What size drill bit for a 1/4-20 tap? For a 1/4-20 UNC tap, the common tap drill is #7, which is 0.2010 in or about 5.105 mm. Open guide What size drill bit for a 10-32 tap? For a 10-32 UNF tap, the common tap drill is #21, which is 0.1590 in or about 4.039 mm. Open guide What size drill bit for an M8 x 1.25 tap? For an M8 x 1.25 tap, the common tap drill is 6.8 mm, which is about 0.2677 in. Open guide
Quick formulas
Metric tap drill formula
Tap drill = major diameter - pitch.
Example: M8 x 1.25 uses 8.00 - 1.25 = 6.75 mm, rounded to 6.8 mm.
Inch tap drill formula
Tap drill = major diameter - (1 / TPI).
Example: 1/4-20 gives about 0.200 in, commonly #7.
Cut taps and form taps
Form taps usually use a larger drill size and create threads by displacing material.
Clearance hole guide
Use clearance drills when a fastener must pass through a part instead of cutting threads.
FAQ
What size drill bit for a 1/4-20 tap?
A common recommendation is a #7 drill bit, which is 0.2010 in or about 5.105 mm.
How do you calculate tap drill size?
For metric threads, subtract the pitch from the major diameter. For inch threads, subtract 1 divided by TPI from the major diameter.
Is tap drill size the same for roll taps?
No. Roll taps usually need a larger hole than cut taps. Check the tap manufacturer data for the material you are working with.
What is the difference between tap drill and clearance drill?
A tap drill prepares a hole for threading. A clearance drill makes a hole large enough for a fastener to pass through.