Why the drill is larger
A cut tap removes material. A roll tap moves material into the thread shape. If the hole is too small, torque rises quickly and the tap can fail.
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Roll tap drill size
Roll taps, also called form taps, do not cut chips. They displace material to form the thread, so the starting hole is larger than the hole used for a cut tap.
M8 x 1.25 Metric
ACTIVE- Tap size:
- M8 x 1.25 Metric
- Recommended tap drill:
- 6.8 mm
- Decimal inch:
- 0.2677 in
- Metric mm:
- 6.8 mm
- Nearest fractional drill:
- 17/64
- Thread standard:
- Metric
Material matters
Form taps work best in ductile materials. They are usually a poor fit for brittle materials because the thread is made by material flow, not chip removal.
Use manufacturer data
Roll tap geometry varies by maker and thread percentage target. Use the tap maker chart when the job calls for a form tap.
15 results
Scope: All families Sort by: Tap size Filters
| Tap size | Thread pitch / TPI | Recommended tap drill | Decimal inch | Metric mm | Clearance drill | Thread type |
|---|---|---|---|---|---|---|
| 10-24 | 24 TPI | #25 | 0.1495 | 3.797 | 3/16 | UNC |
| 1/4-20 | 20 TPI | #7 | 0.2010 | 5.105 | 9/32 | UNC |
| 5/16-18 | 18 TPI | F | 0.2570 | 6.528 | 11/32 | UNC |
| 3/8-16 | 16 TPI | 5/16 | 0.3125 | 7.938 | 13/32 | UNC |
| 1/2-13 | 13 TPI | 27/64 | 0.4219 | 10.716 | 17/32 | UNC |
| 10-32 | 32 TPI | #21 | 0.1590 | 4.039 | 13/64 | UNF |
| 1/4-28 | 28 TPI | #3 | 0.2130 | 5.41 | 9/32 | UNF |
| 3/8-24 | 24 TPI | Q | 0.3320 | 8.433 | 13/32 | UNF |
| M6 x 1.0 | 1.0 mm pitch | 5.0 mm | 0.1969 | 5 | 6.6 mm | Metric |
| M8 x 1.25 | 1.25 mm pitch | 6.8 mm | 0.2677 | 6.8 | 9.0 mm | Metric |
| M10 x 1.5 | 1.5 mm pitch | 8.5 mm | 0.3346 | 8.5 | 11.0 mm | Metric |
| M8 x 1.0 | 1.0 mm pitch | 7.0 mm | 0.2756 | 7 | 9.0 mm | Metric Fine |
| M10 x 1.25 | 1.25 mm pitch | 8.8 mm | 0.3465 | 8.8 | 11.0 mm | Metric Fine |
| 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 |
10-24
UNC24 TPI
#25
0.1495 in
3.797 mm
1/4-20
UNC20 TPI
#7
0.2010 in
5.105 mm
5/16-18
UNC18 TPI
F
0.2570 in
6.528 mm
3/8-16
UNC16 TPI
5/16
0.3125 in
7.938 mm
1/2-13
UNC13 TPI
27/64
0.4219 in
10.716 mm
10-32
UNF32 TPI
#21
0.1590 in
4.039 mm
1/4-28
UNF28 TPI
#3
0.2130 in
5.41 mm
3/8-24
UNF24 TPI
Q
0.3320 in
8.433 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 NPT tap drill chart Find NPT tap drill sizes for tapered pipe threads, including 1/8 NPT and 1/4 NPT drill references. 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 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.