SHOP REFERENCE / CUTTING TAPS
Tap Drill Size Chart
Start with the thread size and pitch or TPI, then drill the pilot hole before cutting internal threads. The tables list common starting drill sizes for cutting taps, including metric coarse/fine and inch UNC/UNF threads.
Open the Drill & Tap AssistantA tap drill makes a hole smaller than the thread’s major diameter. A clearance hole lets an untapped bolt pass through.
Metric tap drills
Pitch is the distance between adjacent thread peaks, in millimeters. Drill sizes shown in millimeters.
| Thread | Pitch | Tap drill |
|---|---|---|
| M3 × 0.5 | 0.5 mm | 2.5 mm |
| M4 × 0.7 | 0.7 mm | 3.3 mm |
| M5 × 0.8 | 0.8 mm | 4.2 mm |
| M6 × 1 | 1 mm | 5 mm |
| M6 × 0.75 | 0.75 mm | 5.25 mm |
| M8 × 1.25 | 1.25 mm | 6.8 mm |
| M8 × 1 | 1 mm | 7 mm |
| M10 × 1.5 | 1.5 mm | 8.5 mm |
| M10 × 1.25 | 1.25 mm | 8.8 mm |
| M10 × 1 | 1 mm | 9 mm |
| M12 × 1.75 | 1.75 mm | 10.3 mm |
| M12 × 1.5 | 1.5 mm | 10.5 mm |
| M12 × 1.25 | 1.25 mm | 10.8 mm |
| M14 × 2 | 2 mm | 12.1 mm |
| M14 × 1.5 | 1.5 mm | 12.5 mm |
| M16 × 2 | 2 mm | 14 mm |
| M16 × 1.5 | 1.5 mm | 14.5 mm |
SAE tap drills
TPI counts threads per inch. Number (#), letter and fractional labels are drill-bit sizes, not thread series.
| Thread | TPI / series | Tap drill |
|---|---|---|
| #6–32 | 32 TPI · UNC | #36 |
| #8–32 | 32 TPI · UNC | #29 |
| #10–24 | 24 TPI · UNC | #25 |
| #10–32 | 32 TPI · UNF | #21 |
| 1/4–20 | 20 TPI · UNC | #7 |
| 1/4–28 | 28 TPI · UNF | #3 |
| 5/16–18 | 18 TPI · UNC | F |
| 5/16–24 | 24 TPI · UNF | I |
| 3/8–16 | 16 TPI · UNC | 5/16 in. |
| 3/8–24 | 24 TPI · UNF | Q |
| 7/16–14 | 14 TPI · UNC | U |
| 7/16–20 | 20 TPI · UNF | 25/64 in. |
| 1/2–13 | 13 TPI · UNC | 27/64 in. |
| 1/2–20 | 20 TPI · UNF | 29/64 in. |
| 5/8–11 | 11 TPI · UNC | 17/32 in. |
| 5/8–18 | 18 TPI · UNF | 37/64 in. |
| 3/4–10 | 10 TPI · UNC | 21/32 in. |
| 3/4–16 | 16 TPI · UNF | 11/16 in. |
HOW TO USE THE CHART
Thread → drill → tap
The pilot drill leaves enough material for a cutting tap to form internal threads. A coarser pitch or lower TPI needs a different hole from a finer thread at the same major diameter: M8 × 1.25 uses 6.8 mm here, while M8 × 1.0 uses 7.0 mm. For inch threads, 1/4–20 uses a #7 drill and 1/4–28 uses a #3 drill.
These drill selections follow LittleMachineShop’s approximately 75% theoretical thread reference for cutting taps. Its chart also gives larger pilot drills for roughly 50% theoretical thread in some materials. Actual hole size and thread engagement depend on material, drill runout and application. Form/roll taps need different pilot holes; follow the tap maker’s and job’s specifications.
Use the Drill & Tap Assistant for reverse lookup or clearance holes → See bolt and socket sizes →Source and scope
Values are transcribed from LittleMachineShop’s metric and inch tap and clearance drill reference (PDF) and its inch tap drill chart, using the 75% cutting-tap column. Guhring’s cutting versus form tap chart shows why drill choices vary with desired thread engagement and tap method. This page gives shop starting points, not engineering requirements for a particular part.