Tap Drill Size Chart: Metric & SAE Thread Tap Drill Sizes
Find the correct pilot hole size before cutting metric, UNC, or UNF threads with a tap.
What Is a Tap Drill Chart?
A tap drill chart shows the correct drill bit size to use before cutting threads with a tap. The drilled hole must be slightly smaller than the thread's nominal diameter. The tap then removes the remaining material to form the threads as it cuts. Using the wrong size causes one of two failures: a hole that's too small breaks the tap, and a hole that's too large produces weak, undersized threads that strip under load.
This page covers the metric tap drill table from M3 to M24, the SAE UNC and UNF tables, the formula behind the numbers, thread engagement percentage, and how to pick between a taper, plug, and bottoming tap for the job at hand.
Metric Tap Drill Table (M3 to M24)
| Thread | Pitch (mm) | Tap Drill | Inches |
|---|---|---|---|
| M3 | 0.5 | 2.5mm | 0.0984 |
| M4 | 0.7 | 3.3mm | 0.1299 |
| M5 | 0.8 | 4.2mm | 0.1654 |
| M6 | 1.0 | 5.0mm | 0.1969 |
| M7 | 1.0 | 6.0mm | 0.2362 |
| M8 | 1.25 | 6.8mm | 0.2677 |
| M10 | 1.5 | 8.5mm | 0.3346 |
| M12 | 1.75 | 10.2mm | 0.4016 |
| M14 | 2.0 | 12.0mm | 0.4724 |
| M16 | 2.0 | 14.0mm | 0.5512 |
| M18 | 2.5 | 15.5mm | 0.6102 |
| M20 | 2.5 | 17.5mm | 0.6890 |
| M22 | 2.5 | 19.5mm | 0.7677 |
| M24 | 3.0 | 21.0mm | 0.8268 |
SAE UNC Tap Drill Table
| Thread | TPI | Tap Drill |
|---|---|---|
| #4 | 40 | #43 (0.0890") |
| #6 | 32 | #36 (0.1065") |
| #8 | 32 | #29 (0.1360") |
| #10 | 24 | #25 (0.1495") |
| 1/4" | 20 | #7 (0.2010") |
| 5/16" | 18 | F (0.2570") |
| 3/8" | 16 | 5/16" (0.3125") |
| 7/16" | 14 | U (0.3680") |
| 1/2" | 13 | 27/64" (0.4219") |
| 9/16" | 12 | 31/64" (0.4844") |
| 5/8" | 11 | 17/32" (0.5313") |
| 3/4" | 10 | 21/32" (0.6563") |
| 7/8" | 9 | 49/64" (0.7656") |
| 1" | 8 | 7/8" (0.8750") |
| 1-1/4" | 7 | 1-7/64" (1.1094") |
| 1-1/2" | 6 | 1-11/32" (1.3438") |
| 2" | 4.5 | 1-25/32" (1.7813") |
SAE UNF Tap Drill Table
| Thread | TPI | Tap Drill |
|---|---|---|
| #4 | 48 | #42 (0.0935") |
| #6 | 40 | #33 (0.1130") |
| #8 | 36 | #29 (0.1360") |
| #10 | 32 | #21 (0.1590") |
| 1/4" | 28 | #3 (0.2130") |
| 5/16" | 24 | I (0.2720") |
| 3/8" | 24 | Q (0.3320") |
| 7/16" | 20 | 25/64" (0.3906") |
| 1/2" | 20 | 29/64" (0.4531") |
| 9/16" | 18 | 33/64" (0.5156") |
| 5/8" | 18 | 37/64" (0.5781") |
| 3/4" | 16 | 11/16" (0.6875") |
| 7/8" | 14 | 13/16" (0.8125") |
| 1" | 12 | 59/64" (0.9219") |
The Tap Drill Formula
For a metric thread at roughly 75% engagement, tap drill diameter equals major diameter minus pitch. M8 × 1.25 gives 8 − 1.25 = 6.75mm, close to the standard 6.8mm tap drill. For an SAE thread, the same target uses the formula: tap drill = major diameter − (1 ÷ TPI × 0.75 × 1.299). A 1/4 inch-20 bolt (TPI 20) works out to roughly 0.2010 inch, which matches the standard #7 tap drill listed above. Most machinists use the published tables rather than calculating by hand, but the formula explains why the numbers land where they do.
Thread Engagement Percentage
75% engagement is the workshop standard. It produces threads strong enough for nearly any fastening job while keeping tap-breakage risk low. 100% engagement is theoretically stronger, but it requires close to twice the cutting torque and breaks taps far more often in practice. 50% engagement is common in soft materials like aluminum, where lower torque is preferred and the material itself isn't the weak point in the joint. The tap drill tables above all assume 75% engagement.
Taper, Plug, and Bottoming Taps
A taper tap has a long chamfer, often 8 to 10 threads, that eases the tap into the hole and keeps it self-aligning. It's the right choice for starting a thread in a through hole. A plug tap has a shorter chamfer, around 3 to 5 threads, and cuts closer to the bottom of a blind hole after a taper tap has started it. A bottoming tap has almost no chamfer, just 1 to 1.5 threads, and finishes threads to within a thread or two of a blind hole's bottom. Most jobs on a through hole only need a plug tap; a blind hole that needs full-depth threads typically requires all three in sequence.
How to Select the Correct Tap Drill Size
Identify the thread needed, for example M8 × 1.25, and find that row in the metric table. Check which drill bits are actually available. If only inch-sized drills are on hand, use the inches column instead of switching to a metric bit mid-job. For soft materials, step up one drill size from the 75% recommendation to reduce cutting torque. For hardened steel, stay at the listed size and use cutting fluid to reduce heat and tap wear.
Common Tap Drill Sizes for Workshop Use
M6 (5.0mm), M8 (6.8mm), M10 (8.5mm), 1/4 inch-20 (#7), 5/16 inch-18 (F), and 3/8 inch-16 (5/16 inch) cover most general workshop tapping. Keep a small tap drill index or a printed copy of this chart next to the tap and die set, since reaching for the wrong bit is the most common cause of a broken tap.
Tap Drill Size FAQ
What size drill bit do I use to tap M6?
A 5.0mm drill bit is standard for tapping an M6 × 1.0 thread, giving approximately 75% thread engagement, the strength level most workshop tapping targets.
What happens if I drill the tap hole too small?
The tap has to remove more material than it's designed for, which sharply increases cutting torque and the risk of snapping the tap off inside the hole. A broken tap is difficult and sometimes impossible to remove without damaging the part.
What happens if I drill the tap hole too large?
The tap engages less material, producing shallow, weak threads that strip out under load. A hole that's too large cannot be fixed by tapping again; the part needs a helical insert, a larger thread, or replacement.
What is the formula for a metric tap drill size?
Tap drill diameter equals the thread's major diameter minus its pitch, for roughly 75% engagement. For M8 × 1.25, that's 8 − 1.25 = 6.75mm, close to the standard 6.8mm tap drill listed in the table above.
Do I need a different tap drill for aluminum versus steel?
Yes, often. Soft materials like aluminum and brass can use a slightly larger tap drill for around 60 to 65% engagement, since the extra thread depth used in steel isn't necessary and the softer material tolerates the reduced contact fine. Hardened steel should stay at or near 75%.
What's the difference between a taper tap, a plug tap, and a bottoming tap?
A taper tap has a long, gradual chamfer on its cutting end and starts a thread easily, but cannot cut all the way to the bottom of a blind hole. A plug tap has a shorter chamfer and reaches deeper. A bottoming tap has almost no chamfer and cuts threads to within one or two threads of a blind hole's bottom, typically used after a taper or plug tap has already started the hole.