How deep should a blind tapped hole be? Deep enough for the thread the screw needs, plus the tapered lead on the end of the tap, plus some room for chips, plus the cone the drill leaves at the bottom. This calculator adds those up for any thread from #0 to 1 in and M1.6 to M24. It starts from the usual engagement rule (about 1 x the diameter in steel and 2 x in aluminium) or a length you enter, and gives the drill depth to the full diameter and to the tip.
Tapped hole depth calculator
How it works
- Engagement = 1 x nominal diameter (steel or cast iron), 2 x (aluminium and other soft metals), or your figure
- Tap reach = engagement + chamfer threads x pitch: 5 threads for a plug tap, 1.5 for a bottoming tap, 10 for a taper tap (the top of each published range)
- Drill depth to full diameter = tap reach + chip room (in threads) x pitch
- Drill point = (tap drill diameter ÷ 2) ÷ tan(half the point angle): about 0.30 x the drill diameter for a 118° point and 0.21 x for 135°
The pitch is 1 ÷ threads per inch for inch threads and the thread pitch in mm for metric. The tap drill is the 75% thread size from the same formula as the tap drill size calculator.
Worked example
A 1 in – 8 UNC bolt that must engage 1 in of thread, tapped with a plug tap, with two threads of chip room and a standard 118° drill:
- Thread: 1-8 UNC
- Thread engagement: Enter my own length
- Engagement length (inches for inch threads, mm for metric): 1
- Tap used to finish the hole: Plug tap (3 to 5 tapered threads)
- Chip room below the tap: 2 threads
- Drill point angle: 118 degrees (general purpose)
The calculator gives:
- Drill depth to the full diameter: 1.875 in (1-7/8", 47.6 mm)
- Depth including the drill point: 2.139 in (2-1/8", 54.3 mm)
- Full-thread depth (engagement): 1 in (1", 25.4 mm), 8 threads
- Depth the tap must reach: 1.625 in (1-5/8", 41.3 mm), allowing 5 chamfer threads on the tap
- Tap drill (75% thread): 0.8782 in
- Drill point length: 0.264 in (1/4", 6.7 mm) for a 118 degree point
The pitch is 1/8 in, so the plug tap’s 5 tapered threads add 5/8 in: the tap has to reach 1-5/8 in to leave a full inch of complete thread. Two more threads of chip room make 1-7/8 in, the depth to drill at the full 7/8 in (0.878 in) diameter. The drill point adds another 0.26 in, so the very bottom of the cone is about 2-1/8 in down. Finish with a bottoming tap (1.5 tapered threads) and the hole could be about 7/16 in shallower.
Blind hole drill depths with a plug tap
| Thread | Steel (1 x D) | Aluminium (2 x D) |
|---|---|---|
| #6-32 UNC | 0.357 in | 0.495 in |
| #8-32 UNC | 0.383 in | 0.547 in |
| #10-24 UNC | 0.482 in | 0.672 in |
| #10-32 UNF | 0.409 in | 0.599 in |
| 1/4-20 UNC | 0.6 in | 0.85 in |
| 5/16-18 UNC | 0.701 in | 1.014 in |
| 3/8-16 UNC | 0.813 in | 1.188 in |
| 1/2-13 UNC | 1.038 in | 1.538 in |
| M3 x 0.5 | 6.5 mm | 9.5 mm |
| M4 x 0.7 | 8.9 mm | 12.9 mm |
| M5 x 0.8 | 10.6 mm | 15.6 mm |
| M6 x 1 | 13 mm | 19 mm |
| M8 x 1.25 (coarse) | 16.8 mm | 24.8 mm |
| M10 x 1.5 (coarse) | 20.5 mm | 30.5 mm |
| M12 x 1.75 (coarse) | 24.3 mm | 36.3 mm |
Notes and limits
- The 1 x and 2 x diameter figures are rules of thumb for a screw to break before the thread strips. Soft or thin-walled parts, plastics and special alloys may need more, and an engineer will calculate critical joints.
- Engagement beyond about 1.5 to 2 x the diameter adds very little strength in the same material: the first few threads carry most of the load.
- Tap chamfer lengths vary by maker. If you know the length of the tapered part of your tap, enter the engagement plus that length as the tap reach instead.
- Clear chips before the tap reaches the bottom. A tap that bottoms out on packed chips is the usual way taps break in blind holes.
- For a through hole none of this applies: drill all the way through and tap from either side.
Frequently asked questions
How deep should a tapped hole be?
The full-thread depth should be about 1 x the screw diameter in steel and 2 x in aluminium. Drill deeper than that by the length of the tap’s tapered lead (3 to 5 threads for a plug tap) plus a couple of threads for chips.
How deep do I drill for a 1 in 8 TPI bolt with 1 in of thread engagement?
About 1-7/8 in at the full 7/8 in drill diameter with a plug tap and two threads of chip room, or about 2-1/8 in to the tip of a 118° drill point.
How many threads should be engaged in a tapped hole?
Think in length rather than count: about 1 x the diameter in steel. For a 1/4-20 screw that is 1/4 in, or 5 threads; in aluminium aim for 1/2 in.
How much does the drill point add to the hole depth?
About 0.3 x the drill diameter for a standard 118° point. For a 0.201 in drill (1/4-20) that is 0.06 in.
Do I need a bottoming tap?
Only if the thread has to run close to the bottom of a blind hole. Start with a taper or plug tap, then finish with the bottoming tap, which has just 1 to 1.5 tapered threads.
Sources
- Nord-Lock Group, "The Experts: Thread engagement in a tapped hole": minimum engagement about 1 x the nominal diameter in steel and 2 x in aluminium
- Wikipedia, "Tap and die": tapered threads on taper (8 to 10), intermediate or plug (3 to 5) and bottoming (1 to 1.5) taps
- ASME B1.1, Unified Inch Screw Threads (UN and UNR Thread Form): major diameters and threads per inch
- ASME B94.11M, Twist Drills, Straight Shank and Taper Shank, Combined Drills and Countersinks (number, letter and fractional drill diameters)
Last reviewed: 16 October 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.