The tap drill for 1-8 UNC is 7/8" (0.8750 in, 22.22 mm), which gives about 77% of full thread depth. The exact 75% hole is 0.8782 in (22.31 mm). The table lists every number, letter, fractional and metric drill that gives between 55% and 90% thread, so you can work from the bits you own.
| Drill | Type | in | mm | Thread | Note |
|---|---|---|---|---|---|
| 55/64" | fraction | 0.8594 | 21.83 | 87% | deep, hard to tap |
| 22.1 mm | metric | 0.8701 | 22.10 | 80% | deep, hard to tap |
| 22.2 mm | metric | 0.8740 | 22.20 | 78% | |
| 7/8" | fraction | 0.8750 | 22.22 | 77% | chart drill |
| 22.3 mm | metric | 0.8780 | 22.30 | 75% | |
| 22.4 mm | metric | 0.8819 | 22.40 | 73% | |
| 22.5 mm | metric | 0.8858 | 22.50 | 70% | |
| 57/64" | fraction | 0.8906 | 22.62 | 67% | easier to tap |
| 29/32" | fraction | 0.9062 | 23.02 | 58% | easier to tap |
About the 1-8 thread
1-8 is the coarse 1 in thread, 8 threads per inch, often described as “1 in 8 TPI”: large anchor rods and hold-down rods, structural and machinery bolts, heavy equipment and agricultural implements. It is the largest size in our per-size set and the point where tapping by hand stops being practical in steel.
7/8 in (0.875 in) is the chart drill at about 77% thread; 57/64 in gives about 67%. A 1 in tap needs a large tap wrench or a machine, a rigid setup and plenty of cutting fluid, and many shops cut threads this size on a lathe, or use a nut or a weld nut instead of tapping the part.
1-8 thread dimensions
| Dimension | Value | Also |
|---|---|---|
| Nominal diameter | 1.0000 in | 25.40 mm |
| Threads per inch | 8 | pitch 0.1250 in (3.175 mm) |
| Basic pitch diameter | 0.9188 in | 23.338 mm |
| Basic minor diameter (nut) | 0.8647 in | 21.963 mm |
| Tensile stress area | 0.6057 in² | 390.80 mm² |
| 1-12 UNF (same diameter): tap drill | 59/64" (0.9219 in) | 72% thread |
Clearance hole for 1 in
The tapped hole is in the part that holds the thread; the part being clamped gets a clearance hole the bolt passes through. For 1 in a normal fit is 1-1/32" (1.0312 in). Never drill the clearance part with the tap drill: the screw will not pass.
| Fit | Drill | Diameter |
|---|---|---|
| Close fit | 1-1/64" | 1.0156 in |
| Normal fit | 1-1/32" | 1.0312 in |
| Loose fit | 1-1/16" | 1.0625 in |
How deep to drill a blind 1-8 hole
| Part material | Full thread wanted | Drill depth (full diameter) | Depth to drill tip (118°) |
|---|---|---|---|
| Steel or cast iron (1 × diameter) | 1.000 in | 1.875 in (about 1-7/8″) | 2.139 in (about 2-1/8″) |
| Aluminium or brass (2 × diameter) | 2.000 in | 2.875 in (about 2-7/8″) | 3.139 in (about 3-1/8″) |
Wrench size and torque for 1 in
- Hex cap screw: 1-1/2 in wrench; heavy hex: 1-5/8 in
- Socket head cap screw: 3/4 in hex key
| Bolt grade | Dry | Lubricated |
|---|---|---|
| Grade 2 | 250 ft-lb | 187 ft-lb |
| Grade 5 | 644 ft-lb | 483 ft-lb |
| Grade 8 | 909 ft-lb | 681 ft-lb |
Threads easily mistaken for 1-8
| Thread | Diameter (in) | Diameter (mm) | Pitch |
|---|---|---|---|
| 1-8 UNC (this page) | 1.0000 | 25.40 | 8 TPI (3.17 mm) |
| 1-12 UNF | 1.0000 | 25.40 | 12 TPI (2.12 mm) |
| M24 x 3 | 0.9449 | 24.00 | 3 mm (8.5 TPI) |
| M24 x 2 | 0.9449 | 24.00 | 2 mm (12.7 TPI) |
Tips for tapping 1-8
- Drill in stages (for example 1/2 in, then 3/4 in, then 7/8 in) so the large bit cuts on centre.
- In aluminium, aim for at least 2 in of engagement (2 x diameter).
- Hex cap screws of this size take a 1-1/2 in wrench; heavy hex 1-5/8 in.
- The 1 in fine thread is 1-12 UNF, and 1-14 (UNS) is also found on some equipment; check the pitch with a gauge before buying a tap.
Frequently asked questions
What size drill bit for a 1-8 tap?
7/8" (0.8750 in), which gives about 77% thread. The exact 75% size is 0.8782 in (22.31 mm).
How deep should a blind 1-8 hole be?
For a full thread of 1 × diameter in steel, drill about 1.875 in (about 1-7/8″) at full diameter with a plug tap, or 2.139 in (about 2-1/8″) to the drill tip; in aluminium, about 2.875 in (about 2-7/8″).
What is the 1-8 tap drill in mm?
22.3 mm is the nearest 0.1 mm drill to the 75% size and gives 75% thread.
What does 1 in 8 TPI mean?
A 1 in diameter thread with 8 threads per inch: the 1-8 UNC coarse thread.
How much thread engagement does a 1-8 bolt need?
About 1 in (1 x diameter) in steel and about 2 in in aluminium, so that the bolt fails before the thread strips. The thread engagement chart explains the rule.
Sources and method
Every dimension and percentage on this page is computed from the formulas listed, not copied from another chart. Values are basic (nominal); real parts are made to tolerance classes (2A/2B for inch, 6g/6H for metric).
- ASME B1.1 and FED-STD-H28/2B (Unified inch), ISO 68-1 and ISO 261 (metric): basic thread profile and stress area formulas.
- Machinery’s Handbook (Industrial Press): percentage-of-thread relation used by our tap drill size calculator.
- ASME B94.11M twist drill series (number, letter, fractional); 0.1 mm metric drill steps.
- ISO 273 metric clearance holes; inch close, normal and loose fits as published in the ASME B18.2.8 clearance classes.
- SAE J429 and ISO 898-1 proof strengths; T = K × D × F torque relation (see our bolt torque chart for the full method).
- Wrench and hex key sizes: ASME B18.2.1, ASME B18.3, ASME B18.6.3, ISO 4014/4017, DIN 933 and ISO 4762 values as widely published.
More thread and fastener tools
- All thread and fastener charts
- UNC and UNF thread chart
- Metric thread pitch chart
- Tap drill chart (inch and metric)
- Clearance hole chart
- Nail size chart
- Wood screw size chart
- Lag screw pilot hole chart
- Bolt torque chart
- Wrench size chart
- Drill bit size chart
- Thread engagement chart
- Nail gun nail sizes by gauge
- Screw and nail size calculator
- Tap drill size calculator
- Clearance hole size calculator
- Tapped hole depth calculator
Last reviewed: 24 October 2026. How we build and check our data: methodology and corrections. Spotted an error? Please let us know.