Determine ISO 273 and ASME B18.2.8 screw clearance hole diameters, Close / Normal / Loose fit tolerances, socket head cap screw counterbores, flat head countersinks, and drill bit sizes.
Major nominal outer diameter of thread shank.
Prevent bolt binding, thread stripping, and CNC machining tolerance stackup failures.
When drilling mating flanges or multi-bolt circles, manufacturing position variations cause bolts to bind if holes are undersized. Choosing the correct Normal or Loose fit eliminates binding across bolt patterns.
Machining a counterbore that matches the screw head exactly leaves zero clearance for standard socket tools or zinc plating buildup. This tool provides standardized DIN 912 and ASME counterbores with proper socket clearance.
Metric flat heads use 90° included angles, while imperial screws use 82°. Using the wrong countersink bit leaves screws protruding above the surface or riding on a sharp edge, severely weakening clamp load.
Placing drilled holes too close to part edges causes tensile breakout when torqued. Our built-in AISC and ASME edge distance checks safeguard structural integrity across aluminum, steel, and plastics.
Five intuitive steps to specify machining-ready clearance hole dimensions.
Toggle between Metric ISO 273 (millimeter) and Imperial ASME B18.2.8 (fractional/numbered).
Choose your screw size from M1.6 to M30, or #0 up to 1" imperial coarse/fine threads.
Select Close for dowels, Normal for standard bolted assemblies, or Loose for thermal expansion.
Select standard Through Hole, Counterbore (SHCS), or Countersink (Flat Head 90°/82°).
Read the exact drill bit size, tap drill, counterbore depth, and copy directly to your CAD drawing.
Built specifically for mechanical engineers, CNC machinists, toolmakers, and fabricators.
Includes precision drill size data across all three standard engineering fit classes (Fine, Medium, Free) matching official ISO and American standards.
Computes standard counterbore diameters and recess depths for DIN 912 and ASME B18.3 socket head cap screws with flush and sub-flush clearances.
Automatically pairs imperial inch dimensions with standard American letter (A-Z), number (#1-80), and fractional drill bit equivalents.
Displays the matching 75% thread depth tap drill size for the internal mating thread, allowing complete through-and-tap machining setup.
Clearance hole diameters and standard drill sizes across standard mechanical fasteners.
| Fastener Size | Close Fit (Fine) | Normal Fit (Medium) | Loose Fit (Free) | SHCS C'Bore Dia | Tap Drill (75%) |
|---|---|---|---|---|---|
| M3 (Metric) | 3.2 mm (#30) | 3.4 mm (#29) | 3.6 mm (#27) | 6.0 mm | 2.5 mm |
| M4 (Metric) | 4.3 mm (#18) | 4.5 mm (#16) | 4.8 mm (#12) | 7.5 mm | 3.3 mm |
| M5 (Metric) | 5.3 mm (#4) | 5.5 mm (7/32") | 5.8 mm (Letter F) | 9.0 mm | 4.2 mm |
| M6 (Metric) | 6.4 mm (Letter E) | 6.6 mm (Letter G) | 7.0 mm (Letter J) | 11.0 mm | 5.0 mm |
| M8 (Metric) | 8.4 mm (Letter Q) | 9.0 mm (Letter T) | 10.0 mm (Letter X) | 15.0 mm | 6.8 mm |
| M10 (Metric) | 10.5 mm (Letter Z) | 11.0 mm (7/16") | 12.0 mm (15/32") | 18.0 mm | 8.5 mm |
| 1/4"-20 (Imperial) | 0.2570" (Letter F) | 0.2660" (Letter H) | 0.2810" (9/32") | 0.438" (7/16") | #7 (0.201") |
| 5/16"-18 (Imperial) | 0.3230" (Letter P) | 0.3320" (Letter Q) | 0.3440" (11/32") | 0.531" (17/32") | Letter F (0.257") |
| 3/8"-16 (Imperial) | 0.3860" (Letter W) | 0.3970" (Letter X) | 0.4060" (13/32") | 0.625" (5/8") | 5/16" (0.3125") |
| 1/2"-13 (Imperial) | 0.5156" (33/64") | 0.5312" (17/32") | 0.5625" (9/16") | 0.812" (13/16") | 27/64" (0.4219") |
Detailed machining, GD&T, and fastener standards guidance.
