Engineering quick reference
Clearance, Counterbore & Countersink Chart
Close / normal / loose clearance holes, socket-head counterbores, and 82° / 90° / 100° countersinks for inch & metric fasteners.
Key: Ø = diameter · C'sink = countersink · C'bore = counterbore · SHCS = socket-head cap screw · FHCS = socket flat (countersunk) head cap screw · Drill = nearest standard drill
Clearance holes — inch · ASME B18.2.8
| Size | Nom Ø | Close | Normal | Loose | |||
|---|---|---|---|---|---|---|---|
| Drill | Ø in | Drill | Ø in | Drill | Ø in | ||
| #2 | 0.0860 | 3/32 | 0.0938 | #38 | 0.1015 | #32 | 0.1160 |
| #4 | 0.1120 | #31 | 0.1200 | #30 | 0.1285 | #27 | 0.1440 |
| #6 | 0.1380 | #23 | 0.1540 | #18 | 0.1695 | #13 | 0.1850 |
| #8 | 0.1640 | #15 | 0.1800 | #9 | 0.1960 | #3 | 0.2130 |
| #10 | 0.1900 | #5 | 0.2055 | #2 | 0.2210 | B | 0.2380 |
| #12* | 0.2160 | #2 | 0.2210 | 15/64 | 0.2344 | 1/4 | 0.2500 |
| 1/4" | 0.2500 | 17/64 | 0.2656 | 9/32 | 0.2813 | 19/64 | 0.2969 |
| 5/16" | 0.3125 | 21/64 | 0.3281 | 11/32 | 0.3438 | 23/64 | 0.3594 |
| 3/8" | 0.3750 | 25/64 | 0.3906 | 13/32 | 0.4063 | 27/64 | 0.4219 |
| 1/2" | 0.5000 | 17/32 | 0.5313 | 9/16 | 0.5625 | 39/64 | 0.6094 |
| 5/8" | 0.6250 | 21/32 | 0.6563 | 11/16 | 0.6875 | 47/64 | 0.7344 |
| 3/4" | 0.7500 | 25/32 | 0.7813 | 13/16 | 0.8125 | 29/32 | 0.9063 |
| 1" | 1.0000 | 1-1/32 | 1.0313 | 1-3/32 | 1.0938 | 1-5/32 | 1.1563 |
Drill + nominal (minimum) hole Ø, inch. Hole size keyed to nominal fastener Ø, not pitch.* #12 is not tabulated in ASME B18.2.8; these are common shop clearance drills, not values from the standard.
Clearance holes — metric · ISO 273
| Size | Nom Ø | Clearance hole Ø mm | ||
|---|---|---|---|---|
| Close (fine) | Normal (med) | Loose (coarse) | ||
| M2 | 2.00 | 2.20 | 2.40 | 2.60 |
| M2.5 | 2.50 | 2.70 | 2.90 | 3.10 |
| M3 | 3.00 | 3.20 | 3.40 | 3.60 |
| M4 | 4.00 | 4.30 | 4.50 | 4.80 |
| M5 | 5.00 | 5.30 | 5.50 | 5.80 |
| M6 | 6.00 | 6.40 | 6.60 | 7.00 |
| M8 | 8.00 | 8.40 | 9.00 | 10.00 |
| M10 | 10.00 | 10.50 | 11.00 | 12.00 |
| M12 | 12.00 | 13.00 | 13.50 | 14.50 |
| M16 | 16.00 | 17.00 | 17.50 | 18.50 |
| M20 | 20.00 | 21.00 | 22.00 | 24.00 |
All values mm. For metric, the clearance hole is the drill Ø — fine / medium / coarse = close / normal / loose.
Countersinks — inch flat-head · 82° & 100° · ASME B18.6.3 / B18.3
| Size | Mach-scr head Ø | 82° C'sink Ø | 100° C'sink Ø | |
|---|---|---|---|---|
| Mach scr | FHCS | |||
| #4 | 0.225 | 0.225 | 0.255 | 0.243 |
| #6 | 0.279 | 0.279 | 0.307 | 0.301 |
| #8 | 0.332 | 0.332 | 0.359 | 0.347 |
| #10 | 0.385 | 0.385 | 0.411 | 0.402 |
| 1/4" | 0.507 | 0.507 | 0.531 | 0.514 |
| 5/16" | 0.635 | 0.635 | 0.656 | 0.611 |
| 3/8" | 0.762 | 0.762 | 0.781 | 0.735 |
| 1/2" | 0.875 | 0.875 | 0.938 | 0.963 |
All values inch. C'sink major Ø = the head Ø of that screw family; the angle sets cone depth, not surface Ø. Machine-screw (B18.6.3) head is smaller than socket FHCS (B18.3).
Countersinks — metric flat-head · 90° · ISO 10642
| Size | Head Ø mm | 90° C'sink Ø mm |
|---|---|---|
| M3 | 6.72 | 6.94 |
| M4 | 8.96 | 9.18 |
| M5 | 11.20 | 11.47 |
| M6 | 13.44 | 13.71 |
| M8 | 17.92 | 18.25 |
| M10 | 22.40 | 22.73 |
Metric flat-head / countersunk socket screws use a 90° included angle — don't substitute an 82° tool. All values mm.
Counterbores — inch SHCS · ASME B18.3
| Size | SHCS head | Counterbore | ||
|---|---|---|---|---|
| Ø max | Ht max | Ø | Depth | |
| #4 | 0.183 | 0.112 | 7/32 (0.219) | 0.112 |
| #6 | 0.226 | 0.138 | 9/32 (0.281) | 0.138 |
| #8 | 0.270 | 0.164 | 5/16 (0.313) | 0.164 |
| #10 | 0.312 | 0.190 | 3/8 (0.375) | 0.190 |
| 1/4" | 0.375 | 0.250 | 7/16 (0.438) | 0.250 |
| 5/16" | 0.469 | 0.312 | 17/32 (0.531) | 0.312 |
| 3/8" | 0.562 | 0.375 | 5/8 (0.625) | 0.375 |
| 1/2" | 0.750 | 0.500 | 13/16 (0.813) | 0.500 |
All values inch. C'bore Ø clears the head with running clearance; depth = max head height for a flush top (go deeper to recess sub-flush).
Counterbores — metric SHCS · ISO 4762
| Size | SHCS head | Counterbore | ||
|---|---|---|---|---|
| Ø max | Ht max | Ø | Depth | |
| M3 | 5.5 mm | 3.0 mm | 6.5 mm | 3.0 mm |
| M4 | 7.0 mm | 4.0 mm | 8.25 mm | 4.0 mm |
| M5 | 8.5 mm | 5.0 mm | 9.75 mm | 5.0 mm |
| M6 | 10.0 mm | 6.0 mm | 11.25 mm | 6.0 mm |
| M8 | 13.0 mm | 8.0 mm | 14.25 mm | 8.0 mm |
| M10 | 16.0 mm | 10.0 mm | 17.25 mm | 10.0 mm |
Metric SHCS per ISO 4762 / DIN 912. Counterbore depth = max head height for flush; Ø gives running clearance around the head.
A clearance hole lets the fastener pass freely; a tap drill (smaller) leaves stock for cutting threads — don't confuse them. Counterbore = flat-bottom pocket for a socket-head; countersink = cone for a flat-head. Full reference and notes:clearance hole chart andcountersink & counterbore chart.
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