Kaimera Industries
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KM-T-05 / Fastening · Hole Prep

Fastener Hole Reference

Clearance · counterbore · countersink. Inch values from ASME B18.2.8 and B18.3, metric from ISO 273 and ISO 10642.

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A free clearance hole and counterbore calculator for inch and metric fasteners. Choose a screw standard, size, head type, and fit class to get the clearance drill size, counterbore diameter and depth for socket head cap screws, and countersink diameter and angle for flat head screws — the numbers you need before the drill touches the part.

Fastener

Head geometry

Head diameter (max)
0.3750in
per ASME B18.3
Head height
0.2500in
= nominal screw dia
Clearance drill — 1/4 screw passes through
9/32=0.2812in
ASME B18.2.8 · normal fit

Counterbore

C'bore diameter
7/16
= 0.4375 in
C'bore depth (min flush)
0.2500in
add a few thou for sub-flush
C'sink relief dia
0.2780in
edge break for fillet clearance
Paired clearance drills from the same B18.3 chart: normal 9/32 (0.2812), close 17/64 (0.2656). These match the B18.2.8 values at this size.
Depth shown seats the head exactly flush at max head height. The countersink-relief diameter is a light edge break at the top of the clearance hole so the screw's under-head fillet clears — it is not a flat-head countersink.

All clearance fits at this size

close
17/64
= 0.2656 in
normal
9/32
= 0.2812 in
loose
19/64
= 0.2969 in
Sources: inch clearance holes per ASME B18.2.8 (#8–1 in; close/normal/loose), cross-checked against two independent published tables; sizes #0–#6 per the ASME B18.3 drill chart (close/normal only). Counterbores per the ASME B18.3 chart (inch) and its metric B18.3.1M-family equivalent. Flat head data per ASME B18.3-2003 (82°, theoretical sharp diameters) and ISO 10642 (90°). Metric clearance values are identical between ISO 273 and ASME B18.2.8.

Not covered here: DIN 974 European counterbore dimensions (slightly smaller than the US values shown), M18/M22 clearance (not in the verified set), tapped-hole/thread-forming drills (that's the thread dimension tool's job), and structural/slotted holes, which are set by connection design rather than these tables. Always verify against the standard or Machinery's Handbook for tolerance-critical work.

§ Common Questions

Frequently asked

What is a normal fit clearance hole?
A normal fit clearance hole gives moderate slop between the fastener and the hole — enough to absorb typical stack-up on multi-hole patterns without letting the joint float. Close fit is for located parts; loose fit is for weldments and rough patterns.
How deep should a counterbore be?
Deep enough that the screw head sits flush or slightly below the surface, plus clearance for the head fillet. The standard head height for the fastener plus a small allowance is the usual target.
What angle is a countersink for flat head screws?
Inch flat head cap screws are typically 82 degrees included; metric flat head screws are typically 90 degrees. Machine screws and aerospace hardware can differ, so check the fastener spec.

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