How this calculator works
A floating fastener can move in both mating holes. A simple preliminary rule places the hole diameter halfway between the shank diameter and effective head bearing diameter. The resulting diametral clearance equals twice the radial bearing overlap. This is a layout rule of thumb, not an ISO clearance-hole table or a strength check. Chamfers and washers change the effective bearing diameter.
Formula
hole = (shank + bearing head)/2; shank = 2 x hole - head; head = 2 x hole - shank; diametral clearance = hole - shank.
Worked example
A 6 mm shank with a 10 mm effective bearing head gives an 8 mm preliminary hole, 2 mm diametral clearance and 1 mm radial head overlap.
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Frequently asked questions
Is this a tap drill size?
No. A clearance hole passes the shank freely; a tap drill leaves material to form an internal thread.
Does this select a standard drill?
No. Select an available drill and verify its actual tolerance, head bearing and positional requirements separately.
Can I use the outside diameter of a washer?
Only if the washer is part of the verified bearing arrangement. A loose washer does not automatically make every hole beneath it acceptable.