How this instrument works
A clearance hole is a hole sized so a bolt or screw can pass through freely without engaging threads in the material it's passing through — used anywhere a fastener needs to slide through one part and thread into (or bolt through) another. Get the hole too small and the fastener won't fit or will bind on threads; get it too large and you lose bearing surface and alignment precision.
ANSI/ASME B18.2.8 is the standard reference for clearance hole sizes in the US, publishing three fit classes for each standard fastener size: close fit (tightest, for precise alignment where minimal play is wanted), normal fit (the general-purpose default for most assembly work), and loose fit (extra clearance for easier assembly or when some misalignment between parts needs to be tolerated).
The published standard gives a min-max range for each size and fit combination rather than one exact number, since real drilling and manufacturing always has some tolerance. This calculator reports the midpoint of that published range as a single representative figure for planning purposes — real shop practice typically drills the nearest standard drill bit size that falls within the published range, which may not land exactly on the midpoint this calculator reports.
- Select your fastener's nominal size from the list (inch sizes from #4 through 3/4 in).
- Select the fit class you need: close, normal, or loose.
- Read the recommended clearance hole diameter.
- Choose the nearest standard drill bit size to that figure, confirming it falls within the published ASME B18.2.8 range for your size and fit.
A 1/4 in bolt with a normal-fit clearance hole
A fabricator needs a clearance hole for a 1/4 in bolt passing through a bracket, using a standard normal fit for general assembly. ASME B18.2.8 publishes a normal-fit range of 0.281-0.290 in for this size; the midpoint is (0.281+0.290)/2 = 0.2855 in — close to a standard 9/32 in drill bit (0.28125 in), which falls near the low end of that published range.
The same 1/4 in bolt in a loose-fit application (where some misalignment tolerance is needed) uses a wider published range of 0.297-0.311 in, and a 3/8 in bolt in a tighter close-fit application uses a much smaller-diameter range of 0.391-0.397 in (midpoint 0.394 in) — illustrating how both fastener size and fit class independently drive the recommended hole diameter.
Questions
Which fit class should I use — close, normal, or loose?
Normal fit is the general-purpose default for most assembly work. Close fit is worth using when precise alignment between parts matters and you want minimal play around the fastener. Loose fit is useful when you need extra clearance for easier assembly, or when the parts being joined may not align perfectly and need some tolerance to accommodate that.
Why does this calculator give a midpoint instead of one exact ASME-specified number?
ASME B18.2.8 publishes a min-max range for each size and fit combination, not a single fixed value, because real drilling always has manufacturing tolerance. This calculator's midpoint is a disclosed simplification for quick reference — in the shop, you'd typically pick the nearest available standard drill bit size that falls within the full published range, not necessarily the exact midpoint.
What drill bit size should I actually use?
Pick the nearest standard fractional, number, or letter drill bit size that falls within the published ASME B18.2.8 range for your fastener size and fit class — it won't always land exactly on this calculator's midpoint figure, but should be close to it and within the acceptable range.
Does this apply to metric fasteners too?
No, this calculator covers standard inch fastener sizes only (from #4 through 3/4 in), per the inch-series tables in ASME B18.2.8. Metric fasteners follow their own separate clearance hole reference tables.