Clearance Hole Calculator

A bolt passes through a hole that is deliberately larger than the bolt — the clearance is what lets assemblies go together, and how much of it is a designed decision. This calculator holds the standard tables: choose your bolt, metric or inch, and your fit — close, normal or loose — and it returns the drill size to make the hole, in decimal and the nearest fractional or letter drill where the inch system applies.

Why Holes Are Bigger Than Bolts

A clearance hole is an admission of how the world is made: parts are drilled in different setups, at different times, by processes with tolerances, and the bolt must still pass through all of them at assembly. The clearance absorbs the accumulated disagreement — hole position error in both parts, drill wander, bolt straightness, coating thickness — and its size is therefore an engineering decision with failure modes on both sides. Too little and the joint won’t assemble (or assembles under force, preload lost to friction and bending); too much and the joint locates by luck, slips under shear until the bolt bears unevenly, and frets. The standards bodies have already had this argument at scale, which is why the tables exist: ISO 273 grades metric clearance holes fine / medium / coarse, and ASME B18.2.8 does the same for inch bolts as close / normal / loose. This calculator is those tables with a front door.

Worked Example: The Ubiquitous M6

The defaults pick the world’s commonest machine bolt: M6. The table answers by fit: close 6.4 mm, normal 6.6 mm, loose 7.0 mm — clearances of 0.4, 0.6 and 1.0 mm over the bolt’s 6 mm. The normal figure is the one burned into fabrication memory, and the reasoning generalises across the table: clearance grows with bolt size (an M20 loose hole is 24 mm, a full 4 mm over) because position errors grow with part size, while the proportion stays civilised. The inch examples rhyme: a 1/4 in bolt takes 0.257 / 0.266 / 0.281 in — the normal 0.266 being the famous “17/64 drill” of every shop wall chart. When a drawing just says “clearance hole”, the normal/medium column is what it means, everywhere on earth, which is a remarkable thing for two standards to agree on without ever having met.

Choosing the Fit: The Assembly Decides

The fit classes map to real assembly situations, and choosing is the engineering this page can’t do for you — only frame. Close is for located joints: dowelled or jig-drilled parts, precision bracketry, anywhere the bolt itself helps position the parts — assembly will be snug and may want a reamer’s honesty rather than a drill’s optimism. Normal is general fabrication’s default: machine-drilled or well-laid-out parts, standard tolerances, the assumption behind structural and mechanical drawings alike (structural steel’s own standard holes live here, at 1/16 in over for most sizes). Loose is for field conditions: parts made in different shops, thermal expansion to absorb, alignment that must be adjustable after assembly — anchor bolts in concrete are the extreme citizens of this class, swimming in oversize holes under plate washers by design. The failure to avoid is drifting loose by accident: a worn drill, an enthusiastic reamer of a hand, and a located joint becomes a rattling one.

Clearance Is Not Tap Drill, and Other Neighbours

Two neighbouring hole types share the drawer and must not share the drill. A tap drill is smaller than the bolt — it leaves the material the thread will be cut into (M6 × 1.0 taps into a 5.0 mm hole) — and confusing the tables produces the classic ruined part: a “clearance” hole the bolt falls through where a thread was meant to be, or a tapped hole attempted in a clearance-size bore. Interference and transition fits go the other direction entirely (holes at or below the shaft size, for press-fitted pins and bushings — a machinist’s discipline with its own standards). This calculator’s jurisdiction is the through-hole for a fastener that must pass freely, which is most holes in most assemblies; where the hole’s job is to grip, locate by press, or carry a thread, different tables govern, and the drawing — not habit — says which.

Structural Steel Speaks a Dialect of This Table

Bridge and building steelwork runs on AISC’s own hole vocabulary, close kin to the normal column: the standard hole is the bolt diameter plus 1/16 in (a 3/4 in bolt takes a 13/16 in hole), the oversize hole adds 1/8 in beyond that (15/16 in for the same bolt), and slotted holes extend one dimension for adjustment — each class with washer and design rules attached, because structural slip and bearing are calculated, not hoped. The dialect matters when the trades meet: a fabricator’s “standard hole” and a machinist’s “normal clearance” are near-identical figures wearing different standards’ clothes, and this calculator’s normal column will keep a conversation honest in either shop.

Making the Hole You Specified

A table value is a target; the drill is an negotiator. Twist drills characteristically cut a few thousandths oversize (two to five in ordinary work, more with a wandering point or a hand-held drill), which quietly moves close fits toward normal and normal toward loose — precision work counters with a drill-then-ream sequence, the drill leaving the last 0.005–0.010 in for the reamer that actually owns the size. Thin sheet adds its own behaviour (the drill grabs and the hole goes tri-lobed; step drills and punches make rounder holes in sheet than jobber drills do). The discipline that survives all of it: specify from the table, measure the first hole with pins or calipers, and adjust the tool, not the standard — the fits in this calculator presume the hole measures what it was called.

The Wall Chart, Digitised

Every machine shop of a certain age has the poster: bolt sizes down the side, drill sizes across the top, the normal column worn pale from pointing. This calculator is that poster with the other two columns restored — because the close and loose answers are the ones nobody memorised, needed precisely when the assembly is unusual enough to have driven you to a calculator in the first place. The poster’s ghost approves: it always knew the standard was a table with three doors, and that most days you only need the middle one — until the day you don’t.

Related Tools

More from the Construction Calculators hub: the Bolt Circle Calculator — where these holes go, in coordinates — and the Arch Calculator for the openings the assemblies frame.

Frequently Asked Questions

What size hole for an M6 bolt?

6.4 mm for a close fit, 6.6 mm normal, 7.0 mm loose — the ISO 273 series this calculator serves for every listed size. If only one figure can be remembered per bolt, remember the normal column: it is the default assumption of drawings, kits and furniture hardware worldwide, and it is engineered so that ordinary fabrication tolerances assemble first time.

What drill for a 1/4 inch bolt?

Normal fit is 0.266 in — the 17/64 in drill (0.2656) that lives in every index for exactly this duty; close is 0.257 (the F drill, 0.257, is the machinist’s standard answer) and loose is 0.281 (9/32). The calculator returns the table value and the nearest 64th; where the exact letter or number drill exists, shops reach for it by name, which is why those drills exist.

How much clearance is too much?

When the joint’s function starts paying for it: shear joints slip until bolts bear (the slip distance is the clearance), located assemblies lose their location, and washers bridge rather than clamp. The loose column is the standards’ considered maximum for ordinary work — beyond it lie designed exceptions (anchor bolts, slotted adjustment holes) that come with plate washers and engineering intent. If a hole needs to be bigger than loose, it should be a slot or a designed detail, not a bigger accident.

Do these sizes suit wood and construction bolts?

With a translation: timber bolts and lag work use their own conventions (carriage bolts in wood often at nominal size or a 1/32–1/16 over, since wood gives; structural steel follows its standard-hole tables, close to this page’s normal column). The machined-metal tables here are the reference the other trades borrow from; in wood, the pilot hole question (for screws and lags) is a different table again, driven by shank and thread rather than clearance.

Why does clearance grow with bolt diameter?

Because position error grows with the size of the parts being joined — a 600 mm plate’s holes cannot be placed as absolutely precisely as a 60 mm bracket’s, and thermal and coating effects scale likewise. The standards encode it as a rising clearance (0.4 mm at M3’s close fit, 4 mm at M20’s loose), roughly logarithmic in spirit: enough to assemble the real world at every scale, never so much that the joint forgets its purpose.

What if my bolt size isn’t listed?

The listed range (M3–M20, 1/4–1 in) covers the overwhelming majority of clearance work. For sizes between, interpolate conservatively toward the next larger bolt’s normal figure; for sizes beyond, the governing standard (ISO 273 or ASME B18.2.8) carries the full series at any engineering library. The principle outlives the table: normal clearance is roughly 8–12% of diameter at small sizes, easing as diameters grow.