Article — Clearance Hole Calculator
Clearance Hole Calculator — Drill Sizes for Bolts and Screws
A clearance hole is a hole drilled larger than the bolt or screw passing through it. The fastener slides through with a small gap; clamping happens at the head and the nut or threaded hole on the other side. Standard sizes come from ISO 273 (metric, three fit classes: close, normal, loose) and ANSI B18.2.8 (inch fasteners). An M6 bolt uses a 6.6 mm clearance hole for normal fit; a 1/4-inch bolt uses 0.281 inch (9/32 drill).
The gap between bolt and hole accommodates manufacturing tolerance on the bolt diameter, position tolerance between mating parts, and small amounts of thermal expansion. Without it, parts rarely line up well enough for the bolt to pass through both holes.
What is a clearance hole?
A clearance hole lets a bolt or screw pass through without engaging threads. The bolt is held by clamping force from its head pressing on one face and a nut (or threaded hole) on the other face. The hole itself plays no structural role beyond letting the fastener through.
This is fundamentally different from a tap drill, which is smaller than the bolt and is later threaded with a tap so the bolt screws directly into the material. Confusing the two produces holes that won't accept threads (clearance too big) or that won't pass a bolt (tap drill too small). Always know which type of hole you're drilling before reaching for a bit.
ISO 273 was first published in 1979, harmonizing nine separate European national standards into a single document. Before it, German DIN, French AFNOR, British BSI, and others all used slightly different clearance gaps — making cross-border fastener interchangeability a nightmare. Modern ISO 273 covers every metric size from M1 to M150.
Clearance hole fit classes — close, normal, loose
ISO 273 defines three fit classes for every bolt size. Close fit gives the smallest gap (about 0.2-0.4 mm), suitable for precision alignment work — jigs, fixtures, optical mounts, electronics. Normal fit sits in the middle (0.4-1.0 mm) and is the production default for general machinery and assembly. Loose fit opens to 1.0-2.0 mm for field work and situations where multiple bolts on different parts have to line up.
Most workshop drilling is normal fit. Use close fit only when the project specifically needs precise positioning. Use loose fit only when you know holes won't line up well — large fabricated flanges, field-installed brackets, structural steel with positional error from welding.
Close 0.2-0.4 mm (precision)Normal 0.4-1.0 mm (default)Loose 1.0-2.0 mm (field)ISO 273 metric clearance holes
For metric bolts, ISO 273 publishes a table of clearance hole diameters by bolt size and fit class. The numbers aren't calculated — they were chosen by committee to balance assembly ease against bolt-head clamp area. An M6 bolt gets a 6.4 mm hole (close), 6.6 mm hole (normal), or 7.0 mm hole (loose).
For M8 the values jump: 8.4 mm close, 9.0 mm normal, 10.0 mm loose — the gap is intentionally larger at M8 and above because larger bolts also tend to have larger positional errors in fabrication. M10 normal clearance is 11 mm; M12 is 13.5 mm; M16 is 17.5 mm.
ANSI B18.2.8 inch clearance holes
For US fasteners, ANSI B18.2.8 covers #4 through 1-1/2 inch bolts with the same three fit classes. A 1/4-inch bolt gets 0.266 inch close fit (17/64 drill), 0.281 inch normal (9/32 drill), or 0.297 inch loose (19/64 drill). A 1/2-inch bolt uses 0.516 / 0.531 / 0.562 inch for close / normal / loose.
Inch clearance holes often land on fractional drill sizes — 1/4 inch normal is exactly 9/32, 1/2 inch normal is 17/32. Inch fasteners and inch drill bits were designed together. The same can't always be said of metric clearance holes, where the standard values sometimes fall between common metric drill sizes (5.5 mm exists, 6.6 mm doesn't — use 6.5 or 7.0).
Clearance hole vs tap drill
A clearance hole is bigger than the bolt; a tap drill is smaller. For an M6 bolt, the clearance hole is 6.6 mm and the tap drill is 5.0 mm — they differ by 1.6 mm, which is half the bolt diameter. Mistaking one for the other is the most common drilling error in DIY and entry-level shops.
Use a clearance hole when the bolt passes through one piece into a nut, threaded insert, or threaded hole in a separate piece. Use a tap drill when the bolt threads directly into the material — typically a thicker piece (twice the bolt diameter or more) of steel, aluminum, or hardwood. Combination holes (clearance through one part, threaded in the other) are common in machine assemblies.
When in doubt, choose normal fit. It works for 90% of projects without needing tighter precision or wider tolerance. Close fit is for jigs, fixtures, and electronics where positioning matters; loose fit is for field assembly of large structures where everything moves. Normal is the "just right" default.
Common clearance hole mistakes
- Drilling a tap drill instead of a clearance hole: 5.0 mm doesn't pass an M6 bolt. Double-check the bit before drilling.
- Drilling too oversized: a hole bigger than half the washer diameter lets the bolt head pull through under load.
- Using inch drill on metric chart: 1/4 inch is not 6 mm; it's 6.35 mm. Match the drill to the standard.
- Drilling clearance through a thin sheet: a 6.6 mm hole in 1 mm sheet metal has no edge engagement — use a smaller hole with a self-tapping screw or add a backer.
- Forgetting the washer: oversized clearance holes need washers to distribute bolt-head pressure across enough surface to prevent pull-through.
- Tight fit on mating parts: when bolting two separately-machined parts together, the holes never line up perfectly. Use normal or loose fit to allow position tolerance.
A 7/32 inch drill (0.219 in) is 5.56 mm — close to an M5 normal clearance (5.5 mm) but not identical. The 0.06 mm difference doesn't matter for normal fit but takes an M5 close fit (5.3 mm) clearly out of spec. For precision work, match the drill set to the fastener standard.
Washers and clearance hole design
The washer's job is to spread the bolt-head pressure across enough material to prevent pull-through and crushing of the underlying surface. Washer outside diameter should be at least twice the clearance hole diameter. For an M6 with 6.6 mm hole, a 13.5 mm OD washer (DIN 125, standard for M6) is the minimum.
On loose-fit holes, washers also keep the bolt head centered. Without one, the head tilts within the oversize hole, concentrating load on one edge and accelerating metal yield around the hole. Always use washers with loose-fit clearance holes. For close fit, washers are optional but recommended on softer materials (aluminum, brass, soft steel).