Clearance Hole Calculator

A clearance hole is a plain, unthreaded through-hole drilled slightly larger than the bolt that goes through it. The bolt's shank slides through freely without the threads biting into the material — only the bolt head (or nut) clamps the parts together. Clearance holes are used on the “bolt passes through” side of a joint; the threads either engage a nut on the other side or a tapped hole in the mating part.

They do opposite jobs. A clearance hole is BIGGER than the bolt — for an M6 bolt the normal-fit clearance hole is 6.6 mm, so the bolt slides through freely. A tap drill hole is SMALLER than the bolt — for M6 it is 5.0 mm — because you then cut M6 threads into it with a tap. Use this calculator for the through-hole side; for the threaded side, use a tap drill finder such as our thread-lab tool.

Per ISO 273, an M6 bolt takes a 6.4 mm hole for close fit, 6.6 mm for normal fit, or 7.0 mm for loose fit. The 6.6 mm normal fit is the default for general workshop assembly. In imperial drill sizes, 6.6 mm equals 0.2598 in — grab a 17/64 in (0.2656 in) or a G (0.261 in) drill, the nearest standard sizes above 6.6 mm. For comparison, the M6 tap drill (threaded hole) is only 5.0 mm.

ISO 273 is the international standard “Fasteners — Clearance holes for bolts and screws”. It defines three clearance series for every standard thread size from M1.6 to M150: the fine series (close fit, minimal play), the medium series (normal fit, the general-purpose default), and the coarse series (loose or free fit, maximum adjustment). The calculator and chart on this page follow ISO 273 values, for example M12 → 13.0 / 13.5 / 14.5 mm.

Use normal fit unless you have a reason not to — it suits general assembly with ordinary drilling tolerances. Choose close fit when parts must align precisely with almost no shift (for example, locating two machined plates). Choose loose fit when you need maximum adjustment during assembly, for field-drilled or welded structures, or when hole positions are uncertain. For an M12 bolt that means 13.0 mm (close), 13.5 mm (normal), or 14.5 mm (loose).

Per ASME B18.2.8 shop practice, a 1/4-20 UNC bolt takes a 0.257 in (F drill) clearance hole for close fit or 0.266 in (H drill) for normal fit — 0.266 in (6.76 mm) is the usual choice. The tap drill for cutting 1/4-20 threads is much smaller: a #7 drill (0.201 in). Mixing these up is the most common mistake: the clearance hole must clear the full 0.250 in shank, while the tap drill leaves material for the threads.

Per ISO 273 (close / normal / loose): M8 → 8.4 / 9.0 / 10.0 mm; M10 → 10.5 / 11.0 / 12.0 mm; M12 → 13.0 / 13.5 / 14.5 mm. The matching coarse-thread tap drills are 6.8 mm (M8), 8.5 mm (M10), and 10.2 mm (M12) — notice each tap drill is well under its bolt diameter, while every clearance hole is above it.

Because the tap drill hole keeps material that becomes the thread. Drilling 5.0 mm and tapping M6 × 1.0 cuts threads about 0.6 mm deep on each side, leaving a strong threaded hole. A clearance hole has no threads at all, so it must clear the bolt's full outside diameter plus a little play — 6.6 mm for M6 normal fit. The gap between the two numbers (5.0 vs 6.6 mm) is exactly the difference between “cut threads here” and “let the bolt pass through”.

It works as a workshop shortcut but it is not the standard size. Example: the M6 normal-fit clearance hole is 6.6 mm, while the M8 tap drill is 6.8 mm — only 0.2 mm over, fine for rough work. But the offset is not consistent across sizes, so for anything that matters use the ISO 273 table values from the calculator above rather than the trick.

That difference is the diametral play — how far the joint can shift sideways. ISO 273 normal-fit play grows with bolt size: 0.4 mm on M3, 0.6 mm on M6, 1.0 mm on M10, 1.5 mm on M12, and 2.0 mm on M20. Close fit roughly halves the play; loose fit roughly doubles it. The calculator shows the play for every size so you can judge whether the joint will still align.

Model the ISO 273 size first, then add compensation: FDM-printed holes typically come out 0.2–0.4 mm smaller than modeled because the plastic shrinks and the first layer sags. For an M6 bolt, model the normal-fit hole at about 6.8–7.0 mm instead of 6.6 mm, then test-fit. For resin (SLA/MSLA) prints, 0.1–0.2 mm extra is usually enough. When precision matters, print slightly small and drill or ream to the final ISO size.

No — you need a tap drill size, not a clearance hole. Use our thread-lab tool: it is a threading workbench with a tap drill finder (metric and UNC/UNF), thread percentage, 3-wire measurement, and NPT sizes. This calculator covers only unthreaded through-holes per ISO 273. A typical bolted joint needs both: a clearance hole in the top part (this tool) and a tapped hole in the bottom part (thread-lab).