Clearance Hole Calculator — ISO 273 Metric Fasteners
Select a metric bolt size (M1.6 to M24) and a fit class to find the ISO 273 recommended clearance hole diameter — the drill size your workshop needs to allow the fastener to pass through freely.
Nominal bolt size
Clearance fit class
ISO 273 recommended drill size for this bolt and fit class
How does this calculator work?
ISO 273 defines three clearance hole sizes for each metric bolt: Close (H12, tightest), Medium (H13, standard), and Free (H14, easiest to assemble). An M6 bolt needs a 6.4 mm (close), 6.6 mm (medium), or 7.0 mm (free) drill. Values cover M1.6 through M24; verify against the full standard for other sizes.
Formula
How this is calculated
A clearance hole is a through-hole that is deliberately larger than the bolt or screw passing through it, so the fastener can slide in without threading the parent material. The hole is slightly oversized relative to the bolt's nominal diameter, and the amount of oversize — the clearance — is defined by a fit class.
ISO 273 (the international standard for metric fastener clearance holes) defines three fit classes: Close (Fine, tolerance H12), Medium (H13), and Free (Coarse, H14). Close fit gives the tightest clearance and the most accurate bolt location — used where alignment between parts matters. Medium fit is the standard engineering choice for general assembly. Free fit gives the most clearance and is easiest to assemble when alignment is not critical, or when manufacturing tolerances on the hole position are wide.
The tabulated values cover nominal bolt sizes from M1.6 to M24 as per ISO 273:1979. For bolts outside this range, consult the full standard directly. The "diametral clearance" is the difference between hole and bolt diameters — the bolt can shift by half this amount in any direction, which sets the positional accuracy of the joint.
Frequently asked questions
Medium fit (H13) is the standard recommendation for general engineering and structural assembly — it gives enough clearance for easy bolt insertion while keeping the fastener reasonably located. Use Close fit when precise hole-to-hole alignment between mating parts is critical. Use Free fit for sheet-metal brackets, rough framing, or when installation access is difficult.
Nominally yes — for example, an M6 medium-fit clearance hole is 6.6 mm, so you drill with a 6.6 mm twist drill. In practice use the nearest standard drill size at or above the ISO value. Drill bits are available in 0.1 mm increments in metric sets, so you can usually hit the exact value.
No — these are ISO metric (M-series) values from ISO 273. Imperial clearance holes follow ASME B18.2.8 and use different thread pitches and nominal diameters. For UNC/UNF bolts, look up the ASME tables or use a dedicated imperial clearance-hole reference.
Also known as
TG we-Calculate Editorial Team. (2026). Clearance Hole Calculator — ISO 273 Metric Fasteners [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/clearance-hole-calculator
TG we-Calculate Editorial Team. "Clearance Hole Calculator — ISO 273 Metric Fasteners." TG we-Calculate. 2026. https://we-calculate.com/calculator/clearance-hole-calculator.
TG we-Calculate Editorial Team, "Clearance Hole Calculator — ISO 273 Metric Fasteners," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/clearance-hole-calculator
@misc{wecalculate_clearance_hole_calculator, title = {Clearance Hole Calculator — ISO 273 Metric Fasteners}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/clearance-hole-calculator}}, year = {2026}, note = {TG we-Calculate} }
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