About

Drilling a through-hole without referencing ISO 273 risks either a press-fit jam or excessive play that defeats the purpose of the fastener joint. This calculator returns the correct clearance hole diameter for metric bolts from M1 through M64 across three fit classes: Fine, Medium, and Coarse. It computes the diametral clearance c = D_hole − d_nom and recommends a standard metric drill bit. The underlying data follows ISO 273:1979 (EN 20273). Tolerances are nominal; actual fit depends on hole roundness, surface finish, and plating thickness on the fastener.

Formulas

The clearance hole diameter D_h is selected from ISO 273 based on the bolt nominal diameter d and the desired fit class. The diametral clearance is:

c = D_h − d

Where D_h = clearance hole diameter mm, d = bolt nominal diameter mm, and c = diametral clearance mm. ISO 273 defines three fit classes. Fine (H12) provides minimal clearance for precise alignment. Medium (H13) is the default for general assembly. Coarse (H14) allows maximum positional tolerance for slotted or oversized holes.

The radial clearance (gap per side) is half the diametral value:

c_r = D_h − d2

Where c_r = radial clearance mm. This value determines the maximum positional misalignment the joint can absorb before the bolt contacts the hole wall.

Reference Data

Bolt Size	Nominal d mm	Fine Hole mm	Medium Hole mm	Coarse Hole mm
M1	1.0	1.1	1.2	1.3
M1.2	1.2	1.3	1.4	1.5
M1.6	1.6	1.7	1.8	2.0
M2	2.0	2.2	2.4	2.6
M2.5	2.5	2.7	2.9	3.1
M3	3.0	3.2	3.4	3.6
M4	4.0	4.3	4.5	4.8
M5	5.0	5.3	5.5	5.8
M6	6.0	6.4	6.6	7.0
M8	8.0	8.4	9.0	10.0
M10	10.0	10.5	11.0	12.0
M12	12.0	13.0	13.5	14.5
M14	14.0	15.0	15.5	16.5
M16	16.0	17.0	17.5	18.5
M18	18.0	19.0	20.0	21.0
M20	20.0	21.0	22.0	24.0
M22	22.0	23.0	24.0	26.0
M24	24.0	25.0	26.0	28.0
M27	27.0	28.0	30.0	32.0
M30	30.0	31.0	33.0	35.0
M33	33.0	34.0	36.0	38.0
M36	36.0	37.0	39.0	42.0
M42	42.0	43.0	45.0	48.0
M48	48.0	50.0	52.0	56.0
M56	56.0	58.0	62.0	66.0
M64	64.0	66.0	70.0	74.0

Frequently Asked Questions

Fine fit (H12) is appropriate when positional accuracy of the bolted joint matters - for example, dowel-free alignment of mating flanges, precision instrument housings, or assemblies where shear load is partially carried by bolt-hole contact. The clearance is typically only 0.1 - 0.5 mm diametrically, which demands CNC-drilled or reamed holes. Hand-drilled holes rarely achieve the roundness required for Fine fit to function as intended.

Yes. Hot-dip galvanised bolts can add 0.05 - 0.15 mm per side to the effective diameter. For an M16 bolt, galvanising may increase the shank to approximately 16.3 mm. If you selected Fine fit (17.0 mm hole), your actual clearance drops to 0.7 mm diametrically, which may cause insertion difficulty in multi-hole patterns. In such cases, upgrade to Medium fit or verify with the coating specification.

Coarse fit provides the starting diameter for slotted (elongated) holes. Per EN 1090-2, short slotted holes use the Coarse diameter as the width and extend the length by 1.5d for bolts up to M24. Long slotted holes extend to 2.5d. The Coarse value from this calculator gives you the correct slot width; you then calculate the slot length separately.

Below M8, the clearance increments are small and roughly proportional (typically 0.2 - 0.5 mm per class). At M8 and above, the ISO 273 table shifts to larger absolute clearances because manufacturing tolerances on hole position become the dominant factor in assembly, not just the hole diameter. A 1.0 mm positional error on an M8 hole pattern is common in fabricated steelwork, so the Medium and Coarse fits accommodate that reality.

The clearance hole diameter does not directly affect preload - that is governed by tightening torque and friction coefficients. However, a larger hole reduces the annular bearing area under the washer or bolt head. For an M12 bolt with a Coarse hole (14.5 mm), the washer must fully cover the hole to distribute clamping force. If the washer outer diameter is marginal, the bearing stress on the clamped part increases, risking embedment in soft materials like aluminium. Always verify that washer OD exceeds the hole diameter by at least 2 mm.

ISO 273 defines hole sizes independent of fastener material. The nominal diameters for stainless steel (A2, A4) and brass metric bolts follow the same ISO 4014/4017 shank dimensions as carbon steel. However, stainless steel bolts have tighter diameter tolerances (6g), so Fine fit works reliably. Brass fasteners may have looser manufacturing tolerances depending on the supplier, making Medium fit a safer default.