A clearance hole size chart is the one table that decides whether a bolted part assembles on the buyer's line or comes back as scrap. And most drawings get it wrong in the same way: they call out the bolt size as the hole size. For an M8 bolt that means a ⌀8 hole — and an M8 bolt will not pass through an 8.0 mm hole.
ISO 273:1979 fixes this. It is the international standard for clearance holes for bolts and screws, it covers thread diameters from M1 up to M150, and it does not give you one number per bolt. It gives you three.
The clearance hole size chart (ISO 273, metric)
| Bolt | Major ⌀ | Close fit (H12) | Normal fit (H13) | Loose fit (H14) |
|---|---|---|---|---|
| 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 |
| M16 | 16.0 | 17.0 | 17.5 | 18.5 |
| M20 | 20.0 | 21.0 | 22.0 | 24.0 |
| M24 | 24.0 | 25.0 | 26.0 | 28.0 |
| M30 | 30.0 | 31.0 | 33.0 | 35.0 |
The three columns are the three series in the standard — fine, medium and coarse — which the trade usually calls close, normal and loose. Each series is paired with a hole tolerance field: H12 for the fine series, H13 for medium, H14 for coarse. Those values are not arbitrary. The scope of ISO 273 states that the diameters result from bearing area calculations made in connection with the ISO bolt and nut product standards, which is why the numbers jump around instead of following a neat plus-one-millimetre rule.
For an M8 bolt the three answers are 8.4 mm, 9.0 mm and 10.0 mm. If your drawing says ⌀8, you have specified an interference fit and the part will not go together.
The clearance hole size chart for inch bolts
If you quote into US shops, the same three-fit logic arrives as inch and number drills. ASME B18.2.8 covers metric clearance holes and is aligned with ISO 273; the inch table below is the shop-practice chart most US machinists work from.
| Bolt | Major ⌀ | Close fit | Normal fit | Loose fit |
|---|---|---|---|---|
| 1/4" | 0.250 | F — 0.2570 | 17/64" — 0.2656 | 9/32" — 0.2812 |
| 5/16" | 0.3125 | 21/64" — 0.3281 | 11/32" — 0.3438 | 3/8" — 0.3750 |
| 3/8" | 0.375 | 25/64" — 0.3906 | 13/32" — 0.4062 | 7/16" — 0.4375 |
| 7/16" | 0.4375 | 29/64" — 0.4531 | 15/32" — 0.4688 | 1/2" — 0.5000 |
| 1/2" | 0.500 | 33/64" — 0.5156 | 17/32" — 0.5312 | 9/16" — 0.5625 |
| 5/8" | 0.625 | 41/64" — 0.6406 | 21/32" — 0.6562 | 11/16" — 0.6875 |
| 3/4" | 0.750 | 49/64" — 0.7656 | 25/32" — 0.7812 | 13/16" — 0.8125 |
| 1" | 1.000 | 1-1/64" — 1.0156 | 1-1/32" — 1.0312 | 1-1/16" — 1.0625 |
Notice the inch chart keeps a clearance margin of roughly 0.015" to 0.031" for a normal fit. The metric chart does the same thing in millimetres. Neither is "the bolt diameter plus a bit". If you quote into both systems, the same buyer confusion that surrounds metric vs imperial fastener threads shows up here too: a 1/4" callout and an M6 callout are close enough to look interchangeable on a quote and far enough apart to scrap a plate.
Myth 1: an M8 bolt fits an 8 mm hole
This is the mistake that costs the most money, because it looks so reasonable. A hole is 8 mm, the bolt is 8 mm, they match.
They do not. The "8" in M8 is the thread's nominal major diameter, and the bolt carries its own tolerance on top of it — an 8.8 or 10.9 hex bolt is allowed to run slightly over the nominal. Drill the hole at exactly 8.0 mm and you have built a press fit by accident. The bolt either will not enter, or it enters with a hammer and galls the thread, or it enters and deforms the hole until the joint is no longer where the drawing says it is.
ISO 273 exists precisely to stop this. For a general-purpose joint with an M8 bolt, the normal series gives you a 9.0 mm hole. That is the number that belongs on the drawing, not 8.
Myth 2: there is one clearance size
The second-most-common assumption is that "clearance hole" means one fixed number per bolt. ISO 273 deliberately refuses to give you that, because clearance is a design decision, not a bolt property.
The three series answer three different questions:
- Close fit (fine series, H12) — the joint must locate precisely and you are willing to accept almost no play. M8: 8.4 mm.
- Normal fit (medium series, H13) — general-purpose assembly. This is the default for the overwhelming majority of industrial joints. M8: 9.0 mm.
- Loose fit (coarse series, H14) — you expect misalignment, field assembly, thermal movement, or you need to slide the part into position before tightening. M8: 10.0 mm.
Two suppliers can both write "clearance hole for M8" on a quote and ship parts that are 1.6 mm different in hole diameter. That difference is invisible on the quote and obvious on the assembly line.
Myth 3: a bigger hole is always safer
If a 9.0 mm hole assembles easily, a 10 mm hole must assemble even more easily — so why not just drill 10 and be safe?
Because the hole is not free space. The bolt head and nut clamp the plate through the material that surrounds the hole, and every millimetre you add to the diameter removes bearing area. Push the hole far enough and the joint stops behaving like a clamped joint: the plates slip under load, the bolt is loaded in shear instead of tension, and the hole elongates over time.
A loose fit is a legitimate engineering choice — slotted holes, alignment during installation, thermal expansion, hand assembly in the field. It is not a default and it is not "safer". On a loaded structural joint, a normal fit with the correct tolerance is safer than a loose fit with none.
Myth 4: ⌀9 on the drawing is a complete specification
A drawing that says ⌀9 tells a machinist the nominal diameter and nothing else. The hole still has to be manufactured, and every manufacturing process has its own spread.
ISO 273 pairs each series with a tolerance field for exactly this reason. At H13, a nominal 9 mm hole is allowed to run from 9.000 mm up to 9.220 mm — the H field is positive-only, so the hole can never come out undersized. At H14 on the same nominal 10 mm hole the allowance is +0.36 mm, up to 10.36 mm. A CNC shop holding ±0.05 mm and a laser cutter holding ±0.2 mm will both read "⌀9" and deliver different parts.
So the complete callout is not ⌀9. It is ⌀9 H13. One extra field, and the shop no longer has to guess what you meant.
Myth 5: clearance holes are a workshop detail, not a buyer-facing spec
In B2B export this is where clearance holes stop being a machining topic and start being a commercial one. Before a purchase order is issued, the buyer's engineer checks the interface: hole diameter, hole pitch, edge distance, plate thickness, and the bolt the part is designed around. If any of those is missing from the drawing or the spec image, the questions start — and every question is a delay, and every unanswered question is a reason to prefer a supplier who answered it before being asked.
The fix is not a longer email. It is putting the measured hole callout on the image the buyer is already looking at: the actual diameter with its tolerance class, the pitch, and the bolt size it is designed for. That is a different job from restyling a photo. A generative image model can repaint a background and invent a plausible-looking dimension; it cannot measure a hole or state a tolerance, because it has no geometry to measure. A measured annotation — diameter pinned to the real feature, tolerance carried as text — is the version an engineer can quote from.
That is the whole value of getting this right: the drawing stops being a source of questions and starts being the answer. If you want the same discipline applied to the rest of the part, an industrial spec diagram is where hole diameter, pitch and edge distance live together, and the same tolerance logic that governs clearance holes also governs H7/h6 fit tolerance on the mating shaft.
What to put on the spec diagram for a clearance hole
Print these six fields and the "what size hole?" thread disappears before it starts:
- Hole diameter with its fit series — ⌀9 H13, not ⌀9. Without the field the callout is ambiguous.
- Bolt size and property class the hole is designed for — M8, class 8.8. A 12.9 bolt and an 8.8 bolt are not interchangeable at the same torque, and the marking on the head is the only place that class is written down — the same logic that makes A2-70 vs A4-80 stainless bolt marking worth reading off the head rather than assuming.
- Hole pitch / centre distance, with its own tolerance — the bolt must pass through two or more holes at once, so pitch error consumes clearance just like diameter error does.
- Edge distance — how much material sits between the hole and the plate edge. This is a design decision, not part of ISO 273, so state the value explicitly.
- Plate thickness and material — bearing area depends on thickness, and two plates of different thickness behave differently at the same hole size.
- Clearance hole or tapped hole — the two are routinely confused, and a tapped hole specified where a clearance hole was meant is scrap, not rework.
Which fit to choose
| Your situation | Series to specify | M8 hole |
|---|---|---|
| Precision assembly, alignment matters more than assembly speed | Close fit, H12 | 8.4 mm |
| General-purpose industrial joint (the default) | Normal fit, H13 | 9.0 mm |
| Field assembly, misalignment expected, or slotted/adjustable joint | Loose fit, H14 | 10.0 mm |
If you have no strong reason to pick otherwise, pick normal. It is the fit the standard's own numbers are built around, and it is the one the buyer's engineer will assume when the drawing does not say.
FAQ
What size clearance hole for an M8 bolt?
A normal (medium series) clearance hole for an M8 bolt is 9.0 mm per ISO 273:1979. A close fit is 8.4 mm and a loose fit is 10.0 mm. The hole is not 8 mm — 8 mm is the bolt's nominal major diameter, and drilling to it creates an interference fit that will not assemble cleanly.
What is the difference between close fit, normal fit and loose fit?
They are the three series in ISO 273, ordered by how much clearance they allow. Close fit (fine series, H12) gives minimum clearance for precise location, normal fit (medium series, H13) is the general-purpose default, and loose fit (coarse series, H14) allows adjustment and misalignment. For M8 the three diameters are 8.4 mm, 9.0 mm and 10.0 mm respectively.
What does H13 mean on a clearance hole?
H13 is the tolerance field ISO 273 pairs with the medium (normal) series. H means the lower limit is the nominal diameter, so the hole can never be undersized, and 13 is the ISO 286 tolerance grade, which sets the upper limit. A 9 mm hole at H13 may measure anywhere from 9.000 mm to 9.220 mm.
Is a clearance hole the same as a tapped hole?
No. A clearance hole is drilled larger than the bolt so the bolt passes through freely; a tapped hole is drilled to the tapping size and then threaded so the bolt grips it. They have different diameters, different tolerances and different callouts, and specifying one where the other was intended usually means scrapping the part.
How do I show a clearance hole on a product image or spec sheet?
Put the measured callout on the image where the buyer already looks: the diameter with its fit series, the pitch with its tolerance, and the bolt size the hole is designed for. The point is that the number must be the real measured diameter rather than an approximate one — a callout an engineer can quote from. A photo editor can move pixels but cannot measure a hole; the value comes from pinning the annotation to the actual feature and carrying the tolerance as text alongside it.
Sources & References
- ISO 273:1979 — Fasteners: Clearance holes for bolts and screws — the standard itself; abstract confirms fine, medium and coarse series for thread diameters from 1 mm up to 150 mm (confirmed current in 2024).
- IS 1821:1987 — Dimensions for clearance holes for bolts and screws (Bureau of Indian Standards) — national adoption identical to ISO 273:1979, including the fine/medium/coarse table and the H12/H13/H14 tolerance fields.
- Engineers Edge — Metric Clearance Holes and Tolerances per ASME B18.2.8-1999 (R2010) — states that metric clearance hole sizes are in agreement with ISO 273, which covers M1 through M150.
- Albany County Fasteners — Clearance Hole Chart (close, normal and loose fit, inch and metric) — reference chart used for the inch-series table above.
- Fuller Fasteners — ISO Tolerances for shafts and holes (ISO 286) — H13 internal tolerance values, including +0.22 mm for the 6–10 mm range and +0.27 mm for 10–18 mm.