Clearance Hole Calculator
Enter your bolt size and choose a fit class to get the recommended clearance hole diameter instantly. Supports metric bolts M1.6 through M36 (ISO 273) and imperial screws #0 through 1-1/2 inch (ASME B18.2.8). Select close fit for precision assemblies, normal fit for general use, or loose fit for easy assembly. The calculator shows the exact hole diameter, the clearance added over the bolt, and a step-by-step breakdown of the calculation.
What is a clearance hole?
A clearance hole is a hole drilled slightly larger than the bolt or screw that passes through it, so the fastener slides through freely without engaging the material. Unlike a tapped hole (which has threads cut into it), a clearance hole simply provides a passage for the bolt shank. The bolt is held in place by a nut on the other side, or by threading into a second part. Clearance holes are used in most bolted joints: one part gets a clearance hole, the other gets a tapped hole or a nut, and tightening the fastener clamps the two parts together.
Close fit, normal fit, and loose fit explained
Engineering standards define three fit classes for clearance holes, each balancing assembly ease against positional accuracy. Close fit adds the smallest clearance over the bolt diameter - typically 0.1 to 0.5 mm for metric bolts - so the bolt can be located precisely. It is used in jigs, fixtures, and precision mechanisms where bolt position needs to be controlled tightly. Normal fit is the standard for general mechanical and structural assemblies: it provides enough clearance for easy assembly in most conditions while keeping the bolt reasonably centered. Loose fit adds the most clearance, making assembly quick even when parts are misaligned, coated with paint, galvanized, or subject to thermal expansion. Most structural steelwork and general construction applications use a normal or loose fit.
Metric standard: ISO 273
For metric fasteners, the international standard ISO 273 defines three clearance series (fine, medium, and coarse) corresponding to close, normal, and loose fit. The standard covers bolt diameters from M1.6 to M150 and specifies both the nominal hole diameter and the allowed tolerance range. The clearance added over the bolt ranges from a fraction of a millimetre for small bolts up to several millimetres for large diameter bolts, reflecting the looser manufacturing tolerances in larger parts. Always check the relevant ISO 273 table for the exact bolt size and fit class before finalising a hole specification.
Imperial standard: ASME B18.2.8
For inch-series fasteners used primarily in North America, ASME B18.2.8 provides the equivalent clearance hole tables covering machine screw sizes from #0 through to 1-1/2 inch diameter bolts. The standard lists nominal drill sizes and minimum/maximum hole diameters for close, normal, and loose fits. Because imperial bolt sizes include both numbered machine screws (#0 through #14) and fractional inch bolts (1/4 inch, 5/16 inch, and so on), it is important to select the correct designation. The hole diameters in this calculator follow ASME B18.2.8 for all inch-series fasteners.
Metric clearance hole sizes - ISO 273 (mm)
| Bolt size | Nominal dia. (mm) | Close fit (mm) | Normal fit (mm) | Loose fit (mm) |
|---|---|---|---|---|
| M1.6 | 1.6 | 1.7 | 1.8 | 2 |
| M2 | 2 | 2.2 | 2.4 | 2.6 |
| M2.5 | 2.5 | 2.7 | 2.9 | 3.1 |
| M3 | 3 | 3.2 | 3.4 | 3.6 |
| M4 | 4 | 4.3 | 4.5 | 4.8 |
| M5 | 5 | 5.3 | 5.5 | 5.8 |
| M6 | 6 | 6.4 | 6.6 | 7 |
| M8 | 8 | 8.4 | 9 | 10 |
| M10 | 10 | 10.5 | 11 | 12 |
| M12 | 12 | 13 | 13.5 | 14.5 |
| M14 | 14 | 15 | 15.5 | 16.5 |
| M16 | 16 | 17 | 17.5 | 18 |
| M20 | 20 | 21 | 22 | 24 |
| M24 | 24 | 25 | 26 | 28 |
| M30 | 30 | 31 | 33 | 35 |
| M36 | 36 | 37 | 39 | 42 |
Standard clearance hole diameters for metric bolts per ISO 273. All dimensions in millimetres.
Frequently asked questions
What is the rule of thumb for clearance hole size?
A common rule of thumb is to add roughly 0.5 mm to 1.0 mm to the bolt nominal diameter for metric bolts, or about 0.015 to 0.030 inches for imperial fasteners. However, for any engineering application you should use the values from the relevant standard (ISO 273 for metric, ASME B18.2.8 for imperial) rather than a rough estimate, since the standards account for manufacturing tolerances and fit requirements.
What is the difference between a clearance hole and a tapped hole?
A clearance hole is smooth and slightly larger than the bolt, allowing the fastener to pass through freely without engaging the material. A tapped hole has threads cut into it that mate with the bolt threads. In a typical bolted joint, one part has a clearance hole and the joint is held together by either a nut on the far side or by threading the bolt into a tapped hole in the second part.
When should I use a close fit versus a normal fit?
Use close fit when accurate bolt positioning is important - for example in jigs, fixtures, precision mechanisms, or when holes need to align closely with holes in a mating part. Normal fit is the standard choice for most mechanical and structural assemblies where precise bolt location is not critical. Loose fit is best when parts are coated (painted, galvanised, plated), when thermal expansion is a concern, or when the assembly needs to be put together quickly in the field.
Does hole diameter include a tolerance range?
Yes. The values in ISO 273 and ASME B18.2.8 include both a nominal hole size and a tolerance range (minimum and maximum). This calculator shows the nominal recommended diameter for each fit class. In practice, the actual hole produced by a drill bit will fall within a tolerance band around that nominal size, which is why the standards specify minimum and maximum diameters as well.
Can I use this calculator for sheet metal screws or wood screws?
No. Sheet metal screws and wood screws are designed to cut their own threads or grip the material - they use a pilot hole, not a clearance hole. This calculator is specifically for through-holes that allow a bolt or machine screw to pass through freely. For sheet metal screws, refer to the manufacturer pilot hole chart; for wood screws, use a pilot hole sized to the core diameter of the screw.
What drill bit size should I use for a clearance hole?
Choose the drill bit whose diameter is closest to (but not smaller than) the recommended clearance hole diameter. For metric bolts, standard metric drill bits (in 0.1 mm increments) usually get you there exactly. For imperial fasteners, the standards specify both a numbered or lettered drill size and the resulting hole diameter, so you can pick the nearest standard drill bit. Always drill a test hole in scrap material first for critical applications.