Clearance Hole Calculator for Screws and Bolts

Clearance Hole Calculator

A clearance hole is a hole drilled slightly larger than a screw or bolt so the fastener can pass through freely, without its threads touching the sides. A clearance hole calculator looks up the correct hole diameter for a given screw or bolt size, using standard engineering tables instead of guesswork.

What is a clearance hole?

Fastener holes come in three basic types.

• A tapped hole has threads cut into it. The screw grips those threads, which is what actually holds two parts together.
• A clearance hole has no threads. It is cut wider than the fastener so the screw or bolt slides through without resistance, then threads into a tapped hole or a nut on the other side.
• An interference-fit hole is smaller than the fastener. The part must be pressed or forced onto it.

A clearance hole that is too small can force threads to cut into it or stop the fastener from passing through at all. A hole that is too large lets the fastener shift out of position and shrinks the flat area under the bolt head or washer that carries the clamping load.

Clearance holes also give room for small misalignments between parts, let assemblies move slightly as materials expand or contract with temperature, and reduce the risk of splitting wood or cracking plastic near the hole.

Clearance hole formula

Clearance hole sizes are not the result of one simple formula. Standards bodies publish a table of allowances, and the allowance grows as the screw gets larger. This calculator uses the normal fit values from ISO 273 (metric fasteners) and ASME B18.2.8 (inch fasteners), the tables most commonly used in general manufacturing, woodworking, and fabrication.

$D_{clearance} = D_{screw} + \text{allowance (from the standard table)}$

For metric screws, the allowance ranges from 0.4 mm on an M2 screw up to 2.0 mm on an M20 or M24 screw — it is not a single fixed value or percentage. Inch-sized screws follow their own table in ASME B18.2.8, with allowances that also increase in steps as the screw gets bigger.

Worked example

An M6 screw has a nominal diameter of 6.0 mm. Its normal-fit clearance hole is 6.6 mm — an allowance of 0.6 mm. A 6.6 mm or 6.5 mm drill bit is the standard choice; a plain 6 mm bit would be too tight.

Metric clearance hole sizes

American numbered screw clearance hole sizes

ASME B18.2.8 lists normal-fit clearance holes for numbered machine screws from #0 to #10, skipping #7 and #9 (sizes that are uncommon for machine screws). It does not define a normal-fit row for #12, so that size is not offered.

Worked example

A #8 screw has a diameter of 0.164 inch. Its normal-fit clearance hole is 0.196 inch, matched by a standard number-9 drill bit. Some drill charts list a smaller figure, 0.177 inch, for a #8 screw. That number comes from a different, tighter "free fit" system based on numbered drill bits, not from the ASME B18.2.8 normal-fit table this calculator uses. Mixing the two systems can produce a hole that binds on the screw.

American fractional screw clearance hole sizes

ASME B18.2.8 does not define a normal-fit row for 9/16", so that size is not offered.

Worked example

A 1/4-inch bolt has a diameter of 0.250 inch. Its normal-fit clearance hole is 0.281 inch, matched by a standard 9/32-inch drill bit. That is an allowance of about 1/32 inch, not 1/16 inch as some rules of thumb suggest.

The calculator converts every inch result to millimeters for display, since 1 inch equals 25.4 mm exactly.

Close fit, normal fit, and loose fit

Standards define more than one class of clearance hole. A close fit uses the smallest allowance and keeps parts aligned as precisely as possible, at the cost of slower assembly. A loose fit uses a larger allowance and gives more room to correct misalignment or absorb thermal expansion, at the cost of a less snug joint. This calculator produces only the normal fit, the middle option used for most general-purpose assembly work in woodworking, metalworking, and machine building.

How to use the clearance hole calculator

1. Choose a fastener type from the dropdown: metric (M2–M24), American numbered (#0–#10), or American fractional (1/4"–1").
2. Read the screw diameter and the recommended clearance hole size, both shown in millimeters.
3. Check the diagram, which draws the screw and the clearance hole to scale so the gap between them is easy to see.
4. Copy the result to paste into a drawing, cut list, or set of notes.

Frequently asked questions

What is the difference between a clearance hole and a tapped hole?

A clearance hole has no threads and lets a fastener pass through freely. A tapped hole has threads cut into it that grip the fastener and hold the joint together. In a typical bolted joint, the clearance hole is in the part being attached, and the tapped hole (or a nut) is in the part it attaches to.

How much bigger should a clearance hole be than the screw?

It depends on the screw size. For metric screws the allowance runs from 0.4 mm on the smallest sizes up to 2.0 mm on the largest, using the normal-fit table. For inch-sized screws, ASME B18.2.8 sets its own allowance for each size, which also grows in steps rather than by a fixed fraction. There is no single percentage that applies to every fastener.

What drill bit should I use for a clearance hole?

Use the exact size if a matching bit is available. If not, choose the next size up rather than down — a slightly larger hole rarely causes problems, while an undersized hole can stop the fastener from passing through or force it to cut new threads.

Why do some charts list a different clearance size for the same screw?

Some published charts use a "close fit" or "free fit" system paired with numbered or letter drill bits, which is tighter than the normal-fit table used here. Mixing values from the two systems can produce a hole that is too tight for the fastener, so it is best to use one system's figures throughout a project.

Does a bigger clearance hole make the joint weaker?

Not within the normal-fit range. Joint strength mainly comes from how tightly the fastener is torqued and how much flat area the bolt head or washer presses against. A hole that is far larger than the normal-fit allowance can reduce that flat area and let the joint shift under load, but a normal-fit hole does not weaken the joint.

References

• ISO 273:1979, Fasteners — Clearance holes for bolts and screws, International Organization for Standardization.
• ASME B18.2.8, Clearance Holes for Bolts, Screws, and Studs, American Society of Mechanical Engineers.