Clearance Hole Calculator

Clearance Hole Calculator for Screws and Bolts

The Clearance Hole Calculator finds the hole diameter that lets a screw or bolt pass through material without the threads biting into the substrate. Use it for woodworking, metalwork, and DIY assembly.

Clearance hole formula

For custom fasteners, clearance hole diameter is the average of the fastener body and head diameters:

$$CH = \frac{D + D_H}{2}$$

where $D$ is fastener diameter and $D_H$ is head diameter.

ASME standard sizes

The ASME B18.2.8 standard lists recommended clearance holes for metric and inch fasteners in close, normal, and loose fit classes. Normal fit is the most common choice for general construction and furniture.

Frequently Asked Questions

What is a clearance hole?

A clearance hole is large enough for the fastener shank and threads to pass through freely. The fastener grips only the material behind the hole, not the substrate with the clearance hole itself.

What is the clearance hole for an M8 bolt?

For an M8 bolt with a 13 mm head, the custom formula gives about 10.5 mm. ASME normal fit recommends a 9.0 mm clearance hole and a 9.0 mm drill for standard M8 fasteners.

What is the difference between close, normal, and loose fit?

Close fit gives the tightest hole for precise alignment. Normal fit is the everyday choice. Loose fit allows more tolerance for rough drilling or misalignment.

Is a clearance hole the same as a pilot hole?

No. A pilot hole guides a screw into material and is smaller than the screw. A clearance hole is larger than the screw so threads do not engage that layer.

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