Clearance Hole
Compute the correct clearance hole size for bolts and screws using fastener and head diameters. Free tool with metric and imperial units plus ASME fit class references.
About This Calculator
About the Clearance Hole Calculator
This calculator helps engineers, machinists, woodworkers, and DIY enthusiasts determine the correct clearance hole diameter for bolts and screws. A clearance hole allows a fastener to pass through a substrate without the threads biting into the material, ensuring proper assembly and clamping force. Using the simple formula CH = (df + dh) / 2, the calculator takes the fastener diameter and head diameter to compute the recommended hole size.
The calculator supports both metric (mm) and imperial (inches) units, making it suitable for international users working with either system. The sizing chart visualizes how clearance hole diameters scale across common fastener sizes, referencing standard values from ASME B18.2.8 and common metric fastener head proportions. The comparison chart shows the relationship between fastener diameter, head diameter, and the resulting clearance hole at a glance.
Regional Notes
US: Inch-series fasteners (#0 through 1.5 in) follow ASME B18.2.8 clearance hole recommendations. Common drill sizes are provided for close, normal, and loose fit classes.
UK: Metric fasteners (M1.6 through M100) per ISO 273 are standard, with clearance hole sizes specified in millimeters for each fit class.
India: IS 4219 specifies clearance hole dimensions for metric fasteners, aligned with ISO 273 standards. M-series bolts (M6, M8, M10, M12, M16) are most common in construction and manufacturing.
Frequently Asked Questions
What is a clearance hole?
A clearance hole is a hole through a substrate that allows a bolt or screw to pass through freely without the threads biting into the material. It is slightly larger than the fastener diameter to provide clearance for assembly.
How do you calculate a clearance hole?
The clearance hole diameter is calculated using the formula CH = (DF + DH) / 2, where DF is the fastener diameter and DH is the fastener head diameter. For example, an M5 bolt with 5 mm diameter and 8.5 mm head gives a clearance hole of (5 + 8.5) / 2 = 6.75 mm.
What is the bolt clearance hole for an M8?
For an M8 bolt with 8 mm diameter and 13 mm head diameter, the clearance hole is (8 + 13) / 2 = 10.5 mm. This falls under the normal fit class per ASME B18.2.8, which specifies a minimum of 8.4 mm and maximum of 8.55 mm for close fit.
What is the difference between close, normal, and loose fit clearance holes?
Per ASME B18.2.8, close fit provides minimal clearance for precise alignment, normal fit offers standard clearance for general applications, and loose fit allows maximum clearance for easy assembly where alignment is less critical. Each fit class specifies a range of acceptable hole diameters for a given fastener size.
What is the clearance hole for a 6-32 screw?
A 6-32 screw has a diameter of 0.1380 inches and a head diameter of approximately 0.270 inches. The clearance hole is (0.138 + 0.270) / 2 = 0.204 inches, which corresponds to a drill size of approximately #7 (0.201 in) for a normal fit.
Does ASME B18.2.8 apply internationally?
ASME B18.2.8 is an American standard, but its clearance hole recommendations are widely used internationally for inch-system fasteners. For metric fasteners, ISO 273 specifies similar clearance hole recommendations. Both standards define close, normal, and loose fit categories for common fastener sizes.
Why does a screw need a clearance hole?
A clearance hole prevents the screw threads from biting into the material being fastened, allowing the screw to pass freely and ensuring proper clamping force when tightened. Without a clearance hole, the screw may bind, strip, or fail to seat properly, compromising the joint integrity.
What is the difference between a bolt and a screw?
Bolts are typically used with nuts to secure items together and have a flat tip, while screws create their own mating threads and are fully inserted into the material. Bolts are common in construction and structural applications, whereas screws are used in furniture, DIY builds, and home repairs.