Clearance Hole Calculator
Estimate a practical pass-through hole diameter from a fastener shank and head diameter, with instant metric or inch results.
Fastener dimensions
Live result
Calculation breakdown
| Quantity | Role in calculation | Value |
|---|---|---|
| Fastener diameter | First diameter | 5.00 mm |
| Fastener head diameter | Second diameter | 8.50 mm |
| Clearance hole | Arithmetic midpoint | 6.75 mm |
How to use the clearance hole calculator
What this calculator does
This calculator estimates a clearance-hole diameter by taking the arithmetic midpoint between the fastener shank diameter and the fastener head diameter. It helps with early layout, drill-size comparison, furniture assembly, prototype work, and general workshop planning. It does not replace a drawing, a manufacturer's specification, or a standards-based fit selection. Formal close, normal, and loose clearance classes are covered by standards such as ASME B18.2.8 for clearance holes.
When to use it
Use the tool when checking whether a screw or bolt can pass through a non-threaded layer, estimating a pilot layout before choosing a drill, comparing a measured fastener with available tooling, or documenting a preliminary workshop calculation. For production work, structural joints, critical alignment, or tightly toleranced assemblies, use the specified standard and an approved drill chart.
How to calculate
- The calculator opens with a complete demonstration: a 5 mm fastener and an 8.5 mm head. The result and a validated example workbook are available immediately.
- Replace Fastener diameter with the shank's outside diameter. Replace Fastener head diameter with the widest head measurement.
- Choose millimeters or inches. Changing the unit converts both current values, the result, the table, and the workbook.
- Read Estimated clearance hole, then review the midpoint, difference, formula line, and calculation breakdown.
- Select Download Excel to export the current typed inputs and outputs. Reset clears the demonstration data and disables export until both required values are valid again.
Input guide
Fastener diameter is required, must be a finite number greater than zero, and uses the selected length unit. Enter plain decimals with a period, such as 5 or 0.196; commas, fractions, unit text, negative values, and scientific notation are rejected. A larger shank diameter raises the estimate. A common mistake is entering thread pitch or nominal trade size instead of the measured outside diameter.
Fastener head diameter is also required and follows the same format and range rules. A realistic value is 8.5 mm for the demonstration fastener. Increasing the head diameter increases the midpoint estimate. Measure the widest relevant head dimension; do not enter head height.
Length unit controls every displayed and exported dimension. Millimeters are the startup setting. The inch conversion uses the exact relationship 1 inch = 25.4 millimeters, consistent with the NIST conversion-factor guidance. Avoid switching units and then retyping an unconverted value unless that is intentional.
Output guide and worked example
Estimated clearance hole is the primary arithmetic estimate in the selected unit. Diameter midpoint repeats that exact identity for clarity. Head-to-shank difference shows how much wider the head is than the shank; a negative value is allowed if the entered dimensions are unusual, but it is a cue to verify the measurements. The formula display substitutes the live numbers into CH = (DF + DH) ÷ 2. The table records each input and the final midpoint, and the summary pills show the unit, difference, and export readiness.
With the startup values, DF = 5.00 mm and DH = 8.50 mm. Their sum is 13.50 mm, and dividing by two gives CH = 6.75 mm. This is the exact first-open result in the page and workbook. It is an estimate, not a prescribed fit. When documenting converted measurements, keep only the precision justified by the source measurement, as explained in NIST's guidance on writing and converting SI units.
How the formula works
The model uses CH = (DF + DH) / 2, where CH is the estimated clearance-hole diameter, DF is the fastener shank diameter, and DH is the head diameter. Mathematically, this is the midpoint of the two entered diameters. It produces a value larger than the smaller input and smaller than the larger input whenever the dimensions differ. That midpoint property is also a useful validation check: if a displayed answer falls outside the two positive inputs, something is wrong.
Actual clearance-hole standards do not generally choose holes by averaging head and shank diameters. They use nominal fastener size, fit class, manufacturing allowance, and tolerance limits. The calculator therefore supports quick estimation and comparison rather than final design approval. For machining, clamp the work securely and follow appropriate machine-guarding practices; OSHA's machine safeguarding guidance includes drill-press precautions.
Practical interpretation
A hole that is too small may bind on the threads or prevent assembly. A hole that is too large may reduce positional control, washer support, or edge distance. Material behavior also matters: thin sheet, brittle plastic, wood, and metal can require different tooling and finishing. Account for drill runout, coating thickness, burr removal, thermal effects, and the tolerance of both the fastener and the hole. For a critical joint, compare the calculated estimate with the governing drawing and the exact close, normal, or loose-fit table.