Clearance Hole Calculator finds bolt or screw clearance drill size using with ISO or ASME fit class, radial margin, max hole size, and tap drill reference.
The Clearance Hole Calculator finds the correct drill size for a bolt or screw to pass through freely, using either Metric ISO 273 or US Customary ASME clearance values. Select your measurement system, fastener size, thread pitch series, and fit class — the tool returns a recommended clearance drill diameter along with all derived clearance values, machining limits, a unit conversion, and a secondary tap drill reference.
Metric outputs follow the ISO 273 close, medium, and coarse clearance series. US Customary outputs follow ASME B18.2.8 close, normal, and loose categories. All values are taken from tabulated standards — not calculated as a freehand oversize percentage.
What the Clearance Hole Calculator Does
The bolt clearance hole calculator takes four inputs — measurement system, fastener thread size, thread pitch series (coarse or fine), and fit class — then looks up the corresponding clearance drill diameter from the applicable standard. From that single diameter it computes diametral clearance, radial margin, clearance area, machining tolerance limits, and a unit conversion.
The tool is scoped to through-hole sizing only. It does not perform thread engagement analysis, joint clamping load calculation, or tolerance-stack engineering.
Clearance Hole Formula
Once the tabulated clearance diameter \(D_h\) is selected for the chosen fit class, the remaining output values are derived as follows.
| Symbol | Definition |
|---|---|
| \(D_h\) | Selected clearance hole diameter — tabulated from ISO 273 or ASME B18.2.8 |
| \(D_f\) | Fastener nominal major diameter |
| \(C_d\) | Diametral clearance — total gap across the hole diameter |
| \(C_r\) | Radial margin — gap on each side of the fastener shank |
| \(A_c\) | Clearance area — annular cross-sectional void around the inserted fastener |
| \(T\) | Tolerance allowance for the selected fit class (ISO H12, H13, or H14) |
| \(D_{\max}\) | Maximum allowable hole diameter before the fit class tolerance is exceeded |
Example: M6 Normal Clearance Hole
The default calculator state uses M6, standard coarse pitch, and Normal fit under ISO 273. The values below correspond directly to the output cards shown in the tool.
Diametral clearance: \(C_d = 6.60 - 6.00 = 0.60\,\text{mm}\).
Radial margin: \(C_r = 0.60 \div 2 = 0.30\,\text{mm}\).
Clearance area: \(A_c = \tfrac{\pi}{4}(6.60^2 - 6.00^2) \approx 5.94\,\text{mm}^2\).
The H13 tolerance allowance \(T = 0.22\,\text{mm}\) gives a maximum allowable hole of \(6.60 + 0.22 = 6.82\,\text{mm}\).
How Fit Class Changes the Hole Size
Fit class is the primary input that trades off alignment precision against assembly ease. Each class maps to a different tabulated drill diameter and a different ISO tolerance grade.
Smallest clearance of the three. Used when hole-to-hole alignment across stacked parts matters and the drilling operation can hold tight positional accuracy. Requires consistent tooling and controlled process conditions.
Standard general-purpose clearance for the majority of bolted assemblies. Balances ease of assembly with acceptable positional tolerance. Suitable for most structural and enclosure applications.
Largest clearance. Used for fast assembly, coated or plated fasteners, slotted holes, or where hole alignment cannot be precisely controlled. Provides additional margin for surface treatments.
What Each Result Means
Each output card in the screw clearance hole calculator addresses a specific engineering question. Card labels below match the tool output exactly.
The tabulated clearance drill diameter for the selected fastener, pitch series, and fit class. This is the dimension to specify on a drawing or program into the machine. It is the basis for all derived outputs.
Total diameter difference between the hole and the fastener shank: \(C_d = D_h - D_f\). Shows total slack across the full diameter. Relevant when checking whether a fastener will pass without binding at assembly.
Gap on each side of the fastener: \(C_r = C_d \div 2\). Useful for estimating how far a bolt can shift laterally inside the hole and for understanding joint float before clamping.
Annular cross-sectional void area: \(A_c = \tfrac{\pi}{4}(D_h^2 - D_f^2)\). Relevant for gasket sizing, sealant volume estimation, and understanding void space in sealed or potted assemblies.
The ISO tolerance grade (H12, H13, H14) or ASME designation for the selected fit class, plus the minimum and maximum allowable hole diameters derived from it. Use these values to set go/no-go gauge limits or inspection criteria.
Direct mathematical conversion of the clearance drill diameter, fastener major diameter, and diametral clearance into the opposing unit system (mm ↔ in). Useful when a drawing is in one unit system but tooling is purchased in another.
Secondary reference only. Shows the tap drill diameter for the same fastener size and pitch — for use only if the hole is intended to be threaded rather than cleared. Do not use this value as the clearance hole result.
Clearance Hole vs Tap Drill
These two hole types serve opposite purposes and are not interchangeable.
Drilled in the through-member of an assembly. Larger than the fastener major diameter by the fit class allowance. The fastener shank does not engage the material — clamping force comes from a nut or from a tapped hole in the receiving member.
Drilled smaller than the fastener major diameter to leave material for the tap to cut threads. Using a clearance drill size in place of a tap drill produces a hole too large to retain threads at any useful engagement length.
Assumptions and Limits
- Clearance diameters are taken from tabulated standard values — not calculated as an arbitrary oversize percentage of fastener diameter.
- Metric values follow ISO 273 close, medium, and coarse series. US Customary values follow ASME B18.2.8 close, normal, and loose categories.
- Machining Limits reference ISO tolerance grades H12, H13, and H14 from the ISO 286 fundamental deviation and tolerance grade system.
- Actual drilled size can vary from nominal due to material hardness, drill geometry, spindle runout, machine rigidity, cutting fluid, and workholding. The calculator does not model process variation.
- Surface coatings, plating, anodizing, and paint reduce effective clearance after application. Add coating thickness allowance before finalizing fit class where required.
- For inspected or critical fits, measure actual holes with a calibrated bore gauge, plug gauge, or CMM. Do not rely solely on drill nominal size.
- This tool is for clearance hole sizing only — not structural fastener design, thread engagement analysis, joint load calculation, or full tolerance-stack engineering.
References
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[1]
ISO 273 — Fasteners: Clearance Holes for Bolts and Screws Defines close, medium, and coarse clearance hole series for metric fasteners. Primary source for all metric tabulated values in this calculator. ↗ iso.org/standard/4192
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[2]
ASME B18.2.8 — Clearance Holes for Bolts, Screws, and Studs (Inch Series) Defines close, normal, and loose clearance categories for US Customary fastener sizes. Primary source for all US Customary tabulated values. ↗ asme.org — B18.2.8
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[3]
ISO 286-1 — Geometrical Product Specifications: Limits and Fits Defines the fundamental deviation and tolerance grade system referenced by the H12, H13, and H14 grades shown in the Machining Limits output. ↗ iso.org/standard/54530
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[4]
Machinery's Handbook (current edition) — Industrial Press Reference for drill and tap sizing tables and clearance hole data applied in deriving the tap drill reference values shown in the tool. ↗ industrialpress.com