The Hole Size and Clearance Calculator helps designers and fabrication buyers select a reference clearance hole for a metric bolt or check the gap between a fastener and an existing hole. Choose a metric fastener and fit class, enter a target diametral clearance, or compare your own fastener and hole diameters. Results include hole diameter, diametral clearance, and radial clearance in millimeters or inches. Use them for preliminary sheet metal assembly design, then confirm finished sizes, positional requirements, and the applicable standard on your drawing.
Nominal thread diameter. M8 means 8 mm.
Use the largest passing diameter, including threads or finish.
Total diameter difference, not clearance on each side.
Use the finished, usable diameter. Smaller holes report interference.
M8 / Normal fit / ASME-based metric reference
H = d + CDesign mode adds your target diametral clearance C to the fastener diameter d.
C = H − dThe total diameter difference. A negative value means geometric interference.
c = (H − d) / 2The centered gap on each side; not a hole-pattern position tolerance.
Lookup: 24 metric sizes, M1.6–M100, using nominal hole diameters transcribed from the AmesWeb ASME B18.2.8 reference table. Confirm the applicable standard and drawing before production. IN converts these metric dimensions; it does not select an inch-fastener standard. Custom modes accept inch-fastener dimensions. No head, countersink, tap-drill, or self-clinching geometry is calculated.
Metric clearance hole lookup returns a reference diameter from the embedded table. Design a hole from clearance adds your chosen diametral allowance to a custom fastener diameter. Check an existing hole subtracts a custom fastener diameter from a finished hole diameter and identifies clearance, zero clearance, or interference.
Use lookup mode for the listed metric fasteners. Use the custom modes for dimensions outside that list, including inch fasteners whose actual diameter and required allowance you already know. Custom mode does not create a standards-based drill recommendation.
The calculator starts in millimeters. Select IN to convert dimensions using exactly 25.4 millimeters per inch. Existing custom input values convert with the units; the metric fastener designation remains unchanged.
Millimeter results display three decimal places and inch results display five. Display precision is not a machining tolerance. Keep the authoritative drawing dimensions and units consistent rather than treating rounded conversions as new requirements.
In lookup mode, choose the nominal metric fastener diameter, then select Close, Normal, or Loose fit. The comparison strip displays the three reference hole diameters for that fastener.
Close fit provides the smallest nominal allowance in this table. Normal fit is a general-purpose starting point. Loose fit provides more allowance, but washer support, edge distance, joint behavior, and alignment still need review. None of these selections defines a precision locating fit or automatically sets a positional tolerance.
In either custom mode, enter the largest diameter that must pass through the hole. For threaded fasteners, consider the thread’s maximum outside diameter and finish rather than only a smaller unthreaded shank. Values must be finite and greater than zero.
If you enter a nominal diameter, the reported gap is nominal too. To check a limiting condition, enter the maximum passing fastener diameter with the minimum usable finished hole diameter.
In design mode, Target diametral clearance means the total diameter difference. A target of 1.000 mm adds 1.000 mm to the fastener diameter, giving a centered radial gap of 0.500 mm on each side. The value can be zero, but zero clearance does not establish a free-passing assembly.
In check mode, Hole diameter means the finished usable bore. It must be positive. A hole smaller than the fastener is a valid geometric comparison and reports interference; it is not silently enlarged to a recommended size. Blank, negative, or invalid inputs suppress the results and show a message.
Results update as you edit the active inputs. Copy results copies the mode, dimensions, clearances, geometric condition, and reference context. Your browser or iframe permissions may block clipboard access; the calculator then tells you to copy the visible results manually.
Reset example restores millimeters, lookup mode, M8, and Normal fit. Custom defaults return to an 8 mm fastener, a 1 mm target diametral clearance, and a 9 mm existing hole.
In lookup mode, this is the nominal reference hole diameter from the selected metric table. For M8 Normal, it is 9.000 mm. It is not a guarantee of a drilled hole’s actual size, and the tool does not assign the standard’s minimum and maximum hole limits.
In design mode, this is your fastener diameter plus your target diametral clearance. An 8.000 mm fastener and 1.000 mm target give a 9.000 mm calculated hole. It is a geometric design value rather than a selected drill-bit size.
In check mode, this repeats your hole input in the active unit system. The calculator keeps it unchanged so you can evaluate clearance or interference against your fastener.
Lookup mode uses the fastener’s nominal metric diameter. Custom modes use your input. A nominal M8 diameter of 8.000 mm does not imply that every manufactured M8 fastener measures exactly 8.000 mm.
This is the signed diameter difference, hole minus fastener. Positive means geometric clearance, zero means equal input diameters, and negative means interference. For an 8.000 mm fastener in a 9.000 mm hole, diametral clearance is 1.000 mm.
This is half the signed diametral difference. The same 8.000 mm fastener in a 9.000 mm hole has 0.500 mm radial clearance when centered. If the parts are off-center, the gap is not equal around the circumference. A negative result is half the diametral interference, not an open gap.
The condition identifies Clearance, Zero clearance, or Interference. It does not evaluate load capacity, fastening torque, fatigue, friction, bearing stress, or the ability to insert a fastener through a complete hole pattern.
H = d + C Design a hole from target diametral clearance
C = H − d Diametral clearance
c = (H − d) / 2 Radial clearance for concentric geometry
H = hole diameter
d = fastener diameter
C = diametral clearance
c = radial clearance
1 in = 25.4 mm
These equations are diameter arithmetic. They do not depend on fastener head diameter. Head seating, counterbores, countersinks, washers, and tool access need separate dimensions.
For a custom 6.350 mm fastener in a 7.000 mm hole, the tool returns 0.650 mm diametral clearance and 0.325 mm radial clearance. A custom 8.000 mm fastener in a 7.800 mm hole returns −0.200 mm diametral clearance, −0.100 mm radial clearance, and Interference.
ASME B18.2.8, Clearance Holes for Bolts, Screws, and Studs covers recommended clearance hole sizes for metric and inch fasteners in close, normal, and loose categories. The public ASME page verifies the standard’s identity and scope; it does not expose the complete numerical tables.
This calculator’s 24 metric rows are a secondary transcription of the nominal drill/hole diameters in the AmesWeb metric clearance hole chart, which identifies ASME B18.2.8-1999 as its source. They were not independently checked against a licensed copy of the full standard. Consult the applicable standard before using these values as contractual production requirements.
Selected entries from the implemented dataset are shown below. All diameters are in millimeters.
| Fastener | Close | Normal | Loose |
|---|---|---|---|
| M1.6 | 1.7 | 1.8 | 2.0 |
| M3 | 3.2 | 3.4 | 3.6 |
| M6 | 6.4 | 6.6 | 7.0 |
| M8 | 8.4 | 9.0 | 10.0 |
| M10 | 10.5 | 11.0 | 12.0 |
| M12 | 13.0 | 13.5 | 14.5 |
| M16 | 17.0 | 17.5 | 18.5 |
| M24 | 25.0 | 26.0 | 28.0 |
| M100 | 104.0 | 107.0 | 112.0 |
The table contains selected standard sizes rather than every diameter between M1.6 and M100. It does not combine ASME values with ISO 273 values. If your drawing calls for another standard, use that standard’s table; matching fit names do not establish identical numerical requirements.
For a simple limiting size check, use the minimum finished hole diameter and maximum passing fastener diameter. Their difference is the minimum diametral clearance for the dimensions entered. The calculator does not generate those limits from a tolerance class or perform a full tolerance stack-up.
Even positive clearance can be insufficient when mating hole centers do not align. Radial clearance is not the permitted center-to-center mismatch for every assembly. A floating fastener passing through two plates, a fixed stud, and a multi-hole rigid pattern have different constraints. Define the datums and position requirements for the actual assembly.
For self-clinching nuts and studs, use the exact hardware family’s installation specifications. ShincoFab’s self-clinching fastener hole size chart explains why those holes cannot be selected using a generic bolt clearance table.
These answers explain common differences between reference tables, nominal CAD dimensions, and finished sheet metal parts. Use them to decide which inputs to check and when a drawing-specific review is needed.
This calculator’s ASME-based metric reference gives 8.4 mm for Close, 9.0 mm for Normal, and 10.0 mm for Loose. Normal gives 1.0 mm nominal diametral clearance relative to an 8.0 mm fastener. The final choice still depends on the applicable standard, size limits, position, finish, and joint requirements.
No. Diametral clearance is the total diameter difference. Radial clearance is half that difference for centered circular geometry. A 1.0 mm diametral difference gives a 0.5 mm centered gap on each side.
A normal through-hole must pass the relevant fastener body or threads; its diameter is not determined by averaging the body and head diameters. Define the head seat, counterbore, countersink, or washer arrangement separately. This tool does not calculate those features.
Enter known inch dimensions in either custom mode after selecting IN. The metric lookup remains a metric dataset, and inch display is its dimensional conversion. The tool does not supply ASME inch-fastener clearance rows or fractional, letter, or numbered drill-bit recommendations.
A negative diametral clearance means the fastener input is larger than the hole input. For an 8.0 mm fastener and 7.8 mm hole, the diameter interference is 0.2 mm. The tool does not predict press-fit force, material deformation, or joint strength.
No. Tap-drill size depends on the thread and tapping process. Self-clinching hardware uses a product-specific mounting-hole size and tolerance. Select the correct reference for the actual feature rather than substituting a clearance-hole value.
Position mismatch, burrs, finish, taper, distortion, or a larger actual passing diameter can prevent insertion. Check the usable finished bore, worst-case fastener size, and mating-part datums. For rigid multi-hole patterns, review the complete assembly rather than one hole in isolation.
No. It reports nominal lookup dimensions or custom geometry. It does not assign minimum and maximum hole limits, an ISO hole tolerance grade, or a production capability. Put the required size and position limits on your drawing and agree on the inspection method.
A drawing-specific design for manufacturability (DFM) review can check whether the hole size, pattern position, finish, hardware, and fabrication sequence support your assembly. Share the part drawing, material and thickness, fastener designation, mating-part details, finish, quantity, and critical size or position requirements.
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