Clearance Hole Calculator

Find the right clearance hole for a bolt or screw

A clearance hole is a plain hole that lets a bolt or screw pass through one part without its threads gripping that part. The fastener threads engage in a nut or a tapped second part. The hole is slightly larger than the fastener's outside diameter, leaving room for assembly and alignment. This clearance hole calculator gives reference diameters for common inch and metric fastener sizes and lets you compare close, normal, and loose fits.

Choose the fastener system, size, and fit. The result shows a suggested hole diameter, the approximate clearance around the bolt, and a drill size rounded up to a practical increment. Metric selections are shown in millimetres; inch selections are shown in decimal inches and a nearby fractional drill size. Use the result to plan a hole in a clearance plate, bracket, or fixture, then confirm it against the drawing and the hardware manufacturer's instructions.

Hole size is one detail in a bolted joint. A larger hole can make alignment easier, but it also changes how parts locate and may affect strength, slip, edge distance, washers, and inspection. This calculator is a shop planning aid. It does not design a structural connection, select a fastener grade, or replace an engineering specification.

How to use the clearance hole calculator

Start by choosing Metric or Inch. Select the nominal bolt or screw size from the list, then select close, normal, or loose fit. Normal is a useful starting point for many general assembly tasks. Close leaves less room and may help parts locate when the holes are accurately made. Loose provides more room for alignment during assembly. The selected result updates when you change any of these options.

Read the recommended hole diameter and the drill suggestion separately. The recommended diameter is the target hole size from the selected fit table. The drill suggestion rounds that value up to a common size step; it is a practical starting point, not a promise that every drill bit will cut an exact finished hole. Material, drill runout, tool wear, workholding, and the operation used can make the actual hole larger or less round.

Check the displayed clearance per side to understand how much room separates the nominal fastener diameter from the selected hole. That value is half of the difference in diameters. For example, a hole that is 0.5 mm wider than a bolt leaves about 0.25 mm of radial clearance on each side when centered. It does not describe positioning tolerance or the accuracy of the hole location.

Clearance hole formula

The calculator uses a tabulated recommended hole diameter for the selected nominal fastener and fit class. The table value is the target diameter. Clearance is the difference between the hole diameter and the nominal fastener diameter. Because that difference is shared around the bolt when centered, radial clearance per side is half the difference.

The drill suggestion rounds the target hole diameter upward to the next 0.5 mm step for metric sizes or 1/64 inch step for inch sizes. Real drill inventories use specific letter, number, fractional, and metric sizes. A nearby available drill may be smaller or larger than the suggested step, so compare the actual tool diameter and required tolerance before drilling. For precision work, use the drawing's limit dimensions and an appropriate finishing process.

• Hole diameter = table value for fastener size and selected fit
• Diametral clearance = hole diameter − nominal fastener diameter
• Clearance per side = diametral clearance ÷ 2
• Drill suggestion = next practical size increment at or above the target diameter

Close, normal, and loose fits explained

A close clearance fit uses a smaller hole than the normal or loose option. It can be useful when accurately made parts need to be brought together with less lateral play. A close hole also leaves less allowance for positional error, coatings, burrs, or slight variation in the fastener. Do not choose it just because a smaller hole sounds stronger; the joint design and manufacturing tolerances need to support it.

A normal clearance fit is a common general-purpose choice when the bolt should pass through a plate without tapping the plate threads. It provides a modest assembly allowance. If several holes must line up across multiple parts, normal clearance may still be too tight when the pattern or fabrication tolerances are large. Follow the drawing when it specifies a hole class or diameter.

A loose fit uses a larger hole and gives extra room for assembly adjustment. This can help when parts are difficult to align or when a bolt must pass through a thicker stack. More clearance can also allow more movement before the parts bear against the fastener. It is not automatically appropriate for a load-bearing connection, locating feature, or joint where slip is restricted.

Metric clearance hole sizes

Metric bolt sizes use an M designation followed by the nominal outside thread diameter in millimetres. An M6 bolt has a nominal diameter of 6 mm; its clearance hole is larger than 6 mm. The selected fit determines the recommended amount of extra room. The calculator includes common coarse metric sizes from M3 through M30, including popular M5, M6, M8, M10, M12, M16, and M20 fasteners.

As a quick reference, an M6 clearance hole is commonly around 6.6 mm for a normal fit, while an M8 normal fit is around 9 mm. An M10 normal fit is around 11 mm. These are table-based reference sizes; the close and loose options differ. Metric clearance tables may distinguish fine, medium, and coarse series or use a specified standard and tolerance class. This calculator presents a practical reference series, so use the applicable standard or engineering drawing for controlled work.

Do not confuse a clearance drill with a tapping drill. A tapping drill is smaller because it leaves material for the tap to cut internal threads. If the part itself must hold the screw threads, choose a tap drill based on the thread size, pitch, material, and required thread engagement. A clearance hole lets the fastener pass through; it does not create a threaded connection.

Inch clearance hole sizes

Inch fasteners may be identified by number sizes, such as #6 or #10, or by fractional diameters, such as 1/4 inch, 3/8 inch, and 1/2 inch. The number size does not state the diameter directly, so select the exact fastener designation rather than estimating from a ruler. The calculator includes common machine screw and fractional bolt sizes.

For a familiar reference, a 1/4-inch bolt commonly uses a 0.281-inch normal clearance hole, close to a 9/32-inch drill. A 3/8-inch bolt commonly uses a 0.406-inch normal hole, close to a 13/32-inch drill. A 1/2-inch bolt commonly uses a 0.531-inch normal hole, which is close to a 17/32-inch drill. The close and loose fit choices use different diameters. Check the table values and required drill carefully instead of assuming every fractional bit is the normal size.

Fastener size and fit conventions can differ between standards and applications. Structural steel holes, aircraft fasteners, precision machine assemblies, wood screws, and sheet metal screws may use their own requirements. The inch references here are for general clearance planning, not a substitute for a governing code or a specified hole tolerance.

Clearance hole versus tapped hole

A clearance hole is larger than the bolt's nominal diameter and lets the fastener slide through the first part. The bolt is retained by a nut or by threads in another part. A tapped hole has internal threads cut into the material, so the screw threads engage directly. Tapped holes require a smaller pilot drill called a tap drill. Using a clearance size where a tapped hole is needed leaves too little material for reliable threads.

A pilot hole is a general term for a starter hole. Its required size depends on what comes next: tapping, driving a wood screw, drilling to a final diameter, or inserting a fastener. A clearance hole should not be selected from a tap drill chart. Likewise, a clearance diameter is not automatically suitable for a self-tapping screw that forms its own threads.

Choosing a drill and making the hole

Use the suggested drill diameter as a starting point, then select a tool that matches the drawing tolerance and material. Standard drill sizes do not include every decimal diameter. In inch work, a fractional, number, or letter drill may be closer to the target. In metric work, choose the nearest available metric drill that meets the required minimum and maximum hole diameter. When the hole must be accurate, drilling undersize and reaming to the final size can improve roundness and control.

Clamp the work securely, mark the center accurately, and support thin sheet to reduce grabbing or distortion. Use a suitable cutting speed and lubricant for the material and drill. Remove burrs without enlarging the hole beyond its limit. Measure critical holes with suitable gauges or measuring tools instead of relying only on the marking printed on a drill bit.

If parts have a coating, plating, paint, or a thick finish, consider whether the finish changes the available clearance. Also check washer coverage, edge distance, nearby holes, and whether the bolt head or nut needs a counterbore or countersink. A plain clearance hole describes the shaft opening; it does not size the recess for a head or washer.

Worked example: choosing a hole for an M8 bolt

Suppose a bracket needs to slide over an M8 bolt, and the drawing permits a normal clearance fit. Choose Metric, M8, and Normal. The reference hole diameter is 9 mm. The nominal bolt diameter is 8 mm, so the diametral clearance is 1 mm. When the bolt is centered, that corresponds to about 0.5 mm clearance per side. The next 0.5 mm drill increment is 9 mm, so the drill suggestion remains 9 mm.

If the same assembly needs more room to align, select Loose and check the larger result against the drawing and joint requirements. If close location is important, the close result is smaller, but it also allows less tolerance for hole-position error. The right choice depends on the assembly, not simply the bolt label. When a production print calls for a specific diameter or tolerance, use that callout even if it differs from this example.

Worked example: a 1/4-inch bolt

For a 1/4-inch bolt with a normal fit, select Inch, 1/4 in, and Normal. The reference hole is 0.281 inch. A common fractional drill close to that diameter is 9/32 inch, or 0.28125 inch. The difference from the 0.250-inch nominal fastener diameter is about 0.031 inch across the diameter, giving roughly 0.016 inch clearance per side when centered.

That small calculation helps explain why a bolt should pass through a clearance plate while still having limited movement. It does not predict the final fit if the hole is drilled off-center, the bolt is bent, or the mating parts are misaligned. Check the entire fastener stack and hole pattern when assembly alignment matters.

Common mistakes to avoid

The most common mistake is choosing a hole equal to the bolt's nominal diameter. A hole the same size as the bolt has no allowance for tolerances and may not assemble freely. The opposite mistake is making every hole oversized for convenience. Oversized holes can change positioning, bearing behavior, and connection design, particularly in structures and moving equipment.

Another frequent mix-up is using the thread pitch or minor thread diameter as the clearance size. Clearance holes are based on the fastener's nominal outside diameter and selected fit. Thread pitch describes the distance between threads; it does not set the plain hole diameter. Do not rely on an online result without confirming the exact fastener series, fit, and application.

Also avoid reading the drill suggestion as an automatic manufacturing tolerance. A drill marked 9 mm may cut slightly larger depending on condition and setup. If the maximum hole diameter matters, use a controlled process and inspect the result. Deburring, countersinking, coating, and reaming can all change the final opening.

When to use a drawing or engineering standard

Use the project drawing first whenever it provides a hole diameter, fit, tolerance, or standard reference. This is especially important for structural steel, pressure equipment, vehicles, lifting hardware, machinery guards, and safety-related assemblies. Some applications require specific edge distances, hole types, washers, bolt grades, installation methods, or inspection records that a basic clearance calculator cannot determine.

If a drawing points to a standard, consult the current edition and the exact table that applies to the fastener. Standards can distinguish nominal sizes, tolerance classes, and hole manufacturing requirements. The calculator is designed for quick planning and learning; it is not a compliance check. If the connection carries significant load or failure could cause injury or damage, have a qualified engineer review the design.

Frequently asked planning questions

What size hole should I drill for a bolt? Start with the bolt's nominal size and the fit required by the design. A normal fit is a common general assembly reference, but the drawing or standard takes priority. What is a standard clearance hole? It is a plain hole larger than the fastener so the bolt passes through without threading into that part. How much larger should the hole be? The amount depends on the fastener size and whether the fit is close, normal, or loose.

Can I use a clearance hole for a wood screw? A clearance hole may be used through the first piece of a two-piece wood joint, but wood screws and self-tapping screws have different pilot-hole needs. Follow the fastener manufacturer's guidance and account for the wood species and screw design. Is a clearance hole the same as a pilot hole? No. Pilot hole is a broad term; clearance hole specifically allows a fastener shank to pass through a part.

Does a bigger hole make a stronger joint? Not by itself. Hole size interacts with material thickness, edge distance, fastener strength, load direction, clamping force, and joint design. Use an approved design method for load-bearing connections. Can I drill the next larger size? Only when the permitted tolerance and joint design allow it. A larger drill may exceed the specified maximum diameter.

Use the result as a starting point

A useful clearance hole lets the fastener pass through as intended while keeping the joint within its design limits. Choose the exact fastener, select the fit, compare the recommended diameter to available drills, and confirm the drawing before cutting. For a simple bracket, that process helps avoid a hole that is too tight to assemble. For a precision or load-bearing connection, rely on the applicable specifications and inspection requirements.

Keep the calculator result with the job notes when it helps the next person understand the selected fit. Record the fastener designation, hole diameter, drill or finishing operation, material, and any tolerance from the print. Clear notes make it easier to repeat the work and catch a mismatch before parts are assembled.