Clearance Hole Chart: Metric & Imperial Drill Size Table (Free PDF)

In the left-hand column of this chart, locate the thread size of the bolt/screw you want to use, then select the series. For very accurate hole placement select the fine series (H12); general assembly applications uses the medium series (H13), while the coarse series (H14) are used where the part must have some positional slack.

Each column in the series show the drill size in millimetres plus the diameter of the clearance hole. The final column convert the medium fit hole to decimal inches for those workshops using imperial measuring tools.

The fastener passes easily through the hole in the part, with a clearance around it. The nut (or tapped hole) and head will then clamp the joint between them, the hole never touching the thread.

ISO 273:1979 provides the standard for these three sizes. It is based off the calculations of the bearing area required for the various ISO nut and bolt product standards. That is where the names come from.

For a hole drilled by hand, it doesn’t matter what tolerance class you use, just the nominal drill size. If you’re using a drawing that calls out a hole tolerance, there are tolerance classes H12, H13, and H14.

Go with the default medium series, which leaves plenty of bearing surface under the bolt head but also clears common socket head cap screws, hex bolts, and washer stack ups.

Go to the fine series if you have tight control over part hole position (e.g., jig drilled vs. hand drilled; CNC vs. torch cut), since a slight position misfire will prevent assembly of multi-bolt patterns at fine clearance.

Go to the coarse series if your holes were site drilled or plated (e.g., hot dip galvanizing) to allow for some tolerance due to zinc buildup on the threads eating into the gap.

This chart is often confused for a tap drill chart, which is something different. The tap drill size is less than the bolt to tap a thread into it, while the clearance hole size is greater than to allow the bolt to pass through without damage. It’s easy to drill a clearance size instead of a thread size and destroy the part.

Also keep in mind that the pitch of the thread doesn’t matter. Only the major diameter matter when determining clearance, such as M10 x 1.25 and M10 x 1.5 requiring an identical clearance hole.

This table does not apply to structural steel bolting if codes like AISC or EN 1090 provide their own slot sizes and hole diameters. It also does not apply when bolts are specifically designed to bear on the wall of the hole, whether they are reamed or fitted.

If you’re using screws with threads that need to bite into the material (e.g., self tapping screws, wood screws), create a pilot hole instead. That’s a separate data set from what I’ve got here, see Tabelic’s pilot hole size chart for those sizes.

The basic diameter is the nominal major diameter in decimal inches. Read down first column until you find your screw size; then check that against the basic diameter. Now read across to your required fit.

Every fit have two columns. The left hand one gives the drill to chuck. That might be a fraction, a letter drill, or a number drill. The right hand column give the decimal diameter of the hole it makes. All these are according to ASME B18.2.8, Clearance Holes for Bolts, Screws, and Studs.

As you might guess, there is corresponding fit classes to these metrics. Normal is your typical everyday go-to fit. This is what to use if a drawing doesn’t indicate otherwise.

Close fits snugly over holes that are precisely positioned, for example, by drilling on a CNC machine or in a jig. Loose fits allows for position errors in multi-hole bolt patterns. It also accommodates thick coatings like hot-dip galvanizing.

For the hole itself, the nominal value given is the minimum diameter suggested. In other words: Drill to this size. The finished hole can be bigger than this by the allowable drilling tolerance. So even if a drill has wandered a bit, or gotten a little worn, the resulting hole will still be conforming.

The closest fractional size isn't equal to either a letter or a number drill. Several near misses cause practical problems.

The normal fit for a #8 screw is a #9 drill (0.196 in). Because of damage margins for threads and plating, a 3/16 drill (0.1875 in) won't consistently clear the screw's diameter of 0.164 in. A 13/64 drill (0.203 in) overshoots the desired fit. Where the chart names a number or letter drill, use it, or check the drill conversion table further down this page for the exact decimal and metric equivalents.

Note how the fractional and screw gauges jump. For instance, a #10 screw (0.190 in) is smaller then 1/4 in. Not the same thing. Most moddern charts don’t include a #12 screw (outdated size of 0.216 in).

Then there are the legacy gauges #7, #9, #11, and #14 that you might encounter on older drawings. Estimate a clearance based off the basic diameter. At small sizes, this would of be about the basic diameter plus 0.010 to 0.016 in for a normal fit. Or upsize to the next gauge listed.

This is not a tap drill chart. This is a chart of bolt and machine screw sizes when they are free running through a component.

A pilot hole for sheet metal screw and wood screw requires the screw to grip into the material. Those sizes are on Tabelic's pilot hole size chart.

Hole tables for structural connections made to bearing and slip critical specs is in the appropriate code and are for that application only. They also do not use this general purpose standard.

The purpose of this table is to allow you to size the entire recess for a hexagon socket head cap screw so that its head will be flush or below surface.

To do this, you first determine your screw size. Next, you drill the normal fit or close fit through-hole. Finally, you counterbore to a minimum diameter X to at least depth H. Additionally, you chamfer the hole entry up to the countersink diameter Y.

All measurements are in millimeters. They conforms to the drill and counterbore recommendations in the machinery's handbook. Also they match catalog information from manufacturers who produce screws per ASME B18.3.1M.

The diameter is X for the counterbore.

There is sufficient clearance for a typical counterbore tool to pilot on the through hole.

Example: M6 screw with 10 mm head fits into a 11.25 mm counterbore.

Height H is the max head height or the nominal screw diameter for socket head cap screws.

An M8 screw has an 8 mm high head and is hidden at H = 8 mm.

If you need to have the screw head recessed under the surface, go a little deeper. In this case you'd want some additional length of screw.

Y is the countersink size, not a decorative feature. This equals the largest possible transition between head and fillet below.

If the sharp hole edge is left unbroken on a part as hard as this screw, it will dig into the fillet. That prevents the head from seating square, the resulting stress riser invites fatigue failure.

Unless the drawing says otherwise, chamfer to a maximum of Y at an included angle of about 60 to 90 degrees. Don’t make it bigger. If you do, the bearing area under the head diminishes.

Note: The close and normal fit holes shown above aren't the same sizes as in the first table (the ISO 273 series). That's American shop practice matched to counterbore tooling.

Here, M6 normal is 6.80 mm vs. ISO 273's medium is 6.60 mm. Instead, European drawings frequently call up DIN 974 counterbores which has a bit different diameters, like 11.00 mm different than 11.25 mm for an M6.

Either convention will do. Don't mix them on the same part. If a part is called out, follow it from the drawing.

Published sources above M36 disagree, so we stop there with the table.

For sizes M42 and larger you use the actualy head diameter from the fastener drawing to size the counterbore. Add 3 mm minimum clearance. Standard head height is assumed when sizing these depths. Flange head screw and low head cap screws requires their own dimensions.

ASME B18.3 specifies inch socket head cap screws. Look up the screw size. Drill the hole through using the normal fit or close drill. Read decimal equivalent from table. Counterbore to a minimum of diameter B, at least as deep as the depth in the depth column. Make the countersink diameter at the entering end no more than C.

In some locations, the plain ASME B18.2.8 clearance chart above does not match drill options. Where it’s a #31, it’s a 1/8 close fit drill here; where it’s a #9 for a normal fit, it’s #10.

The reason is that this appendix data match standard pilot counterbore sizes to each particular hole. Whenever the screw are seated in a counterbore, use this table different than the general chart.

The depth sinks the head just flush, since the thickness of the screw equal the height of the corresponding inch socket head. The head of a 1/2 in screw is 1/2 in high. To make it below-flush, counterbore slightly deeper. Just be sure you have enough length on the screw grip for it to engage all threads once the head sink.

Diameter B provide enough radial clearance (1/32 in to 1/16 in) around the head. This ensures the socket won’t bind against the head when using hex key to locate the socket.

The hole edge has to be chamfered so it doesn’t damage the fillet underneath the head. Diameter C in the table above represent the maximum diameter for the countersink and the chamfer must not go larger than this. If you do, you’ll lose bearing surface for the head. Chamfer the edge whenever the part hardness approaches the screw hardness. On soft material, a slight deburring suffice.

For the counterbore, use an end mill or a counterbored pilot tool. The point of a twist drill create a cone which prevents the flat bottom of the head from seating.

Sizes 1-1/8, 9/16 and #12 is not on the ASME B18.3 list of preferred socket head cap screw sizes.

These are very different types of recess geometry for button head or flat head (countersunk) screws. A flat head require an 82-degree countersink in inches. These should of not been sized by this table.

The table above includes every letter drill, and number drill on the two inch tables above in their precise decimal inch diameter and the millimetre equivalent form. From there you can reference what size is near enough metricaly and replace accordingly in a shop using millimeters. The last column, “used in this chart as,” points to the row it names. This include both the ASME B18.3 counterbore chart and the ASME B18.2.8 clearance chart.

The reason that number drills read opposite is they are smaller as the gauge size goes up. For example, a #51 drill is 0.067 in, whereas a #5 is 0.2055 in.

Beyond number drills are letter drill. Letter drills runs from A through the alphabet, beginning at .234 and increasing in size.

The mistake most people make is reading a #2 drill when the chart indicate a #2 screw. Remember, screws never have the # prefix on them in the drill column, but drills do. The only time you will see screw gauges is in the first column of each table.

Round up for a clearance hole, always go up, not down. A 5.0 mm drill is 0.1969 in. That’s close enough for a #9 clearance hole (0.196 in). 6.0 mm drill will cover the letter B loose fit for a #10 screw.

For close fits, stay with the substitution based off range of about 0.05 mm. You can also step up to the next fit class. Substitutions work equally well going in reverse.

This table will allow you to select number drills for metric clearance hole. In the first table choose the next drill size up from the metric value.

The decimals represent common drill gauge diameters. They does not measure the holes they cut.

Clearance standards specify a minimum hole, hence the name. Because drills are hand held, they tend to be oversized by .05-.15 mm.

The material being drilled and the way the tip is ground determine how much the drill will overcut. For that reason clearance standards has a tolerance above the drill size. You should of checked it.