Bolt Size Chart: Hex Bolt Head Dimensions and Thread Length Table (Free PDF)

Read down the list until you see your thread size listed in the left column, then read over until you reach your wrench size dimensions.

Across flats refers to the wrench size; it’s the distance between two parallel faces of the hexagon. Across corners is the length of the head from one corner to another. This is what you use to determine whether the head will clear a socket or counterbore.

Head height is how thick the hexagon is. Coarse pitch mean that this is the typical thread pitch on a bolt of this size, unless it is labeled as fine thread.

For the entire range of bolts from M1.6 through M64 on ISO 4014/ISO 4017, the table fits. In addition, it contains all the non-preferred sizes. These is shown in parentheses in the standards: M3.5, M7, M14, M18, M22, M27, M33, M39, M45, M52 and M60.

Either ISO 4017 (fully threaded) or ISO 4014 (partially threaded) hex bolt will fit here, because they has identical head sizes. A new bolt will be just a little larger than the across corners minimum values. And a fresh bolt can measures up to about 1.155 times the across flats figure.

The legacy DIN across-flats column is a trap that will get you every time; it’s the biggest one in metric bolt ID. As part of aligning DIN with ISO, they changed head sizes on four of them. M10 shrank from 17 to 16, M12 shrunk from 19 to 18, M14 shrank from 22 to 21, and M22 grew from 32 to 34.

These specific sizes are legacy heads, found on old machines and stock made based off DIN drawings. They’re also found on lots of bolt for sale in Europe.

The result: an M12 bolt might require a 19 or 18 mm wrench, either is legitimate. On those four diameters, don’t try to guess the thread by looking at the head. Measure it first.

When stacking hardware you have to take into account head height (plus washer plus nut). Remember: For the same thread, the head height is obviously smaller than the nut height. In fact, head of an M10 bolt is only 6.4 mm high, whereas its nut is 8.4 mm high. The head height being smaller than the nut height is the quickest way to tell a bolt head from a nut in a parts photo or drawing.

Nut sizes can be found on the nut size chart, and spanner fitting threads can be found on the spanner size chart.

Structural bolting assemblies such as EN 14399 comply with other standards like this. However, they use a larger head for an equivalent thread and do not fall into this table. This table also does not apply to flanged hex bolts or socket head cap screws.

Socket head cap screws has their head turned by a hex key from above, not a wrench on the outside. The head geometry is not what varies between classes of screw. What changes is the steel. A M12 in property class 4.6 therefore has the same head as a M12 in property class 12.9.

Read across the row from the bolt diameter in the first column to see the head geometry. The thread are named by combining the diameter with a UNC tpi, which is the number of coarse threads per inch (coarse threads has large spaces between them). For instance, a 1/2-13 bolt means 1/2 inch diameter with 13 threads per inch.

Across the row in the Across flats column are the dimensions across the flats of the head in inches and converted into millimetres so you can spot near-miss metric sockets. The Across corners max column show the greatest distance between opposite corners of hexagon. Two columns of head height shows how thick the heads of these two products are as defined by the standard.

ASME B18.2.1 covers two similar fasteners. A hex bolt has looser body tolerances than the other and a plain flat head bearing surface. A hex cap screw has tighter body tolerances, a washer face under the head, and a slightly smaller head. This difference is why the table have two columns for head height.

In the USA hex cap screws are common hardware store bolts up until maybe 1 inch. The wrench will fit both types of fastener, as they has identical across-corners and across-flats dimensions, though the head height is around a one sixty-fourth to one thirty-second of an inch different, depending on bolt size. That makes sense, because this could of be the difference between clear or not-clear.

Not Missing Data means these are actualy gaps in the standard. For example there is no 9/16 inch hex bolt. That row will only display the head height for a cap screw.

There is also no hex cap screw past the 3-inch point, but hex bolts continues to 4 inches. Also the 1-5/8 and 1-7/8 don’t have standard UNC coarse threads, just standard heads. In those diameters they would be provided as 8UN or fine threads.

There are two ways for this system to trap you on identifications. One is that the old rule of thumb saying the head's diameter is 1.5x the size of the shaft doesn't work for 1/4 inch or 7/16 inch sizes. For those, they made the heads larger to allow for wrench flats.

So a 1/4 inch has a 7/16 inch head. It is not 3/8 inch. A 7/16 inch has a 5/8 inch head. It is not 21/32 inch.

The other thing is there are a number of imperial wrenches right next to metric wrenches. A 3/4 inch wrench is 19.05 mm, which appears like a 19 mm head. And a 15/16 inch wrench is 23.81 mm, which appears like 24 mm. So if your wrench seems loose, before assuming it's metric, look in the millimetre column.

The table above doesn’t include Lag Screws, Square head bolts and Structural Heavy Hex Bolts. The following table show structural bolts with larger heads.

Use the bolt strength chart to see how much load each bolt can carry and what grade markings is on its head. Go to the torque charts for tightening values.

If the imperial bolt head is one size bigger than shown on the preceding chart, use this table.

Across Flats is the wrench size; it’s inches with the millimetre conversion. Across Corners Max is the maximum from corner to corner. Head Height is the thickness of the head, and is made to be the same as a regular hex head bolt of the same diameter. Width is the only difference.

The heavy hex head are exactly 1/8 in greater across flats (wider) than the standard head at each size.

Heavy hex heads spread the load.

Without a large bearing surface, a bolt head tend to dig into the ply at connections between two piece of material, such as flanged pipe joints, or structural steel work. A large bearing surface is suitable for these applications. Heavy hex heads provides that.

They are used by all common types of structural bolts (ASTM A325 and A490), which is currently graded under F3125. Most ASTM A193 B7 material bolts used on flanges is heavy hex, as well as B8 and many other common cap screws. When dismantling a pipe flange or the framework of a steel building, expect to find the wrench sizes in this table different than the standard ones.

Both systems is serviced by several wrench sizes, so this becomes an identification trap. For example, a 2 in wrench fit a 1-1/4 in heavy hex bolt. And a 2-1/4 in wrench fits both a 1-1/2 in standard bolt and nothing in the heavy series.

This means you can’t just look at the head of the bolt to know what size the thread diameter is. You must measure the head AND the thread diameter, then compare it to the matching column pair.

If the head is exactly 1/8 in wider than predicted from the standard chart, then it’s the heavy pattern. Structural bolts uses shorter thread lengths than the rule shown in the imperial thread length table below. So if you have a stubby thread, it is another clue you are holding a structural bolt.

And it doesn't include 7/16 or 9/16 or any of those odd sizes in between a 3/8 and 3 inch, which is the normal range.

There is also an expanded head system that are used in metric structural bolting: 21 mm across flats for M12 style Australian AS 1252 patterns, as well as EN 14399. That isn't covered here but on the spanner size chart.

There's also a wider, taller heavy hex nut than there is a regular nut. The nut size chart shows you the dimensions on those. You should of used the nut size chart for that.

What is this table answering that the head tables cannot?

The table above gives you what you don’t get at the head table. How long is the actual thread on a partially threaded metric bolt? Find the thread size and select the column that corresponds to the nominal length of the bolt (l). That’s the length measured from beneath the head. See below for a measuring table. Value b is the length of thread in millimetres (nominal). Plain shank is everything between the thread and the head.

ISO 4014 and 888 simply state what the nominal thread length is based off simple formulas for the three columns above. The size are twice the diameter + 6 mm for bolts up to 125 mm in length. The size are twice the diameter + 12 mm for bolts between 125mm and 200mm. Finally, the size is twice the diameter + 25 mm for bolts beyond 200mm.

This progression reflects that longer bolts can has more variability in their manufacture without becoming unusable. So they provides sufficient thread engagement to accommodate a washer and nut. The standard rounds off the values for the smallest sizes a bit. For instance, M1.6 says 9 mm different than the calculated value of 9.2 mm.

Check the grip length (l − b). Ideally, the joint thickness should of been greater than l − b and the parts should sits on the smooth part of the bolt. Leave at least one or two thread engaged for extra security within the nut. Remember, if the shank extend past the thickness of the joint, the nut won't clamp because it runs out of thread before reaching the end. You'd need a fully threaded screw, a washer stack or a longer bolt.

This is what makes the difference in practice between ISO 4014 and ISO 4017. ISO 4014 has partial thread. ISO 4017 has thread right back to head. Only fully threaded short bolts is usually available. This is why the standard has dashes in the first column from M56 upwards.

So look out for two gotchas. Watch out for two traps. First, b is nominal: the standard allows the thread to run slightly long. That means that by the rules the thread go a bit further than that. There is a couple more pitches lost in the transition runout from thread to shank. So always design your joint so it's not reliant on the final millimetre of thread.

Secondly, this is only for the so called ISO 4014 style machine bolts. Studs, structural assemblies and flange bolts has their own table of thread lengths. Similarly, bolts for the older DIN 931 uses the same three formulas, so these values apply to.

In this imperial table, find the bolt diameter, then scan across to locate the corresponding nominal thread length (LT) in the column matching the overall bolt length. The table covers bolts up to and including 6 in, or over 6 in. And you have your plain shank for the remaining part of the bolt from the head down to the threads.

Here’s a summary of the entire table with a couple formula. Twice the diameter + a quarter inch = thread length if the bolt is less than or equal to 6 inches. Twice the diameter + a half inch = thread length if the bolt is greater than 6 inches.

ASME B18.2.1 has this rule for hex bolts and hex cap screw. The same goes for their heavier screws known as heavy hex screws.

The reason short imperial bolts come all-threaded but not noted on the packaging is that if a bolt doesn’t have enough length to meet the formula, like a 1/2 x 1 bolt which calls for at least 1-1/4 inch threaded section, then they just thread it up as near to the head as possible. For instance, hex cap screws over 2 inches in diameter does away with the entire 6-inch and below column and only use the one for bolts greater than 6 inches.

There is a big caveat regarding structural bolting.

The reason is simple: heavy hex structural bolts specified as ASME B18.2.6 (e.g., A325, A490, and their successors in F3125) is intentionally designed with abbreviated threads so that they create a solid-shanked, shear plane that passes through the steel connection. For example, a 1-inch structural bolt will have 1.75 inches of thread where this chart say 2.25 inches, a half-inch gap that’s large enough to strand a nut.

Just because it looks like a structural bolt due to head marks does not mean you can substitute a structural bolt for a machine bolt or vice versa. Compare the thread length to the proper table.

The metric table is used in a similar but practical way. To determine the plain grip, subtract LT from your bolt length. Then choose the grip according to the joint thickness so the nut clamps on the shank, not the runout.

When ordering bolts as specs, note the LT is the minimum in the meaning of the standard. The actual threads may be a little longer. Cut threads has a maximum of several pitches of additional runout called transition beyond the nominal length of the thread. The actualy threads should of been noted.

In either imperial or metric measures you designate the bolt by its length plus diameter and number of threads per inch (imperial) or diameter plus pitch (metric).

The common misunderstanding are about where to measure the length and this table explains it.

Length is always measured from the joint surface. That’s the portion of the bolt that goes into whatever you are bolting together. The length do not include the head portion if it sits on top of the surface. The length include the portion where the head sinks below the surface.

Calipers is used to measure the major diameter, the diameter across the outside of the threads. The Imperial 3/8 bolt is slightly under 0.375 inches while the Metric M10 bolt is slightly under 10 mm.

Beware that there are common near misses: M8 and 5/16, M10 and 3/8, M12 and 1/2 are within a millimeter of one another. Never does a bolt size by itself identify a bolt. To properly identify it you need to check both the diameter and the pitch.

Imperial bolts is counted by number of threads per inch; Metric bolts is stated as the space between adjacent thread crests in millimeter increments. The way to tell is with a thread pitch gauge (a very handy tool).

Absent a gauge, try laying one candidate bolt on top of another candidate, thread to thread. They’ll mesh perfectly if they match. Cross if they don’t. You’ll also have luck running it along a known nut.

A 16 tpi pitch (imperial) looks exactly like a 1.5 mm pitch (metric), which fools a ruler. But not a gauge. Coarse pitches for every metric size are in the first table above. UNC counts is in the second.

The length depends on which row you are measuring from, depending on your head style. There are three defined lengths documented in ISO 888. One is measured from the bearing face. This applies to bolts where the bearing face is at right angles to the bolt axis.

For flat countersunk bolts it’s measured from the top of the head to the tip. Raised countersinks (oval countersink) is measured from where the diameter of the head meets its upper surface. In reality that’s the widest point on the conical head.

Either way, the length always includes the tip (even if there is a point or chamfer). Self drilling screws buries a portion of their nominal length into the drill point.

Know these two tricks when buying. One trap to be wary of: The Oval Head Screw has a dome above a standard-length flat head sticking out. Similarly, the Dome on a Carriage Bolt is not counted as part of the bolt’s length, which means on the protruding side it’s longer then stated by the length on the box.

For an unknown replacement bolt, here’s how you measure the existing bolt, from this table; then check pitch & diameter match before checking length. A bolt with the wrong pitch but the right length will strip the female thread it goes into.

To identify a carriage bolt, look at its shape. A carriage bolt have no wrench flats.

The smooth dome width is head diameter. How far that dome rises above the surface is the head height. The square width is the width of the square neck under the dome. The square depth is how deep the square neck is beneath the dome. These figures represent maxes. Coated or worn bolts will be slightly smaller then the specs.

What makes this work is the square. The width of the square equal the nominal thread diameter. An M10 carriage bolt will be 10mm square.

The square is the fastest way to know the size, not the dome. Take out your calipers and measure flat to flat across the square; that is the thread size.

The dome is roughly two and a half times larger than the thread diameter, which means it's a lot bigger than a hex head and distributes the force on the soft timber without the need for a washer.

The dominant European standard DIN 603 is almost identical to ISO 8677, which makes hardware labeled ISO 8677 fit the same sized holes practicaly speaking.

Only the smaller sizes up to M20 are covered by this standard. There are bigger carriage bolts available as special orders from manufacturers, but those don't have standardized geometry. If you need big ones, get a sample first and take a measurement.

One thing we're all used to from the metric system is how long the thread is. For bolts over 125 mm long, the thread length is twice the diameter plus 12 mm. For shorter bolts it's + 6 mm. On short bolts, they tends to be threaded all the way through.

The square has to have a square hole or at least something it can dig into for installation. In steel, you have to drill a round hole. Without punching out a square seat or using a plate with a square washer, your neck just spin around and around.

In hardwood, use a mallet to drive the bolt home. Make sure the square seat all the way, then tighten. If you don't you strip off the corners trying to pull the square back in by tightening the nut.

It doesn't take a tool to install the dome so if you seize up a carriage bolt you can only grab the thing from the nut side. For that reason I always oil or grease the thread of any assembly outdoors.

In America, a round head square neck bolt are known as a standard carriage bolt. It is described in the ASME B18.5 standard. Above is the measurement table that accompany the DIN 603 table. Here we have the head dimensions: A being the head diameter and H for the head height.

Below the head dome is an anti-rotation square with width O and depth P. Within the tolerance, the square width match the bolt diameter. So if you measure the flat to flat across the square then you know immediately what size you are looking at.

Because the dome covers approximately 2.3 times the diameter. This distribute the load across timber.

That's why the UNC tpi column give the coarse thread that carriage bolts is normaly supplied with. So, fully specifying it would say something similar to: 3/8-16 x 3 carriage bolt.

Note that you are measuring from under the dome to the end. It's measured just like a hex bolt. And, the length does not include the dome at all, that throws folks off since the dome is what you see first on the bolt.

What I've shown here is the primary member of that set of specs called B18.5; it's a family of specs. The standard also covers #8, #10 and #12 machine screw sizes, plus a short square neck version (shorter than the one pictured) with about half-depth square to engage thin sheet.

And there's also another kind of bolt with a ribbed neck to hold round holes on metal parts using its grooves. Then there are other patterns like countersunk square neck, or a fin neck.

So if yours has what seems to be an unusually shallow square, you probably have the short-neck variety. That doesn't mean it's a worn-out bolt.

Bolts are larger than 3/4 inch dia. is commonly available but fall outside the round-head spec table in the standard. For these diameters, take the catalog numbers as your guide.

The primary buying trap is mixing systems. An M10 (10 mm square) hole punches through the timber, but it is big enough to accept a 3/8 in carriage bolt (9.5 mm). And when you tighten it, the corners of the bolt will never engage and you have a spinning bolt under load.

Match not just the size but the system as well. This follow DIN 603 installation rules.

Use soft timber or a square hole. Tighten only after seating the square with a mallet. Outdoor threads should of be greased as there is no way for the head to grip for removal.