Bolt Length Calculator

Bolt Length Calculator that adds grip, washers, nut height and threads past the nut, then picks the stock length and checks thread start, shear planes, RCSC bolts and flange studs.

How a Bolt Length Calculator Sizes a Bolt

A bolt length calculator adds up everything the bolt has to pass through, then adds enough length to fill the nut and show thread past it. The bolt length calculator result is a minimum length, and the bolt you buy is the next stock length at or above it.

$$L_{min}=G+W+H_{nut}+n\times P$$

G is the grip, the total thickness of the parts being clamped. W is the stack of washers, H_nut is the nut height, P is the thread pitch and n is the number of threads you want past the nut. Length is measured from under the head to the tip of the bolt.

That is the bolt length calculation formula for a through bolt with a nut. A cap screw in a tapped hole swaps the nut and protrusion for thread engagement, and a structural bolt swaps them for a code allowance, both covered below.

Worked Example: 1/2-13 Bolt Through Two Plates

Two steel plates, 3/4 in and 1/2 in thick, are bolted with a 1/2-13 UNC hex bolt, a plain washer under the head and another under the nut. The goal is two full threads past a standard hex nut, the default most bolt length calculator tools use.

Part	Thickness	Running Total
Plate 1	0.7500 in	0.7500 in
Plate 2	0.5000 in	1.2500 in
2 × plain washer (0.095 in)	0.1900 in	1.4400 in
Hex nut (7/16 in)	0.4375 in	1.8775 in
2 threads past nut (2 × 0.0769 in)	0.1538 in	2.0313 in

The minimum is 2.0313 in, so the stock length is 2-1/4 in. A 2 in bolt misses by 0.0313 in and shows only 1.6 threads past the nut, while the 2-1/4 in bolt shows 0.3725 in, or 4.8 threads.

How Long Should a Bolt Be Past the Nut?

A bolt is long enough when its end is at least flush with the outer face of the nut, and a good bolt length calculator checks that before anything else. The RCSC structural bolt specification sets exactly that rule as "sufficient thread engagement" in its glossary, so flush is the hard minimum and anything past it is margin.

For general work, aim for one to three full threads past the nut. Two threads covers the chamfered first thread on the bolt tip, which carries little load, and leaves a visible check that the nut is fully engaged.

Too much protrusion causes its own problems, which is why a bolt length calculator picks the shortest stock length that passes. Long tails snag clothing and cables, hit nearby parts, and on a partially threaded bolt they can mean the nut runs out of thread before it clamps.

Make Sure the Threads Reach the Joint

The nut must tighten on full thread, not on the smooth shank. On a partially threaded bolt, that means the thread has to start inside the clamped stack, before the face the nut seats against.

In the 1/2-13 example, the 2-1/4 in bolt has 1.25 in of thread and 1.00 in of smooth shank. The stack under the nut is 1.44 in, so the threads begin 0.44 in inside the joint and the nut seats on full thread.

When the shank reaches past the nut face, the nut jams on the thread run-out and stops clamping. Switch to a fully threaded tap bolt, or add washers to build up the stack, rather than forcing a longer partial-thread bolt.

Bolt Length Calculation Chart by Diameter

The quickest way to size a through bolt without a bolt length calculator is to add a fixed allowance to the grip. The allowances below include a standard hex nut, two plain washers and two threads past the nut, using the nut heights in ASME B18.2.2 and narrow plain washers.

Bolt Size	Hex Nut	2 Plain Washers	2 Threads	Add to Grip
1/4-20	0.2188 in	0.130 in	0.1000 in	0.449 in
5/16-18	0.2656 in	0.130 in	0.1111 in	0.507 in
3/8-16	0.3281 in	0.130 in	0.1250 in	0.583 in
7/16-14	0.3750 in	0.130 in	0.1429 in	0.648 in
1/2-13	0.4375 in	0.190 in	0.1538 in	0.781 in
5/8-11	0.5469 in	0.190 in	0.1818 in	0.919 in
3/4-10	0.6406 in	0.268 in	0.2000 in	1.109 in
7/8-9	0.7500 in	0.268 in	0.2222 in	1.240 in
1-8	0.8594 in	0.268 in	0.2500 in	1.377 in
1-1/4-7	1.0625 in	0.330 in	0.2857 in	1.678 in

Add the allowance to the grip and round up to the next stock length, just as the online bolt length calculator does. Drop the washer column for each washer you leave out, or swap in 0.156 in per hardened F436 washer.

3/4 in Diameter Bolt Length by Grip

For a 3/4-10 hex bolt with two plain washers and a hex nut, the chart below runs the same bolt length calculator math across common grips. The structural column uses the RCSC allowance with one F436 washer instead.

Grip	Minimum Length	Stock Bolt	Structural Bolt (RCSC)
1/2 in	1.609 in	1-3/4 in	1-3/4 in
3/4 in	1.859 in	2 in	2 in
1 in	2.109 in	2-1/4 in	2-1/4 in
1-1/4 in	2.359 in	2-1/2 in	2-1/2 in
1-1/2 in	2.609 in	2-3/4 in	2-3/4 in
2 in	3.109 in	3-1/4 in	3-1/4 in
2-1/2 in	3.609 in	3-3/4 in	3-3/4 in
3 in	4.109 in	4-1/4 in	4-1/4 in
4 in	5.109 in	5-1/4 in	5-1/4 in

For a 3/4 in bolt, the length is the grip plus about 1-1/8 in for general work, then rounded up. Every bolt in this chart starts its thread about 0.77 in inside the joint, so the nut always seats on full thread.

Bolt Size Chart: Nuts, Washers and Thread Length

A bolt length calculator is only as good as the hardware heights it uses. These are the inch values, with hex nut heights checked against Portland Bolt's ASME B18.2.2 hex nut table.

Size	Hex Nut	Heavy Hex Nut	Jam Nut	Plain Washer	Lock Washer	Thread LT (L ≤ 6 in)
1/4	7/32 in	15/64 in	5/32 in	0.065 in	0.062 in	3/4 in
3/8	21/64 in	23/64 in	7/32 in	0.065 in	0.094 in	1 in
1/2	7/16 in	31/64 in	5/16 in	0.095 in	0.125 in	1-1/4 in
5/8	35/64 in	39/64 in	3/8 in	0.095 in	0.156 in	1-1/2 in
3/4	41/64 in	47/64 in	27/64 in	0.134 in	0.188 in	1-3/4 in
7/8	3/4 in	55/64 in	31/64 in	0.134 in	0.219 in	2 in
1	55/64 in	63/64 in	35/64 in	0.134 in	0.250 in	2-1/4 in
1-1/4	1-1/16 in	1-7/32 in	23/32 in	0.165 in	0.312 in	2-3/4 in
1-1/2	1-9/32 in	1-15/32 in	27/32 in	0.165 in	0.375 in	3-1/4 in

A hardened ASTM F436 washer is thicker than a plain washer, about 5/32 in nominal for most sizes. Using plain-washer thickness for an F436 stack under-sizes a 1/2 in bolt by 0.12 in over two washers.

Threaded Bolt Length: Partial vs Full Thread

A threaded bolt length calculator has to know where the threads stop, because a standard hex bolt is threaded only part of its length. Portland Bolt's hex bolt data, per ASME B18.2.1, gives the thread length as a simple rule.

$$L_T=2D+0.25\text{ in}(L\le 6\text{ in})$$

$$L_T=2D+0.5\text{ in}(L>6\text{ in})$$

D is the nominal diameter. A 1/2 in bolt 6 in long or shorter has 1-1/4 in of thread, and a longer one has 1-1/2 in. Portland Bolt notes most stock hex bolts 10 in and longer carry 6 in of thread, so check the actual part on long bolts.

The difference is large on long joints. A 9-1/2 in, 1/2-13 bolt through an 8-1/2 in grip has 0.69 in of thread inside the joint at the ASME length, but 5.19 in if it carries 6 in of thread, enough to put threads into a shear plane between the plates.

Metric hex bolts to ISO 4014 use b = 2d + 6 mm up to 125 mm long, 2d + 12 mm from 125 to 200 mm, and 2d + 25 mm beyond. An M12 bolt up to 125 mm long has 30 mm of thread.

When the grip is short and the bolt is long, the smooth shank can land under the nut. A fully threaded tap bolt (ISO 4017 in metric) solves that, at the cost of having threads inside the joint.

Threads in the Shear Plane

Threads in a shear plane cut a bolt's shear strength, which is why AISC separates bolts with threads excluded (X) from bolts with threads included (N). In the 1/2-13 example, the shear plane sits 0.845 in below the head, and the 1.00 in shank passes it by 0.155 in, so it qualifies as X.

To keep threads out of the shear plane, put the thicker plate on the head side or pick a longer bolt with more shank; a steel bolt length calculator that reports the shank length shows which works. Add washers under the nut to keep the nut on full thread when the shank grows.

Structural Bolt Length Calculator (A325 and A490)

High-strength structural bolts are not sized with a nut-plus-thread rule, so an A325 bolt length calculator works differently from a general one. The 2020 RCSC Specification Table C-2.2 gives a single allowance to add to the grip, and that allowance already covers the heavy hex nut, thread engagement and manufacturing tolerance.

Bolt Diameter	Add to Grip (RCSC Table C-2.2)	General Through-Bolt Allowance
1/2 in	11/16 in	0.781 in
5/8 in	7/8 in	0.919 in
3/4 in	1 in	1.109 in
7/8 in	1-1/8 in	1.240 in
1 in	1-1/4 in	1.377 in
1-1/8 in	1-1/2 in	1.522 in
1-1/4 in	1-5/8 in	1.678 in
1-3/8 in	1-3/4 in	1.835 in
1-1/2 in	1-7/8 in	1.945 in

The RCSC grip excludes washers. Add 5/32 in for each ASTM F436 hardened washer, 5/16 in for each beveled washer and the manufacturer's thickness for a direct tension indicator, then round the total to the nearest 1/4 in for lengths up to 6 in, or the nearest 1/2 in above 6 in.

Example: two 3/4 in plates (1-1/2 in grip) joined with 3/4 in A325 bolts and one F436 washer. The total is 1.5 + 1 + 0.156 = 2.656 in, which rounds to a 2-3/4 in structural bolt.

Nearest means the length can round down. A 3/4 in bolt with a 1.55 in grip and no washers totals 2.55 in and takes a 2-1/2 in bolt, 0.05 in shorter than the sum. RCSC accepts that because Table C-2.2 already carries the bolt and nut tolerances; a 4 in grip with one F436 washer (5.156 in) likewise takes a 5-1/4 in bolt.

The structural bolt length calculator result must still leave the bolt end at least flush with the nut, and RCSC warns against jamming the nut on the thread run-out. Heavy hex structural bolts carry less thread than hex bolts, 1-3/8 in on a 3/4 in bolt per ASME B18.2.6, so the run-out check matters more on short grips.

For A325 bolt length availability, structural heavy hex bolts are commonly stocked in 1/4 in steps, matching the RCSC rounding. Fully threaded A325T bolts are generally limited to lengths of 4 × D or less, which is 3 in for a 3/4 in bolt.

Flange Bolt Length Calculator for Stud Bolts

Pipe flange joints use all-thread stud bolts with a heavy hex nut on each end, and a stud bolt length calculator sizes them from both sides. The stud length is both flanges plus the gasket, plus two nuts and thread past each one.

$$L_{stud}=T_1+T_2+t_g+2\times (H_{nut}+n\times P)$$

For an NPS 4 Class 150 raised-face joint, each flange is 0.88 in thick and a 1/8 in gasket sits between them. With 5/8-11 studs, 39/64 in (0.609 in) heavy hex nuts and two threads past each nut, the minimum is 3.467 in, so the stud is 3-1/2 in.

That matches the 3.50 in raised-face stud length in the Ferguson Industrial ASME B16.5 flange bolting chart. A 12 ft length of threaded rod cut with 1/8 in saw kerfs yields 39 of these studs.

Ring-type joint flanges sit apart by the ring gap, so they need longer studs than raised-face flanges of the same size. The Bolt Circle Calculator lays out the stud holes for the same flanges.

Cap Screw Length in a Tapped Hole

A bolt length calculator treats a cap screw differently. A cap screw threaded into a tapped part has no nut, so the length is the clamped plates and washers plus the thread engagement the base metal needs.

$$L_{min}=G+W+L_e$$

A common guide is engagement of about 1 × D in steel and 2 × D in aluminum, since softer metal needs more thread to match the screw's strength. The Bolt Pull Out Force Calculator checks whether a chosen engagement holds the load.

Example: a 3/8-16 screw clamps a 1/4 in bracket with one 0.065 in washer into aluminum. Minimum length is 0.25 + 0.065 + 0.75 = 1.065 in, so a 1-1/4 in screw engages 0.935 in, or 2.49 × D.

Check the full-thread depth of the tapped hole too, because the screw needs room below its tip. If the screw is longer than the stack plus that depth, it bottoms out and stops clamping, even though the torque wrench clicks.

Bolt Length Calculator in Metric (mm)

A metric bolt length calculator follows the same sum in millimeters. Metric hex nuts to ISO 4032, washers to ISO 7089 and spring lock washers to DIN 127 B have these thicknesses:

Size	Hex Nut (ISO 4032)	Washer (ISO 7089)	Lock Washer (DIN 127 B)	Thread b (L ≤ 125 mm)	Add to Grip
M6 × 1	5.2 mm	1.6 mm	1.6 mm	18 mm	10.4 mm
M8 × 1.25	6.8 mm	1.6 mm	2.0 mm	22 mm	12.5 mm
M10 × 1.5	8.4 mm	2.0 mm	2.2 mm	26 mm	15.4 mm
M12 × 1.75	10.8 mm	2.5 mm	2.5 mm	30 mm	19.3 mm
M16 × 2	14.8 mm	3.0 mm	3.5 mm	38 mm	24.8 mm
M20 × 2.5	18.0 mm	3.0 mm	4.0 mm	46 mm	29.0 mm
M24 × 3	21.5 mm	4.0 mm	5.0 mm	54 mm	35.5 mm
M30 × 3.5	25.6 mm	4.0 mm	6.0 mm	66 mm	40.6 mm
M36 × 4	31.0 mm	5.0 mm	6.0 mm	78 mm	49.0 mm

The "Add to Grip" column is one hex nut, two ISO 7089 washers and two pitches past the nut. Leave out the lock washer column unless you use one; it adds its thickness to the stack.

Example: an M12 bolt clamps 10 mm and 12 mm plates with two washers. The minimum is 22 + 5 + 10.8 + 3.5 = 41.3 mm, so the stock bolt is 45 mm. It shows 7.2 mm past the nut, and its 30 mm thread starts 12 mm inside the joint.

The same metric sizing applies to bolt length calculator UK searches, since UK commercial fasteners are ISO metric. Preloaded structural bolts in the UK follow the EN 14399 series, so use the bolt supplier's length table for those.

Standard Bolt Lengths You Can Buy

There is no single standard length of a bolt; there are standard steps, and every bolt length calculator rounds up to one of them. Inch hex bolts are commonly stocked in 1/4 in steps up to 6 in and 1/2 in steps beyond, so a calculated 2.03 in becomes 2-1/4 in and a calculated 6.2 in becomes 6-1/2 in.

Metric bolts follow preferred lengths of 6, 8, 10, 12, 16, 20, 25 mm and then 5 mm steps to 70 mm, 10 mm steps to 160 mm, and 20 mm steps to 300 mm. In-between sizes such as 14, 18, 22 and 75 mm exist in DIN ranges but are harder to find.

When the next step leaves too much tail, look at a thicker washer or a heavy hex nut instead of cutting, then rerun the bolt length calculator. Both raise the stack slightly and can let a shorter stock bolt work.

Bolt Length Calculator in Excel

A bolt length calculator in Excel takes one formula for the minimum and one for the stock size. Put grip in B1, washer stack in B2, nut height in B3, threads per inch in B4 and threads past the nut in B5:

=B1+B2+B3+B5/B4
=CEILING(B6,IF(B6<=6,0.25,0.5))

The first formula goes in B6 and gives 2.0313 for the 1/2-13 example. The second rounds it to the next 1/4 in step, returning 2.25. For metric, enter pitch in mm instead of TPI, replace B5/B4 with B5*B4, and round with CEILING to 5 mm.

Measuring, Cutting and Sizing Bolts

A bolt length calculator answers only one of the numbers that decide whether a bolt fits. These answers cover how length is measured, how diameter is chosen, and what to do with a bolt that is too long.

How do you measure the length of a bolt?

Measure from the underside of the head to the tip for hex, socket head, carriage and pan head bolts, the same length a bolt length calculator reports. Flat countersunk heads are the exception: measure from the top of the head, because the head sits flush in the part. Studs are measured end to end.

How long is a 1 inch bolt?

A bolt sold as 1 in long measures 1 in from under the head to the tip. If "1 inch bolt" means 1 in diameter, it uses a 55/64 in hex nut and has 2-1/4 in of thread when 6 in long or shorter, so it typically needs at least 1-3/8 in plus the grip.

How do I determine the required bolt size?

Pick the diameter from the load first, then the length from the joint. The diameter comes from the tensile stress area and grade, which the Bolt Area Calculator gives for any size, and the bolt length calculator then sizes the length for that diameter.

How to choose a bolt size for a hole?

The hole sets the largest bolt that fits. A normal clearance hole is about 1/16 in over the bolt diameter for inch bolts; the Clearance Hole Calculator gives close, normal and loose fits for each size.

What size is a 12 mm bolt in inches?

A 12 mm bolt is 0.472 in in diameter, just under 1/2 in. It is not interchangeable with 1/2-13: M12 coarse thread has a 1.75 mm pitch, while 1/2-13 has a 1.954 mm pitch, so the nuts will not run on each other.

How do I cut a bolt to length?

Thread a nut on past the cut mark first, cut with a hacksaw or cut-off wheel, then file a small chamfer on the end. Backing the nut off over the cut chases the damaged thread back into shape so a new nut starts cleanly.

How do I know if my bolt is the right length after tightening?

Look at the nut face. The bolt end should be at least flush, ideally with one to three threads showing, and the nut should not have stopped against the smooth shank. Once tightened, torque it to spec with the Bolt Torque Calculator.

What is the formula for a screw rather than a bolt?

Wood and sheet metal screws are sized by embedment, not by a nut. The Screw Length Calculator handles those, and the Solid Rivet Length Calculator covers rivets, which need extra length to form the shop head.