Thread Engagement Chart: Minimum Length by Size and Material

Thread engagement is the length of full thread that a screw or bolt actually shares with the tapped hole or nut. Too little and the thread strips before the bolt reaches its strength; beyond a certain length, extra thread adds almost nothing, because the first few threads carry most of the load. This chart gives that length two ways for common inch and metric threads: the theoretical minimum for a bolt and tapped part of equal strength, worked out from the standard engagement formula, and the shop rules of 1, 1.5 and 2 times the diameter that most people use for steel, cast iron and aluminium. Comparing the two shows how much margin the rules of thumb carry.

The short answer

Steel bolt in steel: at least 1 × the bolt diameter of full thread (a 3/8 in bolt, 3/8 in of thread).
Steel bolt in cast iron: about 1.5 × the diameter.
Steel bolt in aluminium, brass or other soft metal: about 2 × the diameter, or a steel thread insert.
Through a nut: a standard hex nut is roughly 0.8 to 0.9 × the diameter thick and is designed to be stronger than the bolt in the matching grade, so a full nut with a thread or two showing beyond it is enough.
Plastics and wood: these rules do not apply; use inserts or the fastener maker’s data.

Inch thread engagement chart

Find a size

Thread	Minimum, equal strength	As multiple of D	Threads engaged	Steel (1 × D)	Cast iron (1.5 × D)	Aluminium (2 × D)
#4-40 UNC	0.060 in	0.54 × D	2.4	0.112 in	0.168 in	0.224 in
#6-32 UNC	0.074 in	0.54 × D	2.4	0.138 in	0.207 in	0.276 in
#8-32 UNC	0.091 in	0.56 × D	2.9	0.164 in	0.246 in	0.328 in
#10-24 UNC	0.103 in	0.54 × D	2.5	0.190 in	0.285 in	0.380 in
#10-32 UNF	0.109 in	0.57 × D	3.5	0.190 in	0.285 in	0.380 in
1/4-20 UNC	0.138 in	0.55 × D	2.8	0.250 in	0.375 in	0.500 in
1/4-28 UNF	0.146 in	0.58 × D	4.1	0.250 in	0.375 in	0.500 in
5/16-18 UNC	0.176 in	0.56 × D	3.2	0.312 in	0.469 in	0.625 in
3/8-16 UNC	0.214 in	0.57 × D	3.4	0.375 in	0.562 in	0.750 in
3/8-24 UNF	0.226 in	0.60 × D	5.4	0.375 in	0.562 in	0.750 in
7/16-14 UNC	0.251 in	0.57 × D	3.5	0.438 in	0.656 in	0.875 in
1/2-13 UNC	0.289 in	0.58 × D	3.8	0.500 in	0.750 in	1.000 in
1/2-20 UNF	0.305 in	0.61 × D	6.1	0.500 in	0.750 in	1.000 in
5/8-11 UNC	0.364 in	0.58 × D	4.0	0.625 in	0.938 in	1.250 in
3/4-10 UNC	0.442 in	0.59 × D	4.4	0.750 in	1.125 in	1.500 in
7/8-9 UNC	0.519 in	0.59 × D	4.7	0.875 in	1.312 in	1.750 in
1-8 UNC	0.595 in	0.59 × D	4.8	1.000 in	1.500 in	2.000 in

Minimum = Le from the formula below at basic dimensions (a lower bound; real tolerances lengthen it). Rule-of-thumb columns are 1, 1.5 and 2 × the nominal diameter. Our arithmetic.

Metric thread engagement chart

Find a size

Thread	Minimum, equal strength	As multiple of D	Threads engaged	Steel (1 × D)	Cast iron (1.5 × D)	Aluminium (2 × D)
M3 x 0.5	1.7 mm	0.58 × D	3.5	3 mm	4 mm	6 mm
M4 x 0.7	2.3 mm	0.57 × D	3.3	4 mm	6 mm	8 mm
M5 x 0.8	2.9 mm	0.58 × D	3.6	5 mm	8 mm	10 mm
M6 x 1	3.5 mm	0.58 × D	3.5	6 mm	9 mm	12 mm
M8 x 1.25	4.7 mm	0.58 × D	3.7	8 mm	12 mm	16 mm
M10 x 1.5	5.9 mm	0.59 × D	3.9	10 mm	15 mm	20 mm
M12 x 1.75	7.1 mm	0.59 × D	4.0	12 mm	18 mm	24 mm
M16 x 2	9.6 mm	0.60 × D	4.8	16 mm	24 mm	32 mm
M20 x 2.5	12.0 mm	0.60 × D	4.8	20 mm	30 mm	40 mm
M24 x 3	14.4 mm	0.60 × D	4.8	24 mm	36 mm	48 mm

Same formula with metric basic dimensions (ISO 68-1) and stress areas (ISO 898-1). Our arithmetic.

The engagement formula

When the bolt and the tapped part are the same strength, the length of engagement at which the bolt breaks in tension just before the internal thread strips is:

Le = 2 A_t ÷ ( π K_n [ ½ + 0.57735 n (E_s − K_n) ] )

A_t = tensile stress area of the bolt
K_n = minor diameter of the internal thread (here the basic value, D − 1.082532 P)
E_s = pitch diameter of the external thread (here the basic value, D − 0.649519 P)
n = threads per inch (or per mm for metric)

For a 3/8-16 bolt: A_t = 0.0775 in², K_n = 0.3073 in and E_s = 0.3344 in, so Le = 0.214 in, or 0.57 × the diameter and 3.4 threads. That is the length at which the threads and the bolt are equally strong in theory. Real parts are made to tolerance, with a slightly larger minor diameter in the hole and a slightly smaller pitch diameter on the bolt, and both make the needed length longer. The 1 × D rule for steel is 1.7 to 1.9 times the theoretical figure, which covers tolerance, an imperfect tapped thread and a bolt that is a grade stronger than the part.

When the tapped part is weaker than the bolt

The rules of thumb grow with the softness of the tapped part: 1 × D in steel, about 1.5 × D in cast iron and 2 × D in aluminium. They do not grow in direct proportion to tensile strength. 6061-T6 aluminium has a minimum tensile strength of 38,000 psi, under a third of a Grade 5 bolt’s 120,000 psi, yet the usual rule only doubles the steel length. One reason is geometry: the internal thread strips by shearing at the bolt’s major diameter, so at the same length it has a larger shear area than the bolt’s own threads. The formal method in the engineering references compares those two shear areas and the shear strengths of both materials, using the tolerance limits of the actual thread class. For a critical joint in a soft or unusual material, use that method, a thread insert, or the fastener or part maker’s figure.

Aluminium and magnesium: 2 × D of tapped thread, or a helical-coil or key-locking steel insert where the part will be assembled often.
Cast iron: about 1.5 × D; gray iron is brittle, so avoid over-tightening and do not rely on a short thread at the edge of a casting.
Mild steel plate: 1 × D is the usual minimum for Grade 5 and Grade 8 bolts.

Worked example: a 1 in 8 TPI bolt in steel

For a 1-8 UNC bolt the equal-strength minimum is 0.59 in (4.8 threads). The steel rule asks for 1 in, about 1.7 times as much. To drill the blind hole for 1 in of full thread with a plug tap, our tapped hole depth calculator adds the tap’s 5 tapered threads (0.625 in) and 2 threads of chip room (0.25 in): drill 1-7/8 in at full diameter, or about 2-1/8 in to the tip of a 118° point.

Practical notes

More than about 2 × D of engagement in the same material adds very little strength: the load is concentrated on the first threads.
Thin plate: if the material is thinner than the required engagement, use a nut, a weld nut, a rivet nut or a clinch nut instead of tapping.
Thread inserts (helical coil or key-locking) move the load to a larger diameter in the soft part and are the standard fix for aluminium and magnesium.
A tapped thread cut with a worn tap or an oversize drill (low thread percentage) needs more length; see the thread % column on the tap drill chart.
For structural, lifting, pressure or vehicle parts, the engagement is set by the designer or the code; this chart is not a substitute for that.

Frequently asked questions

What is the minimum thread engagement for a bolt?

In steel, at least 1 × the nominal diameter; in cast iron about 1.5 ×; in aluminium about 2 ×. The theoretical minimum for equal-strength parts is about 0.5 to 0.6 × the diameter at basic dimensions, so the rules carry roughly a factor of two of margin.

What is the minimum thread engagement for a 3/8 bolt?

3/8 in in steel and 3/4 in in aluminium by the usual rules. The equal-strength theoretical minimum for 3/8-16 is 0.214 in.

How many threads should be engaged?

Count length, not threads: fine threads have more threads in the same length. As a check, 1 × D of a coarse thread is about 5 threads for 1/4-20 and 8 threads for 1-8.

Does more thread engagement make a joint stronger?

Only up to the point where the bolt breaks before the thread strips. Beyond about 1.5 to 2 × D in the same material, extra length adds almost nothing.

Sources and method

The minimum lengths are computed from the engagement formula at basic dimensions; the rule-of-thumb columns are multiples of the nominal diameter. All arithmetic is ours and can be checked from the formula on this page.

Machinery’s Handbook (Industrial Press), “Formulas for length of engagement” of screw threads; the same relation appears in FED-STD-H28/2B, Screw-Thread Standards for Federal Services (a US Government work).
ASME B1.1 and ISO 68-1 basic thread profiles; ISO 898-1 and ASME B1.1 tensile stress area formulas (computed on our UNC/UNF and metric charts).
Minimum tensile strengths as commonly published: SAE J429 Grade 5, 120,000 psi (1/4 to 1 in); 6061-T6 aluminium extrusions (ASTM B221), 38,000 psi.
Engagement rules of thumb (1 × D steel, 2 × D aluminium): Nord-Lock Group, “Thread engagement in a tapped hole”, as cited by our tapped hole depth calculator.

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Last reviewed: 24 October 2026. How we build and check our data: methodology and corrections. Spotted an error? Please let us know.