Find how much thread you need so the threads do not strip.
Use this if you have it. Otherwise it is estimated from tensile strength.
Threads in lifting gear, pressure parts and structures follow their own codes, so use this as a first check.
Example Result
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The calculated result will appear here after you press Calculate.
A bolt in a tapped hole can fail in two ways: the bolt snaps, or the thread pulls out. A snapped bolt is usually the better failure, because it is easy to see. This calculator finds the length of thread needed for the bolt to fail first. Enter the bolt, its strength and the shear strength of the hole material. The answer gets longer as the hole material gets weaker.
Choose the thread, or enter another size and pitch.
Enter the bolt tensile strength.
Enter the tensile or shear strength of the hole material.
Keep the default safety factor of 1.5, or enter your own. It cannot go below 1.0.
Press Calculate, then add the imperfect threads at the screw tip to the result.
By O.E. — Partner, chairman of the board and CEO of a metal products manufacturer for 21 years; the company exports to 45 countries.
Drafted with AI assistance; calculations checked against primary sources and tested with the site's own code. About FlowUtil
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NASA's Fastener Design Manual gives the load that strips a tapped hole:
P = π × d_m × F_s × L ÷ 3
Why divide by 3? If the threads mated perfectly, half of the cylinder would be metal from the hole. NASA uses ⅓ to allow for mismatch between the threads.
Set this equal to the bolt's breaking load, A_t × S_u, solve for L, and add a safety factor:
L = SF × 3 × A_t × S_u ÷ (π × d_m × F_s)
This page uses SF = 1.5 unless you enter another value, and it will not accept less than 1.0.
For M12 coarse, A_t = 84.3 mm², d_m = 10.86 mm, and the bolt's tensile strength is 800 MPa. The bolt can break at about 84.3 × 800 = 67,400 N. Using NASA's approximation of shear strength as 0.6 × tensile strength for steel:
| Hole material | Shear strength | SF 1.0 | SF 1.5 |
|---|---|---|---|
| Steel, 800 MPa tensile | 480 MPa | 12.4 mm (1.0 × d) | 18.5 mm (1.5 × d) |
| Steel, 490 MPa tensile | 294 MPa | 20.2 mm (1.7 × d) | 30.2 mm (2.5 × d) |
| Aluminium 6061-T6 | 207 MPa | 28.6 mm (2.4 × d) | 42.9 mm (3.6 × d) |
The familiar rule of "one diameter in steel" is the SF 1.0 result when the hole and the bolt are equally strong. In softer material the answer grows fast. Take the shear strength from your material data, as castings and heat-affected zones can be weaker than the bar value.
In inch units, a 1/2-13 Grade 5 bolt (A_t = 0.1419 in², 120 ksi, d_m ≈ 0.450 in) into A36 steel with F_s = 34.8 ksi needs 3 × 0.1419 × 120 ÷ (π × 0.450 × 34.8) = 1.04 in at SF 1.0, or 1.56 in at SF 1.5.
A strong hole material pushes the formula down, but that does not make a short thread safe. Below about one diameter, a few imperfect threads make up much of the engagement, and the bolt's own threads may strip instead. Treat 1 × d as the floor. This calculator will not report less.
If the bolt is oversized for the job, size for the real load, including any preload:
L = SF × 3 × P ÷ (π × d_m × F_s)
For 10 kN into 6061-T6 with the M12 at SF 1.5, L = 6.4 mm, so the floor of 12 mm applies. Be careful here. Preload counts, and an M12 tightened to 75% of proof load is already carrying 36.7 kN, as the bolt stress calculator shows. Sizing for the full bolt strength is the safer habit, since it keeps the failure on the bolt.
Count only full threads. Merrick warns that a screw can have up to 3 imperfect threads at its end, and that screw lengths are toleranced short. Add those to your depth, and allow for the tap's lead-in with the tap drill size calculator.
Machinery's Handbook gives a more detailed shear area based on thread tolerances. It usually gives a shorter length than NASA's, so NASA's formula is the cautious one to use here.
Threads in lifting gear, pressure parts and structures follow their own codes, so use this as a first check. To work out total bolt length, use the screw length calculator.
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