Thread engagement length calculator
A tapped hole should fail by breaking the bolt, not by stripping the threads. That sets a minimum engagement length that depends on the material.
MPa
MPa
Minimum engagement length
11.75mm
- As a multiple of the diameter
- 1.175
- Common rule-of-thumb depth
- 15.0
- Threads engaged
- 7.84
- Bolt breaking load
- 46 392
- Tensile stress area
- 57.990 mm2
Show the working
- Major diameter = 10
- Pitch = 1.5
- Bolt tensile strength = 800
- Tapped material shear strength = 200
- Tensile stress area = pi/4 * (10 - 0.938194 * 1.5)^2 = 57.9896
- Bolt breaking load = 57.9896 * 800 = 46 391.7
- minor = 10 - 1.082532 * 1.5 = 8.3762
- shear_per_mm = pi * 8.3762 * 0.75 = 19.736
- Minimum engagement length = 200 > 0 ? 46 391.7 / (19.736 * 200) : 0 = 11.7531
- As a multiple of the diameter = 10 > 0 ? 11.7531 / 10 : 0 = 1.175 31
- Common rule-of-thumb depth = 10 * (200 >= 250 ? 1 : 200 >= 150 ? 1.5 : 2) = 15
- Threads engaged = 1.5 > 0 ? 11.7531 / 1.5 : 0 = 7.835 39
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- bolt breaking load = tensile stress area x tensile strength
- thread shear area per mm of engagement is about 0.75 pi x minor diameter
- length = bolt load / (shear area per mm x shear strength)
The classic rules — one diameter into steel, one and a half into cast iron, two into aluminium — are this calculation with typical strengths substituted.
Beyond about two diameters the extra threads carry almost no load; the first three threads take most of it.