How it computes
The proof load comes first: the tensile stress area of the thread times the nominal stress under proof load for the class, both from ISO 898-1:2013.
Fp = As · Sp
The target preload is a fraction of it, and the torque follows from the short-form torque equation:
Fi = (target / 100) · Fp
T = K · Fi · d
The verdict compares Fi with Fp. Proof load is the load a fastener must withstand without permanent set, so a preload equal to it is within, and only a preload above it exceeds.
Below the result, the page also shows the proof load ISO 898-1:2013 prints for the same thread and class, in Table 5 (coarse) or Table 7 (fine), with the difference. The two are not the same number: the calculator uses the exact As, while the table rounds As to three significant figures and rounds the load again. The difference is small but not zero, and the verdict uses the calculated value.
What it assumes
- K is assumed. The nut factor lumps thread friction, underhead friction and geometry into one number. It is not a property of the bolt, and it is the largest uncertainty in the result. The presets beside the field are common shop practice, not standardized: 0.20 for dry, unplated steel (the default), 0.15 for oiled, unplated steel, and 0.11 for steel lubricated with molybdenum disulphide paste. Where the torque matters, measure K on the actual parts, finish and lubricant.
- T = K·F·d is common shop practice, not standardized. It does not separate thread and underhead friction, and it takes no account of the joint.
- Nominal values. As is the defined tensile stress area, and Sp is the nominal proof stress. Neither is a measured property of a particular bolt.
- Not hot-dip galvanized, not structural bolting. ISO 898-1:2013 Table 5 qualifies two groups of its proof loads, and the calculator uses the general value for both. Hot-dip galvanized M8 and M10 bolts are cut to thread tolerance 6az to make room for the zinc, and reduced values from ISO 10684:2004, Annex A apply instead (footnote c): here the calculator overstates the proof load. Structural class 8.8 bolts from M12 take the higher-strength band, so their proof loads are higher, 50 700 N for M12 rather than 48 900 N (footnote d): here it understates it.
- Tension only. The verdict compares the preload that remains after tightening with proof load. It does not include the torsional stress the bolt carries while it is being tightened, which adds to the tensile stress at that moment.
- ISO metric threads, steel property classes. The thread and class tables cover M3 to M24, and the classes ISO 898-1:2013 specifies for carbon and alloy steel. Stainless steel (ISO 3506) is out of scope.
Worked example
M10 × 1.5, class 10.9, K = 0.20, target 65 % of proof load:
- As = 57.99 mm², from the ISO 68-1:2023 profile
- Sp = 830 MPa, class 10.9, ISO 898-1:2013
- Fp = 57.99 × 830 = 48 131 N
- Fi = 0.65 × 48 131 = 31 285 N, shown as 31.3 kN
- T = 0.20 × 31 285 × 10 = 62 571 N·mm, shown as 62.6 N·m
The same case is pinned as a test, with each step worked by hand, in
src/lib/calc/preload.test.ts.