A Bolt Carries Load on Its Stress Area, Not Its Shank
A bolt does not carry load on its shank diameter. It carries it on the tensile stress area — a smaller effective section that accounts for the thread cutting into the material — and using the wrong one overstates a joint's capacity substantially.
The difference, in numbers
The thread pitch reference derives the stress area from the ISO 898 formula rather than reading it from a table:
| Thread | Coarse pitch | Shank area (πd²/4) | Stress area | Loss |
|---|---|---|---|---|
| M6 | 1.0 mm | 28.27 mm² | 20.12 mm² | 28.8% |
| M8 | 1.25 mm | 50.27 mm² | 36.61 mm² | 27.2% |
| M10 | 1.5 mm | 78.54 mm² | 57.99 mm² | 26.2% |
| M12 | 1.75 mm | 113.10 mm² | 84.27 mm² | 25.5% |
| M16 | 2.0 mm | 201.06 mm² | 156.67 mm² | 22.1% |
| M20 | 2.5 mm | 314.16 mm² | 244.79 mm² | 22.1% |
Between a fifth and nearly a third of the apparent section is not there. Size a joint on the shank and you have overstated it by that margin — and the error is proportionally worst on the small fasteners, which is where people are least likely to check.
Why smaller threads lose more
The formula subtracts a fixed multiple of the pitch from the diameter before squaring. On an M6 that subtraction is a large fraction of a small diameter; on an M20 it is a small fraction of a large one. So the proportional penalty shrinks as bolts get bigger, from 28.8% at M6 down to 22.1% at M20.
Which is a useful intuition: the thread costs you relatively more on small fasteners, and small fasteners are where an optimistic assumption is most likely to matter.
It is the same number the torque tool uses
Worth noting because duplicated implementations are exactly how tools drift apart. The stress area behind the bolt torque calculator's preload figure and the one in the thread reference are the same derivation, so a preload computed in one place matches a lookup in the other.
That matters in practice: an M10×1.5 at 57.99 mm² and a class 8.8 proof strength of 580 MPa gives a 75% preload of 25,225 N whichever page you started from.
Coarse and fine are not interchangeable
A fine thread has a smaller pitch, so it subtracts less and ends up with a larger stress area than the coarse thread of the same nominal diameter. On paper a fine thread is therefore slightly stronger in tension.
That is not the whole story, and it is not a reason to substitute. Fine threads are more easily cross-threaded, more sensitive to damage and dirt, and behave differently in soft materials and under vibration. Coarse threads are the default for good reasons. The choice belongs to whoever specified the joint.
Using the reference
The practical uses are identification and specification. Given a fastener, the table matches nominal diameter and pitch and tells you the tap drill for the coarse thread and the load-carrying area. Given a joint requirement, it tells you what a candidate size actually offers.
Two cautions. Tap-drill figures are for a standard thread-fit class and vary with the material being cut — confirm against the fit you need. And any structural or safety-critical joint takes its sizes from the designing engineer or the manufacturer's specification, not from a general table.
Where the formula comes from
The ISO 898 stress area takes the mean of the pitch and minor diameters as the effective load-carrying diameter, which works out as As = π/4 · (d − 0.9382p)² for standard metric threads.
The 0.9382 factor is what encodes the thread geometry, and it is why pitch appears at all: a coarser thread cuts deeper and removes more section. Deriving the area rather than tabulating it means every size and pitch combination is available, including fine threads and sizes that are not in a standard table.
Tap drills are a starting point
The reference gives a recommended tap-drill diameter for each coarse thread, which produces a thread of roughly the usual engagement. That figure assumes a standard fit class and a material that cuts predictably.
Softer and gummier materials, and threads in thin sections, both justify departing from it — and a thread cut in aluminium behaves differently from the same thread in steel. Confirm the drill against the fit class and material rather than treating the table figure as fixed.
Where thread strength stops being the limit
A bolt's stress area matters only while the bolt is the weakest part of the joint. Frequently it is not: the female thread can strip before the bolt breaks, particularly in aluminium or in a shallow blind hole, and the clamped material can yield or crush before either.
Thread engagement length is the usual control — enough engagement that the bolt is the failure mode rather than the tapped hole. That is a joint design question, and it is one more reason a general table informs a decision without making it.