§1The ISO metric system
The ISO metric thread is the world’s standard fastener thread — a 60° form, sized in millimetres, with the pitch stated directly rather than counted per inch.
It shares the unified inch thread’s 60° flank angle and general geometry, so the two are near-identical in form and differ chiefly in units and in how they are specified. Where the inch system counts threads per inch, metric names the pitch — the millimetres between adjacent threads — which is more direct and reads the intuitive way round: a bigger pitch number means a coarser thread, the opposite of the inch system’s reciprocal count. Two other things make the metric system pleasant to work with: the strength is stamped on the head as a two-number property class that decodes arithmetically (§5), and the stress area follows a formula that reproduces the standard tables exactly (§4). Between them, a metric bolt’s capacity can be worked out from its markings alone.
Contents§2Reading a designation
A metric thread is written M, then the nominal diameter, then — if it is not the standard coarse pitch — a multiplication sign and the pitch.
M10 × 1.5 means a nominal major diameter of 10 mm with 1.5 mm between threads. Because 1.5 mm is the standard coarse pitch for a 10 mm thread, the same fastener is usually written simply M10 — the pitch being implied. A fine thread must always be stated, so M10 × 1.25 is unambiguous: 10 mm diameter, fine 1.25 mm pitch. A fuller designation can carry the tolerance class too (M10 × 1.5 – 6g, where 6g is the common external-thread class, the metric counterpart of the inch system’s 2A). The habit to build is to read the second number as a distance, not a count: M10 × 1.5 has threads 1.5 mm apart, so a single turn of the nut advances it 1.5 mm — which is exactly the quantity that drives the torque relation and the angle-control method on the torque-and-tension page.
Contents§3Coarse and fine
Each metric size has one standard coarse pitch and one or more finer options, and the choice follows the same logic as the inch system’s UNC and UNF.
Coarse — M10 × 1.5, M8 × 1.25, M12 × 1.75 — is the default: quicker to assemble, more forgiving of dirt and damage, deeper-threaded so it strips less readily in soft materials, and the pitch you get if none is named. Fine — M10 × 1.25, M12 × 1.5 — has a larger stress area and so more tensile strength (§4), holds adjustment more precisely because each turn advances less, and resists vibration slightly better; against that it is slower to run down, easier to cross-thread, and its shallow threads are poorer in soft metal. The rule is unchanged from the inch page: coarse for general work and soft materials; fine for strength, fine adjustment and thin sections. Because coarse is the unstated default, a fine thread must always be written out in full — and mixing the two in one assembly is a classic and expensive mistake, since an M10 × 1.25 nut will start onto an M10 × 1.5 bolt and then jam.
Contents§4The tensile stress area
A bolt’s strength is reckoned not on its nominal diameter but on a tensile stress area that allows for the metal the threads cut away — and the formula reproduces the standard tables exactly.
The bracketed term is an effective diameter lying between the thread’s root and pitch diameters, which is where a bolt in tension actually fails. For M10 × 1.5: As = (π/4) × (10 − 0.9382 × 1.5)² = (π/4) × 8.593² = 58.0 mm² — appreciably less than the 78.5 mm² of a plain 10 mm bar, which is why a bolt is weaker than its nominal size suggests. Run the same formula across the range and it lands on the standard values every time: M6 → 20.1 mm², M8 → 36.6 mm², M12 → 84.3 mm², M16 → 157 mm². That agreement is worth noticing: the tabulated stress areas are not arbitrary, and one formula and a pocket calculator will reproduce them for any size — which, combined with the property class of §5, means any metric bolt’s capacity can be computed from first principles.
§5The property class
The two numbers stamped on a metric bolt head are not a part code — they are an arithmetic statement of its strength, and they decode in one step.
The first number × 100 is the ultimate tensile strength in N/mm²; the second number ÷ 10 is the ratio of yield to tensile. So 8.8 (the hero) means 8 × 100 = 800 N/mm² tensile, yielding at 0.8 of that = 640 N/mm². Multiply by the stress area and the bolt is fully characterised: an M10 8.8 yields at 640 × 58.0 = 37.1 kN and finally breaks at 800 × 58.0 = 46.4 kN. The same arithmetic runs up the range: 10.9 → 1000 N/mm² tensile, 900 N/mm² yield; 12.9 → 1200 N/mm² tensile, 1080 N/mm² yield. This is the system’s real elegance — where the inch system asks you to count lines and look up a grade, metric prints the numbers themselves, and two multiplications give the load. It is also the figure the torque-and-tension page’s 75%-of-proof preload target is taken from.
§6Choosing a class
Higher classes are stronger but not automatically better — hardness brings brittleness and notch sensitivity, so the class is chosen for the duty.
| Class | Tensile / yield (N/mm²) | Where it belongs |
|---|---|---|
| 4.6 | 400 / 240 | low-carbon commercial bolts; light, non-critical work |
| 8.8 | 800 / 640 | the general engineering standard — structural and machine work |
| 10.9 | 1000 / 900 | highly loaded joints where 8.8 will not do |
| 12.9 | 1200 / 1080 | socket head cap screws, high-duty machine assembly |
| Classes 8.8 and above are quenched-and-tempered alloy steels — the very heat treatment of the materials section — and as it warns, the strength is bought with toughness. A 12.9 bolt yields at 1080 N/mm² but is harder, more brittle, more sensitive to notches and thread damage, and more prone to hydrogen embrittlement if plated. So 8.8 is the sensible default for general work, 10.9 and 12.9 are reached for when the load genuinely demands them, and stainless classes (A2-70, A4-80) trade strength for corrosion resistance. Choose the class the joint needs, not the strongest available. | ||
§7Quick reference
The working core of the page on one card rack.
Designation
M10 × 1.5 = Ø10, pitch 1.5
coarse pitch implied if omitted
Stress area
As = (π/4)(d − 0.9382p)²
M10 → 58.0 mm²
Class
first × 100 = tensile
× second/10 = yield
M10 8.8
640 × 58 = 37.1 kN yield
800 × 58 = 46.4 kN ultimate
Choosing
8.8 the default
12.9 strong but brittle