Wrench Size Chart: Metric Bolts (M6–M100) and Inch Bolts (¼″–4″)

Metric: across-flats size & thread pitch

A/F = width across flats. The "old" column is shown only where the historic DIN 931 value differs from today's ISO value — on older plant, check both. The fine-thread column lists the common pitches per ISO 261 / DIN 13, coarsest first; bracketed values are to be avoided where possible per ISO 261, and still finer pitches are standardised but unusual in plant engineering.

"A/F flange nut" is the metric heavy hex nut to ASME B18.2.4.6M (materials A194M/A563M) as fitted on metric-bolted ASME flanges. In every size it sits exactly one step higher in the ISO across-flats series than the standard nut — M20 therefore takes 34 instead of 30, M64 takes 100 instead of 95. Only the standard's preferred sizes are included (M12–M100); "—" means: not listed in B18.2.4.6M.

* Sizes above M64 fall outside the scope of DIN EN ISO 4014/4017/4032. Their across-flats sizes follow the ISO 272 series (nuts: DIN 934); on large-diameter bolts head dimensions are sometimes made to drawing — verify before planning tooling.

Why the rule of thumb "wrench size = 1.5 × M" almost always works

Anyone estimating the wrench size of a metric bolt on site multiplies the thread diameter by 1.5. Between M12 and M24 that is not an approximation but exactly the standard value: M12 → 18, M14 → 21, M16 → 24, M18 → 27, M20 → 30, M24 → 36 mm. The reason is mechanical: the required tightening torque grows roughly with the cube of the thread diameter (preload ~ d², lever arm ~ d) — and so does the load capacity of the hexagon across its flats. The A/F-to-diameter ratio therefore has to stay nearly constant; ISO 272 fixed it at around 1.5 and rounded the values onto a deliberately short series of wrench sizes, so that a manageable set of sockets covers the whole range.

That ratio is only constant in the middle of the range. Below M12 it is higher (M6: 1.67 · M8: 1.63 · M10: 1.60), between M27 and M52 it creeps up to 1.56 (M45/M48: A/F 70/75 instead of 67.5/72), and from M64 upwards it drops below 1.5 — at M85 it is down to 1.41 (A/F 120 instead of 127.5). For large-diameter bolting the rule therefore returns values that are too big. As an estimate it stays usable across the range; as a basis for ordering it does not.

The best-known genuine exception is M22: 1.5 × 22 = 33 mm — a wrench size that does not even exist in the ISO series. The standard value is 34 mm. M22 is also the only size whose across-flats dimension was moved upwards by the change of standards. Not quite clean either: M30 (46 instead of 45), M36 (55 instead of 54) and M6/M8/M10, where the rule comes out 1 mm short.

The reason the rule misleads on existing plant is that change of standards: in February 1992 DIN 931-1/DIN 933 were replaced by DIN EN 24014/24017 (today DIN EN ISO 4014/4017, widths across flats per ISO 272:1982). Four sizes changed: M10 17 → 16, M12 19 → 18, M14 22 → 21 and M22 32 → 34 mm. Older plant is therefore mixed — on a plant built in the 1980s an M12 takes the 19 mm, not the 18 mm. Only one direction is critical: an undersized socket will not go onto the head, whereas an oversized one (34 on an old 32 mm M22) appears to seat but rounds the corners under hydraulic torque — after which the joint can only be opened with a nut splitter.

The third deviation is deliberate — and it affects precisely the joints that matter in plant engineering. Three series intentionally run larger: high-strength structural assemblies to EN 14399-4/-6 (formerly DIN 6914/6915) with M20 → 32, M22 → 36, M24 → 41 mm (roughly 1.6 × M); the metric heavy hex nut to ASME B18.2.4.6M, which consistently sits one ISO step higher (column "A/F flange nut"); and the nuts for reduced-shank studs to DIN 2510-5 form NF — the classic high-temperature flange bolting — where sizes up to M20 use larger across-flats dimensions because of the greater bearing area required (M12 → 22, M16 → 27, M20 → 32 mm), reverting to the ISO values from M24 up. Pure flange standards such as EN 1515-1/-4 (PN flanges) or EN 1092-1, by contrast, prescribe no across-flats sizes of their own; they merely select bolt and nut standards, and the across-flats size follows from those.

Because hydraulic torque wrenches and cassettes follow the across-flats size and not the thread, the 1.5 rule leads straight to the wrong cassette on all of these series. Bottom line: 1.5 × M is fine for reaching into the right drawer; before hydraulic tooling goes on, the across-flats size gets measured — always at M22, at M10/M12/M14 on legacy plant, on structural, heavy hex and reduced-shank stud joints, and above M64.

Inch: wrench sizes for imperial bolts (UNC / 8-UN, ¼″–4″)

In petrochemical plants, in ASME-code plant engineering and on US/UK equipment, flange joints are imperial. There the 1.5 rule is practically the design principle: from ½″ upwards the across-flats size of a hex bolt to ASME B18.2.1 is exactly 1.5 × nominal diameter — without exception up to 4″ (⅜″ happens to match as well; ¼″, 5⁄16″ and 7⁄16″ deviate). The heavy hex version usual in flange work (ASME B18.2.2, e.g. ASTM A194 Gr. 2H nuts on A193 B7 stud bolts) sits exactly ⅛″ above it.

The 8-UN column is the flange bolting case: stud bolts for ASME B16.5 flanges (ASTM A193 B7/B8, A320) are made with 8 threads per inch throughout from 1″ upwards instead of UNC — a 4″ stud bolt is therefore 4″-8UN, not 4″-4UNC. That does not change the across-flats size: 6″ on the hex, 6⅛″ on the heavy hex nut. In flange work heavy hex is used from ½″ upwards; below that, the regular hex series applies.

What this means for sockets, cassettes and tensioners

In controlled bolting, tool selection follows the across-flats size, not the thread. The hydraulic square drive torque wrench (square drive typically ¾″ to 2½″) takes impact sockets: one tool body covers many sizes, only the socket is swapped — and it is picked by across-flats size. The low-profile or cassette wrench for tight spaces works with ring cassettes instead, and there every across-flats size needs its own cassette. That is exactly why the deviations in the tables above feed straight into the tool list: an M24 joint needs the 36 mm cassette with a standard ISO nut, but the 41 mm one with a high-strength structural set or a metric heavy hex flange nut. And anyone planning by 1.5 × M will not have the 34 mm for M22 in the set at all.

With bolt tensioning it is the other way round: there the thread is what counts. A tensioner is a hydraulic bolt tensioning tool — a preloading tool for bolts. It consists of a pressure body (hydraulic cylinder), bridge, puller/insert and nut driver; to change bolt size you do not swap the pressure body but the adaptor kit made up of puller/insert, bridge and nut driver. The puller/insert screws onto the free thread end and therefore has to match exactly — diameter and pitch. For imperial flange bolting that means the puller/insert follows the bolt thread series: UNC up to 1″ and the 8-UN series from 1 1/8″ upwards. Adaptor kits for ASME B16.5 flanges are consequently 8-UN as a rule — a 4″ stud bolt is 4″-8UN, not 4″-4UNC. On metric joints it is the coarse thread, but on large-diameter studs frequently the fine thread: an M100 × 6 puller/insert will not fit an M100 × 4 bolt. That is why the fine-thread pitch is in the table above.

Two further dimensions decide whether tensioning is possible at all. First, the free thread length above the nut: the puller/insert needs thread to grip, and roughly one bolt diameter above the nut face is required — for torque tightening two threads suffice. So anyone converting a joint from torquing to tensioning has to order the bolts about one diameter longer; it cannot be conjured up afterwards. Second, the nut driver: it grips the nut across its flats and therefore has to match the across-flats size. A tensioner consequently needs both figures — thread for the puller/insert, across-flats size for the nut driver. On top of that come the height available above the nut and the bolt spacing in the flange: on a tight bolt circle not every joint is reachable with standard tooling, and the work is then done with special adaptors or in two passes over every second bolt.

Why there is no universal torque table

Tightening torque cannot be read from bolt size alone. It depends on strength class (e.g. 8.8, 10.9, B7), friction and lubrication (friction coefficient µ) and the required preload — in torque-controlled tightening up to ~90 % of the torque is lost to friction. For pressurised flange joints, torque or preload is therefore calculated per joint (EN 1591-1, ASME PCC-1) and documented.

More precise than torque control is bolt tensioning (hydraulic axial bolt elongation): preload scatter typically ±5–10 % instead of ±25–30 %. Real HSOS project values give the order of magnitude: vessel flange with 1¾″ bolts → 4,800 Nm; HP pre-heater with M72 → 10,200 Nm.

FAQ: methods, tooling and bolts