Verified by Rachel Mayfield, Supply Chain Analyst - July 2026
An M12 hex nut (ISO 4032) measures 18.0 mm across flats, so it takes a 18.0 mm spanner or socket. Standard nut thickness is 10.8 mm maximum, the thread is M12 x 1.75 coarse, and the thin (jam) version per ISO 4035 is 6.0 mm thick. The full specification table, thin-nut data and DIN comparison are below.
M12 nut specification table
| Property | Value | Standard |
|---|
| Thread | M12 x 1.75 (coarse) | ISO 261 |
| Fine pitch option(s) | M12 x 1.5 / 1.25 | ISO 261 |
| Width across flats A/F (spanner size) | 18.0 mm | ISO 4032 |
| Legacy DIN 934 A/F (old stock) | 19.0 mm | DIN 934 |
| Width across corners (min) | 20.03 mm | ISO 4032 |
| Nut thickness (max), standard nut | 10.8 mm | ISO 4032 |
| Thin / jam nut thickness (max) | 6.0 mm | ISO 4035 |
M12 nut spanner size
The M12 hex nut takes a 18.0 mm spanner (width across flats per ISO 4032). Across corners the hexagon measures at least 20.03 mm, which is the clearance you need for a socket wall or a recessed nut pocket.
Watch for legacy stock: the older DIN 934 standard gave the M12 nut a 19.0 mm A/F instead of the current ISO 18.0 mm (the widths changed with ISO 272 in 1982). Old UK drawings and mixed fastener bins still contain DIN-size nuts, so if a 18.0 mm spanner slips on an M12 nut, try 19.0 mm before assuming the nut is damaged.
Standard, thin and nyloc M12 nuts
The standard M12 nut (ISO 4032, style 1) is 10.8 mm thick and is the default for general fastening. The thin or jam nut (ISO 4035) is 6.0 mm thick and is used for locking a standard nut in place or where axial space is tight - it is not a substitute for a full nut in a loaded joint. Thin nuts sold to the older DIN 439 standard keep the legacy 19.0 mm hexagon on this size, so check which standard the supplier quotes. Nyloc (nylon-insert) and other prevailing-torque nuts use the same hexagon as the standard nut but are taller; check the manufacturer's datasheet for exact heights.
M12 nut and bolt pairing
Match the property class of nut and bolt per ISO 898: a class 8 nut pairs with a class 8.8 bolt, class 10 with 10.9, class 12 with 12.9 (ISO 898-2). A nut of a higher class may always substitute a lower one, never the reverse - a lower-class nut on a high-class bolt shifts failure into thread stripping, which gives no visual warning before letting go. Proof-load values per class and nut style are tabulated in ISO 898-2; use the manufacturer's certified figures for critical joints. The mating bolt's full dimensions, torque values and tap drills are on the M12 bolt specifications card.
Related reference charts
Related standards
- ISO 4032 - Hexagon regular nuts, style 1, product grades A and B
- ISO 4035 - Hexagon thin nuts, chamfered
- ISO 898-2 - Mechanical properties of nuts with specified property classes
- ISO 261 - ISO general-purpose metric screw threads
- DIN 934 - Legacy hexagon nut standard (superseded, still in circulation)
Frequently Asked Questions
What spanner fits an M12 nut?
An M12 hex nut per ISO 4032 measures 18.0 mm across flats, so it takes a 18.0 mm spanner or socket. Legacy DIN 934 M12 nuts measure 19.0 mm across flats, so old stock may need a 19.0 mm spanner.
How thick is an M12 nut?
A standard M12 hex nut (ISO 4032, style 1) is 10.8 mm thick maximum. The thin or jam version (ISO 4035) is 6.0 mm thick.
What is the difference between ISO 4032 and DIN 934 M12 nuts?
For the M12 size the only practical difference is the hexagon: ISO 4032 uses 18.0 mm across flats while DIN 934 uses 19.0 mm. Thread, thickness and strength classes are interchangeable in practice, but the spanner size differs.
Which nut fits an M12 bolt?
A standard M12 bolt takes an M12 x 1.75 coarse-pitch nut (ISO 4032). Match the property class: class 8 nut for an 8.8 bolt, class 10 for 10.9, class 12 for 12.9. Fine-pitch bolts (M12 x 1.5 / 1.25) need matching fine-pitch nuts.
What class of nut goes with a class 8.8 M12 bolt?
Use a class 8 nut with a class 8.8 bolt (ISO 898-2). A higher-class nut may always substitute a lower one, never the reverse - a weaker nut on a strong bolt risks thread stripping below the bolt's rated load.