Verified by Rachel Mayfield, Supply Chain Analyst - July 2026
An M18 hex nut (ISO 4032) measures 27.0 mm across flats, so it takes a 27.0 mm spanner or socket. Standard nut thickness is 15.8 mm maximum, the thread is M18 x 2.5 coarse, and the thin (jam) version per ISO 4035 is 9.0 mm thick. The full specification table, thin-nut data and DIN comparison are below.
M18 nut specification table
| Property | Value | Standard |
|---|
| Thread | M18 x 2.5 (coarse) | ISO 261 |
| Fine pitch option(s) | M18 x 2.0 / 1.5 | ISO 261 |
| Width across flats A/F (spanner size) | 27.0 mm | ISO 4032 |
| Width across corners (min) | 29.56 mm | ISO 4032 |
| Nut thickness (max), standard nut | 15.8 mm | ISO 4032 |
| Thin / jam nut thickness (max) | 9.0 mm | ISO 4035 |
M18 nut spanner size
The M18 hex nut takes a 27.0 mm spanner (width across flats per ISO 4032). Across corners the hexagon measures at least 29.56 mm, which is the clearance you need for a socket wall or a recessed nut pocket.
The M18 nut has the same 27.0 mm hexagon in both ISO 4032 and the older DIN 934 standard, so any M18 nut in UK stock takes the same 27.0 mm spanner.
Standard, thin and nyloc M18 nuts
The standard M18 nut (ISO 4032, style 1) is 15.8 mm thick and is the default for general fastening. The thin or jam nut (ISO 4035) is 9.0 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. 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.
M18 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 M18 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 M18 nut?
An M18 hex nut per ISO 4032 measures 27.0 mm across flats, so it takes a 27.0 mm spanner or socket.
How thick is an M18 nut?
A standard M18 hex nut (ISO 4032, style 1) is 15.8 mm thick maximum. The thin or jam version (ISO 4035) is 9.0 mm thick.
What is the difference between ISO 4032 and DIN 934 M18 nuts?
For the M18 size there is no practical difference: both ISO 4032 and DIN 934 use a 27.0 mm hexagon with the same M18 x 2.5 thread. The standards differ only on a few sizes (M10, M12, M14, M22).
Which nut fits an M18 bolt?
A standard M18 bolt takes an M18 x 2.5 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 (M18 x 2.0 / 1.5) need matching fine-pitch nuts.
What class of nut goes with a class 8.8 M18 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.