M20 Nut Dimensions: A/F, Thickness & Spanner Size

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Reviewed by MLC Editorial Team | 16 August 2026

Verified by Rachel Mayfield, Supply Chain Analyst - July 2026

An M20 hex nut (ISO 4032) measures 30.0 mm across flats, so it takes a 30.0 mm spanner or socket. Standard nut thickness is 18.0 mm maximum, the thread is M20 x 2.5 coarse, and the thin (jam) version per ISO 4035 is 10.0 mm thick. The full specification table, thin-nut data and DIN comparison are below.

M20 nut specification table

Property	Value	Standard
Thread	M20 x 2.5 (coarse)	ISO 261
Fine pitch option(s)	M20 x 2.0 / 1.5	ISO 261
Width across flats A/F (spanner size)	30.0 mm	ISO 4032
Width across corners (min)	32.95 mm	ISO 4032
Nut thickness (max), standard nut	18.0 mm	ISO 4032
Thin / jam nut thickness (max)	10.0 mm	ISO 4035

M20 nut spanner size

The M20 hex nut takes a 30.0 mm spanner (width across flats per ISO 4032). Across corners the hexagon measures at least 32.95 mm, which is the clearance you need for a socket wall or a recessed nut pocket.

The M20 nut has the same 30.0 mm hexagon in both ISO 4032 and the older DIN 934 standard, so any M20 nut in UK stock takes the same 30.0 mm spanner.

Standard, thin and nyloc M20 nuts

The standard M20 nut (ISO 4032, style 1) is 18.0 mm thick and is the default for general fastening. The thin or jam nut (ISO 4035) is 10.0 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.

M20 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 M20 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)

References

ISO 4032:2012 - Hexagon regular nuts, style 1
ISO 4035:2012 - Hexagon thin nuts, chamfered
ISO 898-2:2022 - Mechanical properties of nuts

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Frequently Asked Questions

What spanner fits an M20 nut?

An M20 hex nut per ISO 4032 measures 30.0 mm across flats, so it takes a 30.0 mm spanner or socket.

How thick is an M20 nut?

A standard M20 hex nut (ISO 4032, style 1) is 18.0 mm thick maximum. The thin or jam version (ISO 4035) is 10.0 mm thick.

What is the difference between ISO 4032 and DIN 934 M20 nuts?

For the M20 size there is no practical difference: both ISO 4032 and DIN 934 use a 30.0 mm hexagon with the same M20 x 2.5 thread. The standards differ only on a few sizes (M10, M12, M14, M22).

Which nut fits an M20 bolt?

A standard M20 bolt takes an M20 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 (M20 x 2.0 / 1.5) need matching fine-pitch nuts.

What class of nut goes with a class 8.8 M20 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.

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