What spanner for M10 bolt?

M10 bolt uses a 17 mm spanner and 8 mm hex key.

What spanner for M12 bolt?

M12 bolt uses a 19 mm spanner and 10 mm hex key.

What hex key for M16 bolt?

M16 bolt uses a 14 mm hex key and 24 mm spanner.

Metric Bolt Size Chart

M3–M48 · Spanner sizes · Hex key sizes · Printable PDF

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Quick Lookup

Bolt size

Spanner size

17 mm

Hex key (Allen)

8 mm

Shop M10 Bolts on Amazon Shop 17 mm Spanner on Amazon Shop 8 mm Hex Key on Amazon

Bolt	Spanner	Hex Key	Category
M3	5.5 mm	2.5 mm	Light fixings
M4	7 mm	3 mm	Light fixings
M5	8 mm	4 mm	Light fixings
M6	10 mm	5 mm	Light fixings
M8	13 mm	6 mm	Most common on tractors
M10	17 mm	8 mm	Most common on tractors
M12	19 mm	10 mm	Most common on tractors
M14	22 mm	12 mm	Most common on tractors
M16	24 mm	14 mm	Heavy machinery
M18	27 mm	14 mm	Heavy machinery
M20	30 mm	17 mm	Heavy machinery
M22	32 mm	17 mm	Heavy machinery
M24	36 mm	19 mm	Heavy machinery
M27	41 mm	19 mm	Large implements
M30	46 mm	22 mm	Large implements
M33	50 mm	22 mm	Large implements
M36	55 mm	24 mm	Large implements
M39	60 mm	24 mm	Large implements
M42	65 mm	27 mm	Large implements
M45	70 mm	27 mm	Large implements
M48	75 mm	30 mm	Large implements

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Most Common

Workshop Tips

M10 is king

M10 (17 mm spanner) is the single most common bolt on tractors and farm machinery.

Hex key = Allen key

Hex key sizes are for socket head cap screws. Standard hex head bolts use a spanner.

Torque varies

Spanner size is only part of the picture — tightening torque depends on bolt grade (8.8, 10.9 etc).

Metric Bolt Spanner & Hex Key Sizes — M3 to M48

This free reference chart covers every standard metric bolt size from M3 to M48. Find the correct spanner (AF across flats) and hex key (Allen key) size instantly — no formula needed.

Most used bolts on UK farm machinery

M8 — 13 mm spanner · guards, brackets, smaller fittings
M10 — 17 mm spanner · the most common bolt on tractors
M12 — 19 mm spanner · linkage pins, implement attachment
M16 — 24 mm spanner · wheel bolts, heavy structural joints
M20 — 30 mm spanner · drawbar bolts, large implement joints

Frequently Asked Questions

What spanner fits M10?: A 17 mm spanner. M10 is the most common bolt size on tractors and farm equipment.
What is the hex key for M12?: A 10 mm hex key (Allen key) is used for M12 socket head cap screws.
Do all M10 bolts use 17 mm?: Standard ISO hex head M10 bolts use 17 mm. Some DIN bolts and older designs may use 16 mm — always check with the actual fitting.