WRENCH SIZE SELECTOR
What Size Wrench for This Bolt?
A bolt’s thread size doesn’t set its wrench size — the head does. An M10 takes 16 mm to ISO 4014, 17 mm to withdrawn DIN, or 14 mm to a JIS small-series head. Select by bolt size and standard, or measure across the flats.
TWO INPUTS · THREE OUTPUTS
How to Use the Wrench Size Selector
Pick a thread size and head standard, or measure the head. The selector returns a wrench size you can put on the bolt.
Pick Your
Input Method
Select a thread size and head standard, or enter a measured width across flats in mm or inches.
Read Three
Outputs
You get the wrench size, the socket size, and the same value in the other measurement system.
It Names
the Standard
An answer of “16 mm” is only right for an ISO 4014 head. The selector states which standard it assumed.
It Rounds Up,
Never Down
When a measurement falls between two sizes, the selector returns the next size up. Smaller won’t seat.
Confirm Before
High Torque
Check the answer against the bolt’s head marking, or by measuring, before you lean on the wrench.
WRENCH SIZE SELECTOR
How to Use the Wrench Size Selector
Pick a thread size and head standard, or measure the head across the flats. The selector returns a wrench size you can put on the bolt.
WRENCH SIZE
16
mm
ISO 4014 / 4017 M10 head. Measure the bolt to confirm.
Wrench
16 mm
Socket
16 mm
Equivalent
0.630 in
Hex Key
8 mm
BOLT SIZE TO WRENCH SIZE
Bolt Size to Wrench Size by Standard
DIMENSION
A/F · Across Flats
This is the data the selector uses, given as text so you can read it without the tool and check the tool against it. Metric hex bolts, width across flats in millimetres.
Standard Basis
ISO 4014
Current metric hex
Superseded
DIN 931
Still in the field
Sizes That Differ
4
M10 · M12 · M14 · M22
Hex Key Column
ISO 4762
Socket head, not hex head
| Thread | ISO 4014 / 4017 (current) | DIN 931 / 933 (withdrawn) | Hex Key |
|---|---|---|---|
| M3 | 5.5 | 5.5 | 2.5 |
| M4 | 7 | 7 | 3 |
| M5 | 8 | 8 | 4 |
| M6 | 10 | 10 | 5 |
| M8 | 13 | 13 | 6 |
| M10 Differs | 16 | 17 | 8 |
| M12 Differs | 18 | 19 | 10 |
| M14 Differs | 21 | 22 | 12 |
| M16 | 24 | 24 | 14 |
| M18 | 27 | 27 | 14 |
| M20 | 30 | 30 | 17 |
| M22 Differs | 34 | 32 | 17 |
| M24 | 36 | 36 | 19 |
!
The four marked rows are where the standards disagree, and they're the entire reason this page exists. At every other size, ISO and DIN give the same wrench. The hex key column is a different head type: those figures are for socket head cap screws of the same thread size, to ISO 4762, not for hex head bolts.
THE PROBLEM CASE
M10: The Bolt With Three Wrench Sizes
Take M10, the most common metric fastener in maintenance and automotive work, and the one that generates the most confusion. Three different heads carry the same thread size, and all three are in service right now.
FASTENER
M10 · Same Thread
ISO 4014 / 4017
16 mm
DIN 931 / 933
17 mm
JIS B 1180 small series
14 mm
Allen key (all three)
8 mm
THREE HEADS · ONE THREAD
ISO 4014
Current standard
16
mm
VS
DIN 931
Withdrawn, still in stock
17
mm
1 mm
between the two most common heads — enough
to strip the hex or round the corners
If you ordered M10 bolts a year ago and reorder this
year, the head may have changed.
The 17 mm wrench that worked on the old stock may not fit the new, and nothing on the invoice will have told you. Three seconds with a caliper across the flats ends the ambiguity. That measurement is the wrench size, and no standard can contradict it.
METRIC ↔ SAE SUBSTITUTION
Near-Swap Sizes: When a Metric Wrench Meets an SAE Head
Sooner or later you're holding a fractional head and a metric set, or the reverse. Some pairings are effectively identical; others will damage the fastener. The clearance column below is the metric wrench size minus the exact head size. A positive number means the wrench is larger than the head and will seat. A negative number means the wrench is undersized and won't go on.
| SAE Head | Exact mm | Nearest Metric | Clearance | Read |
|---|---|---|---|---|
| 5/16" | 7.9375 | 8 mm | +0.06 | Effectively identical |
| 3/8" | 9.5250 | 10 mm | +0.48 | Too loose; use the 3/8" |
| 7/16" | 11.1125 | 11 mm | −0.11 | Undersized; won't seat |
| 1/2" | 12.7000 | 13 mm | +0.30 | Turns; rounds under load |
| 9/16" | 14.2875 | 14 mm | −0.29 | Undersized; won't seat |
| 5/8" | 15.8750 | 16 mm | +0.13 | Good |
| 11/16" | 17.4625 | 18 mm | +0.54 | Too loose |
| 3/4" | 19.0500 | 19 mm | −0.05 | Effectively identical in practice |
| 13/16" | 20.6375 | 21 mm | +0.36 | Turns; not for torque |
| 7/8" | 22.2250 | 22 mm | −0.23 | Undersized |
| 15/16" | 23.8125 | 24 mm | +0.19 | Fair |
| 1" | 25.4000 | 25 mm | −0.40 | Undersized |
| 1-1/16" | 26.9875 | 27 mm | +0.01 | Effectively identical |
| 1-3/8" | 34.9250 | 35 mm | +0.08 | Effectively identical |
Three pairs are genuinely interchangeable.
5/16" and 8 mm, 1-1/16" and 27 mm, and 1-3/8" and 35 mm — all three differ by less than a tenth of a millimetre. The damage cases matter more: a 13 mm on a 1/2" head leaves 0.30 mm of play, enough to turn a loose fastener and to round a tight one. An 18 mm on an 11/16" head is 0.54 mm of play and shouldn't be attempted at all.
Every clearance figure comes from the exact legal
definition of the inch, 25.4 mm, which the BIPM maintains
as part of the SI and NIST Special Publication 811
restates for US practice. The fit readings are our own
working convention, not a standard. No standards body
publishes substitution bands. The clearance figure is the
hard number; the reading is practical guidance.
PRODUCTION REALITY
What Drives the Head Size You Actually Get
A head size is a production decision constrained by a standard, not a physical constant. A line running to ISO 4014 turns out a 16 mm head at M10. A line running to the withdrawn DIN drawing turns out 17 mm. The difference is the tooling, and the tooling is chosen before the first bolt is forged.
01
Tooling Decides
The head is set before the first bolt
02
Market Fit
Head size belongs on the spec sheet
03
Material Cost
Smaller head, less steel per part
04
Buyer Choice
Which standard to make the bolt to
!
Head size is a market-fit specification.
If your market's installers carry 17 mm wrenches out of habit, an ISO-spec M10 program will generate complaints that have nothing to do with quality, hardness or finish. Choosing between an ISO and a DIN head is a real cost-and-fit trade-off, not a formality. Which standard a bolt is made to is something a buyer can specify.
RANGE PLANNING
Wrench Sizes Your Range Should Carry
The short answer for most markets: cover the metric sizes that surround the divergent rows, because a range built only to current ISO sizing will miss the DIN-era stock still in service. Carrying the near-swap pairs matters as much as carrying the nominal sizes.
01
Cover the Divergent Rows
Both sides of M10, M12, M14 and M22 — a range built only to current ISO sizing will miss the DIN-era stock still in service.
02
Carry the Near-Swap Pairs
The genuinely interchangeable metric/SAE pairs matter as much as the nominal sizes.
03
Specify, Don't Shelf-Pick
Sizing, finish and branding are all part of the conversation when building a range for a market.
BUILDING A RANGE
Send us the wrench sizes you want to stock and the trades you're serving.
We'll tell you what we'd put in the opening order.
Frequently Asked Questions (FAQs)
Find answers to common questions about our hand tools, their care, and usage. If you need additional information, our customer support team is ready to assist you.
What size wrench do I need for a 10mm bolt?
It depends on the head standard. An M10 bolt takes a 16 mm wrench to ISO 4014, a 17 mm wrench to the withdrawn DIN 931, and a 14 mm wrench if it's a JIS small-series head. Measure across the flats to be certain.
Is wrench size the same as bolt size?
No. Wrench size is the head's width across flats, and bolt size is the thread diameter. A 10 mm bolt doesn't take a 10 mm wrench. The two are linked only by the standard that fixed the head dimension.
How do I measure a bolt head for wrench size?
Measure across two opposite flat sides, not corner to corner. That measurement is your wrench size. Use a caliper for anything you'll torque, and if the reading falls between two sizes, take the larger one.
Is a 13mm wrench the same as 1/2 inch?
No, and it's a near miss that causes real damage. A 1/2" head measures 12.70 mm, so a 13 mm wrench leaves 0.30 mm of play. It'll turn a loose fastener and round a tight one. Use the 1/2" wrench.
Is socket size the same as wrench size?
Yes. Both are sized by the width across flats, so a 13 mm wrench and a 13 mm socket fit the same head. Drive size is a separate dimension, and the socket size chart covers it.
Why doesn't my 10mm wrench fit an M10 bolt?
Because M10 names the thread, not the head. Depending on the standard the bolt was made to, an M10 head measures 16, 17 or 14 mm across the flats. If your 10 mm wrench won't go on, that's expected. It never would have.