SOCKET SIZE REFERENCE
Metric to SAE Socket Conversion
One inch equals 25.4 millimeters exactly. The conversion is a single division — but metric and SAE sockets are not interchangeable just because two sizes are close. A 3/8-inch socket is 9.53 mm, not 10 mm, and forcing it onto a 10 mm fastener rounds the head.
COMMON POINT OF CONFUSION
Drive Size vs Socket Size
A 3/8-inch drive and a 3/8-inch socket are not the same thing. The shared number is the source of most of the confusion around this conversion.
What the
Drive Is
The square on the ratchet. Its size — 1/4, 3/8 or 1/2 in — describes the width of that square, not the opening that grips the fastener.
What the
Socket Is
The opening that grips the fastener. Socket size refers to this opening — the measurement that matches a hex head across the flats.
Two Different
Dimensions
A 3/8-inch drive describes the ratchet’s square. A 3/8-inch socket describes the opening. The number matches; the physical dimension does not.
Drive
Coverage
A 3/8-inch drive set typically covers socket sizes from roughly 8 mm to 19 mm. The 3/8 in refers to the drive, not to the sockets in the case.
Standard
Reference
ISO 1174 defines the driving squares for hand socket tools — the dimension behind the drive number on every ratchet and extension.
SOCKET SIZE CONVERTER
Metric to SAE Socket Converter
The conversion is one number. inches = mm ÷ 25.4 and mm = inches × 25.4. The 25.4 factor is exact by definition — but a close conversion is not the same as a safe substitute.
INPUT PARAMETERS
Socket Size
mm
Example: 13 mm socket
CONVERTED RESULT
Live Result
Equivalent in inches
0.512
in
Converted from 13 mm using inches = mm ÷ 25.4.
Nearest SAE Fraction
1/2″
Fractional Size (mm)
12.70 mm
Size Difference
−0.30 mm
Fit Verdict
Not Safe
EQUATION
in = mm ÷ 25.4
13 ÷ 25.4 = 0.512
IN
13 mm
Metric socket
÷
25.4
Exact factor
OUT
0.512 in
SAE equivalent
Close is not the same as interchangeable.
A 1/2″ SAE socket is 12.70 mm — 0.30 mm smaller than the 13 mm fastener. The socket will sit loose, walk across the flats under load and round the head. Use a 13 mm socket.
CONVERSION REFERENCE
Common Metric Sockets in SAE
Reference conversions for common metric sockets. The “nearest SAE” column shows the closest standard imperial size — which is often close but rarely interchangeable.
Metric
Decimal inch
Nearest SAE
Difference
8 mm
0.315″
5/16″ (0.313″)
−0.05 mm
10 mm
0.394″
13/32″ (0.406″)
+0.32 mm
13 mm
0.512″
1/2″ (0.500″)
−0.30 mm
15 mm
0.591″
19/32″ (0.594″)
+0.08 mm
16 mm
0.630″
5/8″ (0.625″)
−0.13 mm
17 mm
0.669″
11/16″ (0.688″)
+0.48 mm
18 mm
0.709″
11/16″ (0.688″)
−0.53 mm
19 mm
0.748″
3/4″ (0.750″)
+0.05 mm
Only a difference under about ±0.10 mm is safe.
Beyond that the socket will either slip (too loose) or not seat fully (too tight). The 25.4 mm factor is exact by definition, following the international yard and pound agreement as published by NIST, resting on the metre as defined in the BIPM SI system.
METRIC TO SAE
Socket Conversion Chart
FACTOR
1 in = 25.4 mm
Common metric socket sizes, their closest SAE fraction, the exact inch size in millimeters, and a fit assessment. Exact means within about 0.10 mm; close means within about half a millimeter and usable only for a test-fit at low torque; do not substitute means the pairing is the classic one that rounds fasteners.
Exact
≤ 0.10 mm
Safe fit
Close
≤ 0.50 mm
Test-fit only
Do Not Substitute
> 0.30 mm
Rounds the head
Exact Factor
25.4
mm per inch
| Metric | Closest SAE | Exact (mm) | Fit |
|---|---|---|---|
| 6 mm | 1/4 in | 6.35 | Close |
| 8 mm | 5/16 in | 7.94 | Exact |
| 10 mm | 3/8 in | 9.53 | Do not substitute |
| 11 mm | 7/16 in | 11.11 | Close |
| 13 mm | 1/2 in | 12.70 | Do not substitute |
| 14 mm | 9/16 in | 14.29 | Close |
| 16 mm | 5/8 in | 15.88 | Close |
| 17 mm | 11/16 in | 17.46 | Close |
| 19 mm | 3/4 in | 19.05 | Exact |
| 21 mm | 13/16 in | 20.64 | Close |
| 22 mm | 7/8 in | 22.23 | Close |
| 24 mm | 15/16 in | 23.81 | Close |
| 27 mm | 1-1/16 in | 26.99 | Exact |
| 30 mm | 1-3/16 in | 30.16 | Close |
!
Metric-only sizes. Metric sizes such as 7, 9, 12, 15, and 18 mm have no SAE equivalent at all. For these, the metric socket is the only correct option — which is why a range serving mixed markets carries both systems rather than relying on the chart to bridge the gap.
REVERSE CONVERSION
SAE to Metric Socket Conversion
The reverse direction uses the same factor, inverted. As before, a close match is a starting point for a test-fit — not a safe substitute under torque.
| SAE | Exact (mm) | Closest Metric |
|---|---|---|
| 1/4 in | 6.35 | 6 mm |
| 5/16 in | 7.94 | 8 mm |
| 3/8 in | 9.53 | 10 mm — do not substitute |
| 7/16 in | 11.11 | 11 mm |
| 1/2 in | 12.70 | 13 mm — do not substitute |
| 9/16 in | 14.29 | 14 mm |
| 5/8 in | 15.88 | 16 mm |
| 11/16 in | 17.46 | 17 mm |
| 3/4 in | 19.05 | 19 mm |
| 13/16 in | 20.64 | 21 mm |
| 7/8 in | 22.23 | 22 mm |
| 15/16 in | 23.81 | 24 mm |
| 1 in | 25.40 | 25 mm (close) |
FIT SAFETY
Which Sizes Are Safe to Substitute
2
Classic pairings
that round fasteners
The rule is simple: under torque, use the correct system for the fastener. Close is for a test-fit on a non-critical fastener, not for a wheel nut or a head bolt.
RISKY PAIR 01
3/8 in on 10 mm
3/8 in in mm
9.53 mm
10 mm fastener
10.00 mm
Difference
−0.47 mm
Risk
Rounds the corners
RISKY PAIR 02
1/2 in on 13 mm
1/2 in in mm
12.70 mm
13 mm fastener
13.00 mm
Difference
−0.30 mm
Risk
Rounds the corners
Two pairings do the most damage, because they feel
like they fit and then round the fastener.
In both cases the SAE socket is smaller than the metric fastener. The socket feels seated, then walks across the flats under load and rounds the corners.
3
Three practices reduce the risk when you must get close.
Use a 6-point socket — it grips the flats and rounds less than a 12-point. Test-fit by hand before applying torque. And if the fastener is load-bearing, stop and use the correct size.
01
6-Point Over 12-Point
A 6-point socket grips the flats and rounds the head less than a 12-point under the same misalignment.
02
Test-Fit by Hand
Seat the socket by hand before applying torque. If it rocks or fits loosely, stop.
03
Stop for Load-Bearing
Wheel nuts, head bolts and structural joints demand the correct size. Do not substitute.
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 SAE Socket Is Equivalent to 10 mm?
There is no true equivalent. 3/8 in (9.53 mm) is the closest and is 0.47 mm smaller, so it rounds a 10 mm fastener under torque. Use a 10 mm socket.
Is 3/8 the Same as 10 mm?
No. 3/8 in equals 9.53 mm. The two sizes are close enough to feel like they fit, but the 3/8 in socket is smaller and rounds the fastener.
Are Metric and SAE Sockets Interchangeable?
Not reliably. A few pairs are effectively exact (8 mm and 5/16 in, 19 mm and 3/4 in, 27 mm and 1-1/16 in), but most are only close, and two common pairs (10 mm and 3/8 in, 13 mm and 1/2 in) round fasteners. Use the correct system under torque.
Is a 13 mm Socket the Same as 1/2 Inch?
No. 1/2 in equals 12.70 mm, which is 0.30 mm smaller than 13 mm. The 1/2 in socket often seats, but it rounds a 13 mm fastener under load.
What Does SAE Stand For in Sockets?
SAE stands for the Society of Automotive Engineers, which standardized the inch-based fractional sizing system for fasteners and sockets in North America. SAE socket sizes are inch fractions; metric socket sizes are millimeters.
How Do I Convert Metric Socket Sizes to Standard?
Divide the millimeter size by 25.4 to get inches, then read the result as the nearest fraction. For example, 14 mm ÷ 25.4 = 0.551 in, closest to 9/16 in (14.29 mm).