Socket size converter
Metric and imperial socket and spanner sizes, and which ones swap safely.
13 mm in inches
0.51″
Nearest imperial socket: 1/2″, 0.3 mm smaller: don’t swap
- Millimetres
- 13 mm
- Nearest 1/64″
- 33/64″
Show workings
- Inches13 ÷ 25.4 = 0.51″ ≈ 33/64″
How it works
An inch is exactly 25.4 mm, so a 9/16″ socket is 0.5625 × 25.4 = 14.29 mm. Metric and imperial sizes rarely match exactly; when they're within about 0.1 mm, they generally swap.
Loose fit (0.1–0.25 mm out) may be fine on a nut that isn't tight, but can round off a seized or high-torque fastener. Beyond that, use the right size. A socket smaller than the nut won't fit at all.
Worked example
19 mm vs 3/4″ (19.05 mm): only 0.05 mm apart, so they swap. 13 mm vs 1/2″ (12.7 mm): 0.3 mm apart, so a 1/2″ socket won't go on a 13 mm nut and a 13 mm socket on a 1/2″ nut is too loose.
Imperial to metric chart
| Imperial | mm | Nearest metric | Fit |
|---|---|---|---|
| 1/4″ | 6.35 | 6 mm | Don’t swap |
| 9/32″ | 7.14 | 7 mm | Loose fit |
| 5/16″ | 7.94 | 8 mm | Swaps |
| 11/32″ | 8.73 | 9 mm | Don’t swap |
| 3/8″ | 9.52 | 10 mm | Don’t swap |
| 7/16″ | 11.11 | 11 mm | Loose fit |
| 1/2″ | 12.7 | 13 mm | Don’t swap |
| 9/16″ | 14.29 | 14 mm | Don’t swap |
| 19/32″ | 15.08 | 15 mm | Swaps |
| 5/8″ | 15.88 | 16 mm | Loose fit |
| 11/16″ | 17.46 | 17 mm | Don’t swap |
| 3/4″ | 19.05 | 19 mm | Swaps |
| 13/16″ | 20.64 | 21 mm | Don’t swap |
| 7/8″ | 22.23 | 22 mm | Loose fit |
| 15/16″ | 23.81 | 24 mm | Loose fit |
| 1″ | 25.4 | 25 mm | Don’t swap |
| 1 1/16″ | 26.99 | 27 mm | Swaps |
| 1 1/8″ | 28.58 | 29 mm | Don’t swap |
| 1 3/16″ | 30.16 | 30 mm | Loose fit |
| 1 1/4″ | 31.75 | 32 mm | Loose fit |
Metric to imperial chart
| Metric | Inches | Nearest imperial | Fit |
|---|---|---|---|
| 5 mm | 0.2″ | 1/4″ (6.35 mm) | Don’t swap |
| 5.5 mm | 0.22″ | 1/4″ (6.35 mm) | Don’t swap |
| 6 mm | 0.24″ | 1/4″ (6.35 mm) | Don’t swap |
| 7 mm | 0.28″ | 9/32″ (7.14 mm) | Loose fit |
| 8 mm | 0.32″ | 5/16″ (7.94 mm) | Swaps |
| 9 mm | 0.35″ | 11/32″ (8.73 mm) | Don’t swap |
| 10 mm | 0.39″ | 3/8″ (9.52 mm) | Don’t swap |
| 11 mm | 0.43″ | 7/16″ (11.11 mm) | Loose fit |
| 12 mm | 0.47″ | 1/2″ (12.7 mm) | Don’t swap |
| 13 mm | 0.51″ | 1/2″ (12.7 mm) | Don’t swap |
| 14 mm | 0.55″ | 9/16″ (14.29 mm) | Don’t swap |
| 15 mm | 0.59″ | 19/32″ (15.08 mm) | Swaps |
| 16 mm | 0.63″ | 5/8″ (15.88 mm) | Loose fit |
| 17 mm | 0.67″ | 11/16″ (17.46 mm) | Don’t swap |
| 18 mm | 0.71″ | 11/16″ (17.46 mm) | Don’t swap |
| 19 mm | 0.75″ | 3/4″ (19.05 mm) | Swaps |
| 20 mm | 0.79″ | 13/16″ (20.64 mm) | Don’t swap |
| 21 mm | 0.83″ | 13/16″ (20.64 mm) | Don’t swap |
| 22 mm | 0.87″ | 7/8″ (22.23 mm) | Loose fit |
| 23 mm | 0.91″ | 7/8″ (22.23 mm) | Don’t swap |
| 24 mm | 0.95″ | 15/16″ (23.81 mm) | Loose fit |
| 25 mm | 0.98″ | 1″ (25.4 mm) | Don’t swap |
| 26 mm | 1.02″ | 1″ (25.4 mm) | Don’t swap |
| 27 mm | 1.06″ | 1 1/16″ (26.99 mm) | Swaps |
| 28 mm | 1.1″ | 1 1/8″ (28.58 mm) | Don’t swap |
| 29 mm | 1.14″ | 1 1/8″ (28.58 mm) | Don’t swap |
| 30 mm | 1.18″ | 1 3/16″ (30.16 mm) | Loose fit |
| 32 mm | 1.26″ | 1 1/4″ (31.75 mm) | Loose fit |
Questions
Which metric and imperial sockets are interchangeable?
The closest pairs are 8 mm ≈ 5/16″, 19 mm ≈ 3/4″, 27 mm ≈ 1 1/16″, and 11 mm ≈ 7/16″ (loose). See the chart below.
What does AF mean on a spanner?
"Across flats": the distance between opposite flat sides of the nut, which is what socket sizes measure. Imperial spanners are often marked AF.
Is Whitworth the same as AF?
No. Whitworth sizes (found on older British machinery) are named after the bolt diameter, not the nut, so they don't match AF or metric sizes.