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Inch (fraction) vs. metric: Measures and tools

The following table (Table 1) gives an overview of the most important inch dimensions in fractions, their conversion into millimeters and the corresponding wrench sizes for AF tools ("Across Flats") in millimeters.

Nominal measure
Inch [in]

Conversion base
Inch [in]

Equivalent in
Millimeters [mm]

Inch-AF-Tools
Effective wrench size [mm]

1⁄4

8⁄32

6.35

6.40

5⁄16

10⁄32

7.94

8.03

11⁄32

11⁄32

8.73

8.81

3⁄8

12⁄32

9.53

9.60

13⁄32

13⁄32

10.32

10.39

7⁄16

14⁄32

11.11

11.18

1⁄2

16⁄32

12.70

12.80

9⁄16

18⁄32

14.29

14.38

19⁄32

19⁄32

15.08

15.18

5⁄8

20⁄32

15.88

15.98

21⁄32

21⁄32

16.67

16.77

11⁄16

22⁄32

17.46

17.58

3⁄4

24⁄32

19.05

19.18

25⁄32

25⁄32

19.84

19.97

13⁄16

26⁄32

20.64

20.78

7⁄8

28⁄32

22.23

22.35

15⁄16

30⁄32

23.81

23.98

1

32⁄32

25.40

25.55

1 1⁄16

1 2⁄32

26.99

27.13

1 1⁄8

1 4⁄32

28.58

28.75

1 3⁄16

1 6⁄32

30.16

30.34

1 1⁄4

1 8⁄32

31.75

31.93

Table 1) The most important inch dimensions, their conversion into millimeters and the corresponding wrench sizes for AF tools

Table 2 gives an overview of the equivalent spanner sizes for different systems (metric; inch: Across-Flats (AF), Whitworth, BA). Example: A 5/16\" AF spanner can be replaced with an 8 mm spanner.

AF – mm

mm – Whitworth

mm – BA

AF – Whitworth

5 = 6BA

7 = 3BA

5⁄16 = 8

8 = 2BA

9 = 1BA

11⁄32 = 1⁄8

7⁄16 = 11

5⁄8 = 16

18 = 3⁄8

21 = 7⁄16

7⁄8 = 22

15⁄16 = 24

1 = 9⁄16

1 1⁄16 = 27

28 = 5⁄8

The tables can also be downloaded as a PDF file:

Yes, download table, please.