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1999-11-24, Rev. 1999-11-28.

All countries in the world except USA use millimeters (mm).   USA citizens:  To convert mm to inch, divide by 25.400.

Standard USA Bolt Hex Head and Hex Nut Dimensions  (All dimensions in units of mm unless otherwise specified.)

  Hex Head Standard Hex Nut Heavy Hex Nut Reduced Standard Hex Nut MS21043 Reduced Hex Nut
USA Size Desig- nation Standard Hex Head Socket Size Width Across Flats,
W
Width Across Corners,
C
Nut Socket Size (inch) Nut Height, H Width Across Flats, W Width Across Corners, C Socket Size (inch) Socket Size (inch) Nut Socket Size (inch) Nut Height, H Width Across Flats, W Width Across Corners, C Washer Face Diameter,
Dw
                             
0 not app.                          
1 not app.                          
2 not app.     0.1875 1.65 4.65 5.18              
3 not app.     0.1875 1.85 4.70 5.10              
4 not app.     0.2500 2.25 6.25 7.05     0.15625 2.96   4.343 5.233
6 not app.     0.3125 2.85 7.85 8.95   0.2500 0.18750 3.36   5.258 6.198
8 0.2500 6.30 7.10 0.34375 3.05 8.50 9.80     0.21875 4.24   6.198 7.366
10 0.3125 7.80 8.85 0.3750 3.10 9.10 11.70     0.25000 4.49   7.036 8.382
12                            
0.2500 0.4375 11.00 12.40 0.4375 5.60 11.00 12.60 0.5000   0.31250 5.44   8.814 10.668
0.3125 0.5000 12.30 14.10 0.5000 6.85 12.30 14.25 0.5625   0.37500 6.63   10.643 13.208
0.3750 0.5625 14.10 16.10 0.5625 8.15 14.10 16.10 0.6875   0.43750       15.748
0.4375 0.6250 15.95 18.05 0.6875 9.25 17.20 19.80              
0.5000 0.7500     0.7500       0.8750            
0.5625 0.8750     0.8750                    
0.6250 0.9375     0.9375       1.0625            
0.7500 1.1250     1.1250       1.2500            
0.8750 1.3125     1.3125       1.4375            
1.0000 1.5000     1.5000       1.6250            
1.1250 1.6875     1.6875       1.8125            
1.2500 1.8750     1.8750       2.0000            
1.3750 2.0625     2.0625       2.1875            
1.5000 2.2500     2.2500       2.3750            
                             

For a hexagon with non-rounded corners, C = W / cos(30°);  and C = 2a, where a is length of hexagon side.