Nut lengths are 3x thread diameter
M5; length 15mm; across flats 8mm
M6; length 18mm; across flats 10mm
M8; length 24mm; across flats 13mm
M10; length 30mm; across flats 17mm
M12; length 36mm; across flats 19mm
M16; length 48mm; across flats 24mm
M20; length 60mm; across flats 30mm