nut and bolted terminology

[i2i]

1/4 - 20 x 3/4

Sounds complicated, but all you have to do is break it down into the three parts.

1/4 = outside diameter of thread = 1/4" = 6.35mm
20 = the pitch of the thread 20 threads per inch
3/4 = the length of the thread = 3/4" = 19mm
Hex head, that's the shape of the bolt head which in this case is a cylindrical head that has a hexagonal hole for an allen key

If you're looking to replace the whole thing with metric, ie. if you're making it from scratch it would be 6mm x 20mm Hex head/cap head.

The original bolt is probably 1/4 unf x 3/4"


Edit, i've just noticed a few american sites are using "hex head" to describe a standard hexagonal bolt (six sided bolt head)