How you calculate your GRID ( HTML/CSS )

First, we decide the desired space between columns, in this case 20, then the number of columns (12) and last the width of the container (1000).

Container: 1000.

Gutters: 20.

Columns: 12.

With these values we can start calculating.

Multiply 20 by 11 (12 – 1 as we don't want the last column to have any space):

20 * 11 = 220
Subtract 220 from 1000:

1000 – 220 = 780
Divide the result 780 by 12:

780 / 12 = 65