calc() function

You can use the CSS calc() function to subtract e.g. the padding or border-width of an element from its actual width.

This ist the outer div with a total width of 100%. All inner divs have 8px padding both left and right as well as an 2px border around. All inner divs also have float: left.

width: calc(
 25%
 - 8px*2
 - 2px*2
 - 4px*2
)

width: calc(
 60%
 - 8px*2 /* (padding left/right) */
 - 2px*2 /* (border left/right) */
 - 4px*2 /* (margin left/right) */
)

width: calc(
 15%
 - 8px*2
 - 2px*2
 - 4px*2
)

Set outer width: 100%, 90%, 75%, 65%, 530px