Its worth pointing out that this all works because of the rule: * { box-sizing: border-box; } This rule enables you to set a % width of a column and specify

Its worth pointing out that this all works because of the rule: * { box-sizing: border-box; } This rule enables you to set a % width of a column and specify a padding in pixels without pushing the width of the columns beyond the 100%. (I spent ages trying to replicate the bootstrap grid system and just couldn't get it to work until I discovered this rule!)