by http://www.freshblurbs.com/fontsizes

font-size: .625em; = 10px
font-size: .6875em; = 11px
font-size: .75em; = 12px
font-size: .8125em; = 13px
font-size: .875em; = 14px
font-size: .9375em; = 15px
font-size: 1.0625em; = 17px
font-size: 1.125em; = 18px
font-size: 1.188em; = 19px
font-size: 1.25em; = 20px

font-size: 1.313em; = 21px
font-size: 1.375em; = 22px
font-size: 1.438em; = 23px
font-size: 1.5em; = 24px
font-size: 1.563em; = 25px
font-size: 1.625em; = 26px
font-size: 1.688em; = 27px
font-size: 1.75em; = 28px
font-size: 1.813em; = 29px
font-size: 1.875em; = 30px