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1 em = default font-size of the browser which is generally 16px

now default font-size can be set to 10px to make calculations of em easier by the following trick:
body {
font-size: 62.5%;
}
(62.5% of 16px = 10px)

Now 14px will be simply 1.4em and so forth.

Also note, line-height is always relative to font-size, so if 1.5em or 150% will be 1.5 times the font-size.