The default pixel size for a browser is usually 16px.
You can change this at any time defining the font-size attribute in the body selector of your CSS to make your custome defualt size.
The em unit value is the default value for that font-size.
1.0em = 16px,2.0em = 32px and0.5em = 8px
You can see what I'm going for here, right?
So, to simplify in mathematical terms, we can find out the em size for 8px using this expression:
16px (default size = 100%) / 10 (our percentage base) * 5 (50% of the default font size) = 8px
Here is a fast lookup table with some usual conversions you can use - remeber: this table is only good for, if you had not reset the default size in the body selector:
px em % 8px 0.50em 50% 9px 0.56em 56% 10px 0.62em 62% 11px 0.68em 68% 12px 0.75em 75% 13px 0.81em 81% 14px 0.87em 87% 15px 0.93em 93% 16px 1.00em 100% 17 1.06em 106% 18px 1.12em 112% 19px 1.18em 118% 20px 1.25em 125% 21px 1.31em 131% 22px 1.37em 137% 23px 1.43em 143% 24px 1.50em 150%
Or use the PX to EM online conversion tool: http://www.w3schools.com/cssref/css_pxtoemconversion.asp
See bellow an article from W3 Schools about CSS Units
CSS has several different units for expressing a length.
Many CSS properties take "length" values, such as width, margin, padding, font-size, border-width, etc.
Length is a number followed by a length unit, such as 10px, 2em, etc.
A whitespace cannot appear between the number and the unit. However, if the value is 0, the unit can be omitted.
For some CSS properties, negative lengths are allowed.
There are two types of length units: relative and absolute.
Relative length units specify a length relative to another length property. Relative length units scales better between different rendering mediums.
Unit Description em Relative to the font-size of the element (2em means 2 times the size of the current font) ex Relative to the x-height of the current font (rarely used) ch Relative to width of the "0" (zero) rem Relative to font-size of the root element vw Relative to 1% of the width of the viewport vh Relative to 1% of the height of the viewport vmin Relative to 1% of viewport's* smaller dimension vmax Relative to 1% of viewport's* larger dimension
The absolute length units are fixed and a length expressed in any of these will appear as exactly that size.
Absolute length units are not recommended for use on screen, because screen sizes vary so much. However, they can be used if the output medium is known, such as for print layout.
Unit Description cm centimeters mm millimeters in inches (1in = 96px = 2.54cm) px pixels (1px = 1/96th of 1in) pt points (1pt = 1/72 of 1in) pc picas (1pc = 12 pt)
CSS Units: http://www.w3schools.com/cssref/css_units.asp
