CSS Units

/**
 * Font Notes
 *  1em == 16px in all browsers.
 *  All browsers can scale fonts with their sizes specified in pixels except IE8 and lower.
 *  Don’t know about IE9+ yet. Thus you should use em whenever possible so visitors can adjust
 *  their text size to their needs. Just use this formula:
 *
 *  pixel size / 16 = em size
 *
 *  Or use points instead with the formula:
 *
 *  pixel size * .75 = pt size
 *
 *  100% == 1em == 0.166044 inches == 16px == 12pt
 *  1in == 72pt == 96px
 */

1in == 72pt / 12pt = 6
1in == 144px

8.5in = 1224px
11in = 1584px

7.5in = 1080px
10in = 1440px

% / 100 = em

px * .75 = pt
px * 6.25 = %
px * 0.0625 = em (if default or inherited font size of 16px)
px * 0.0625 = rem (if default font size of 16px)
px * 0.010416666666667 = in
pt * 1.333~ = px
% * .01 = em

16px * .75 = 12pt / 72 = 0.166in

16px = 12pt = 

1in / 72pt = 0.013888888888889

16px = 12pt = in

1in = 72pt * 1.333333333333333 = 95.9999999999976px
1in = 72pt * 1.333~ = 96px

1in = 95.9999999999976px * .0625em = 5.99999999999985em

Conversion Chart

	px	em	ex	%	pt	in
px	1.000	0.06~	0.06~	6.250	0.750	0.01~
em	16.000	1.000	1.000	100.000	12.000	0.13~
ex	8.00~	0.50~	1.000	100.000	12.000	0.13~
%	0.160	0.100	0.05~	1.000	0.120	8.640
pt	1.33~	0.083~	0.041~	8.33~	1.000	0.01~
in	95.99~	5.99~	3.00~	599.99~	72.000	1.000

Defaulting to 96dpi. Used to think it was either 96dpi and 72ppi but ratified as 96dpi by the W3C?

ch: character width, based on the width of a zero or ½ of em if not defined in the font.
cm: centimeter, where 1cm = 10mm
em: calculated font-size height. Em (typography)
ex: x-height, where the 1ex is the height of the lowercase letter x or ½ of em if not defined in the font.
in: inches
mm: millimeter, where 1mm = 0.1cm
pc: pica, where 1pc is 12pt
pt: point, where 1pt = 1/72 inch
px: pixel, as in screen resolution
rem: root em, where 1rem is always 100% of the root element em size
vh: viewport height
vm: viewport minimum side length in IE9 and some older browsers
vmax: viewport maximum side length
vmin: viewport minimum side length
vw: viewport width

Unit Conversions

http://www.emdpi.com/fontsize.html Approximate Conversion from Points to Pixels (and Ems and %)