Difference between pixel and em (px vs em)

Lots of web developer have this question what is the difference between pixel and em unit in CSS. Well it can be simply explained by a simple px to em conversion formula.

1 ÷ parent font size (px) × required pixels = em equivalent

Here is a px to em conversion table:

pxfont-size	emequivalent	* Roundedto 3dp	1pxin ems	Notes
11	0.689	*	0.091
12	0.750		0.083
13	0.814	*	0.077
14	0.875		0.071
15	0.938	*	0.067
16	1.000		0.063	Browser standard default
17	1.064	*	0.059
18	1.125		0.056
19	1.188	*	0.053
20	1.250		0.050
21	1.313	*	0.048
22	1.375		0.046
23	1.438	*	0.044
24	1.500		0.042
25	1.563	*	0.040
26	1.625		0.039
27	1.688	*	0.037
28	1.750		0.036
29	1.813	*	0.035
30	1.875		0.033