James Whittaker Calculator that converts pixel based mockups into em based layout. Vertical rhythm calculator.
A pixel is the ‘dot’ on your computer screen. Displays are measured in pixels across and down like 1024 × 768. These dots are unscalable and fixed in size. In some situations this is desirable and it would be better to set your measurements as pixels.
An em is a relative measurement based on the square of the font size at that particular point. As font sizes can vary across differing systems and displays the result is that the em measurement will always be relative and scalable.