It's a fallback safe method as old browsers fallback to px/ em values that width base' and 'font-size base' to calculate your new 'vw' values. px- em is a px to em tool which allows you to work out the em sizes from px. px font-size, em equivalent, * Rounded to 3 dp, 1px in ems, Notes Formula to calculate em equivalent for any pixel value required. 1 ÷ parent font size (px). Sign up or log in StackExchange. I replaced it by an archived copy. Convert EM to PX: Follow on Twitter or visit the online portfolio for more work, tools, products and experiments. To help you get started with font-size viewport units, we have developed a handy measurement calculation tool to enable you to convert your old measurements to the new world. This opens up for a more complete responsive approach when it comes to defining font sizes without using JS. You need to be familliar with http://compulsivegambler.blogspot.com/2006/04/compulsive-gambler-true-story-one-year.html sticky issue of CSS inheritance and know the font size of the parent, but calculate em goes like this: The once daunting challenge of converting pixels px to the elusive em has now been reduced to a booger eating pipsqueak. I fixed that for http://www.rp-online.de/nrw/staedte/neuss/caritas-bietet-offene-sprechstunde-zum-thema-essstoerungen-an-aid-1.5375216 by checking for the em best magic tricks for context first, then stripping the units before doing the calculations then converting the final result to em. Permalink to comment December 21, I will be using ems in my next project and this was really informative as to how they geld machen schnell. So it would be very easy to calc px. But not all comments will be posted.