When I write and speak, I often talk about using em units to size layouts giving the user the ability to resize the entire layout if they change their font size. In fact, chapter 5 of the book I co-authored with Greg Rewis is an entire project based on em units. Until all browsers zoom, this is my personal preferred method. My personal site is built using this method.
Last year, I wrote a blog post about Scaling fonts using em units and how I saw that affecting different types of users. A comment was made recently, and I wanted to address it in a new post while sharing a couple charts I made for myself. The comment said:
“A simple calculation as 1pt equals 1px at 72dpi (MAC)
1pt equals 1.33px at 96dpi (PC}
12px (pt) font in MAC is therefore 16px in a 96dpi PC.
If you have a parent element with a 11 pt font then to convert px dimensions to em you divide them by 14.63 (11pt x1.33) Dont interchange pt and px on a PC.”
First I'd like to address using points for the screen on either a Mac or a PC. Simply don't do it. Ever. Points are for print. I regularly use points (as well as inches and serif fonts) when I create a print style sheet. I updated an article recently called From screen to print: Creating a print CSS in Dreamweaver published both at Community MX and Adobe's Devnet Center. One thing it addresses is how to use pts for print style sheets.
Until 2000, Macs used a 12px browser default and PCs used a 16px default. We got used to seeing different things (and due to the 72dpi vs 96dpi difference, it was further compounded if points were defined). I'm under the impression (but can't find anything quickly to back it up) that the 72/96dpi issue is no longer a problem. If anyone can point me to a recent reference confirming or denying that, I'd love it. Either way, all current browsers on both platforms now use 16px as their font-size default.
A while back, I was messing around for my own entertainment (you know, partying on a Saturday night or something) and created a couple of charts that might be helpful. The first is font size equivalencies. This chart shows that setting the font to 100%, 1em, 16px, 12pt and medium are equivalent in the same font. It also shows how font sizes can vary greatly from font to font.
The second chart was an exercise showing divs sized using em-units with a variety of font sizing. The red line down the right side is a graphic at 800px. The body's font size is set to 100% (16px). The first three divs show the default font sizing with the divs set to three different widths. The second section shows all divs set to 50em with the font-sizing (placed on the div) set to different units. It's an interesting exercise if you'd like to more clearly understand the effect of font sizes on the size of an em unit.
The interesting thing to me in the second chart, based on the gentleman's comment in my blog, was that viewing this chart on both a mac and a pc the lengths of the divs are the same. Yes, there's a very slight difference in actual font size on the PC – two of the lines of text slightly wrap to the next line – but the 50em width of the div is still exact on both platforms. Are we really that different anymore?