Photoshop tracking and kerning: what do the number values mean?

Question

Working on a site design where I'm trying to translate a tracking value of "100" to the CSS letter-spacing property.

When I specify a tracking value of "100", what am I saying? Definitely doesn't seem like percent or pixels.

I tend to eyeball to get what I want, but I'd rather run a conversion and not have to think about it if that's possible.

Answer 1

The answers given in the question linked to in AndrewH’s comment are correct.

As noted on this Photoshop help page:¹

Tracking and kerning are both measured in 1/1000 em, a unit of measure that is relative to the current type size. In a 6‑point font, 1 em equals 6 points; in a 10‑point font, 1 em equals 10 points. Kerning and tracking are strictly proportional to the current type size.

In other words, you can achieve the same CSS letter-spacing by dividing the Photoshop tracking value by 1,000 and using the quotient, in ems, as the letter-spacing value.

^{1 The page linked to is for Photoshop, but the same holds true for all other Adobe products. According to Wikipedia, Quark has 1/200 em as its base unit.}