Units - Font Sizes

DIN 16507-2
This draft standard defines (among many others) the following two font measures:

Font size (German: Schriftgröße)
This is the baseline distance for which the font was designed. A font should normally be identified and selected by this size, because the intended baseline distance is much more relevant for practical layout work than the actual dimensions of certain characters.

Font height (German: Oberhöhe)
This is the height in mm of letters such as k or H. Typically, the font height is around 72% of the font size, but this is of course at the discretion of the font designer.

If we write say “Helvetica 5.0”, then this means we have a font that was designed for a 5 mm line spacing. It will typically have an H that is 3.6 mm or 10.2 points tall (72% of 5 mm). Calculations become trivial: in a 60 mm high column, we can write exactly 60 mm / 5 mm = 12 lines. The baselines of text become neatly aligned with a millimeter grid, and if millimeters are used to describe both font size and font height, their relationship becomes easier to handle than if different units such as mm and points were used. Layout designers do not have to juggle any more with conversion factors such as 72.27 and 25.4. If you write “Helvetica 5.00/5.25” then this means that you use exactly the same font as above, but with 0.25 mm more baseline skip than it was designed for.

DIN 16507-2 contains a list of preferred metric font sizes, together with the corresponding preferred 72% font heights in mm. The table below shows in addition to these values from the standard also the corresponding preferred 72% font heights in Postscript points for easier comparison with the old font sizes. Note: the point sizes of US fonts do not always refer to the k/H height that is defined by DIN as the font height. Some font manufacturers (e.g., Knuth) also refer to the size of taller characters such as “(”, so be careful not to convert incompatible measurements and try to find out the baseline distance for which a font was originally designed of you want to convert properly to metric sizes.

Again: The font size refers to the baseline distance for which the font was designed, and is used to generally identify the font. The font height is the actual height of characters such as H or k. The font height is typically 72% of the font size as a preferred value, but this is of course left to the discretion of the font designer. One writes “Courier 6.0” to get the Courier font designed for 6 mm baseline distance (where the height of an H is typically 4.3 mm or 12.2 pt), and one writes “Courier 6.0/9.0” to get the same font but to use it with 50% more space between the lines.


Other proposals

Authors such as (Ó Brógáin, 1983) argue instead that font sizes should be based on the x-height. This clearly has a number of advantages:

The x-height can easily be measured directly in a sample printout and is not an invisible dimension found only in font data.
Whether two fonts appear to have the same height is primarily determined by whether their x-heights match.

The concept of x-height is also present in Greek and Cyrillic typography. Hebrew, Arabic and Devanagari glyphs can be aligned quite nicely with a Roman x-height, too.
Recommended baseline distances are often related to the x-height (2 seems to be a very common factor), therefore using the x-height as a reference dimension will in practice often also lead to round measurements for the baseline distance.

Possible disadvantages of using the x-height as the reference dimension for denoting a font height might:

In some applications (e.g., traffic and warning signs in some countries), text is commonly written uppercase only, or consists only of digits, so there are no visible lowercase letters as a measurement reference. However, as the fonts used in these applications usually also include lowercase characters, therefore the x-height is always well-defined (except for some numeric-only display devices such as 7-segment LEDs).

Chinese characters are more easily aligned with the H-height of Roman characters. However, as East Asian fonts typically also contain Roman lowercase characters, the x-height is always a well-defined quantity here, too.

Draft proposals for an international standard on font sizes drawn up in the late 1970s were based on the height of capital letters and did not find international agreement.

Both x-height and the size of capital letters is an attribute stored in existing font files, therefore scaling font sizes such that the x-height or H-height matches a specified length in millimeters is quite trivial to implement on top of existing font management mechanisms.
An idea that might at least be worth considering is to define a series of preferred font sizes. Unlike the values given in DIN 16507-2, this could be a geometric series in which the quotient of neighboring sizes approximates a root of the square root of two. The international standard paper sizes were designed to be magnified and reduced by factors of sqrt(2) or sqrt(sqrt(2)), and for example standard technical drawing pen sizes follow the same progression. Such a series of standard font sizes could either be designated in a millimeter length and made available via pull-down menus, or it could be designated by an index number, as is already done for ISO paper sizes.