I don't know if there's a canonical answer, but maybe some theories that you can point me to that I can read about. Do most designers just play it by ear? Or go from the gut?
Many designers will leave applications at their default setting - which is typically 120% of type size (at least for Adobe software).
How or when to alter this is highly dependent upon many things:
- Actual typeface
- Some typefaces inherently demand more or less line spacing. 120% for some typefaces can cause ascenders/descenders to overlap. While for other typefaces 120% may create large spacing almost equal to an addition line. IT's all in how the font was encoded.
- Target demographic
- Younger audiences have far less trouble tracking lines of text. So the older the demographic the more additional line spacing is beneficial.
- Desired aesthetics
- If their is a desire to make text unpleasant to read, as in disclaimers or legal-ease, tightening line spacing can make text seem less important. Conversely, increasing line spacing can add an air of openness and ease.
Ultimately line spacing is a choice a designer should consciously make, the same as choosing colors or choosing type sizes. You are correct in assuming there's no canonical answer. Each piece is unique and requires an individual assessment as to what is appropriate.
The goal in most instances is to ensure glyphs do not overlap or would not overlap - i.e. even though a
g and a
b aren't right above one another and other glyphs allow these to display without overlap, if they were on top of one another they shouldn't overlap. Of course, one may consciously choose to overlap glyphs for a specific aesthetic.
In terms of increased line-spacing, the goal for most text is to ensure readers can track one line to the next without losing their place. Rhythm is important and any increase in line-spacing should help to assist rhythm, not hinder it.
Personally, I tend to prefer less than 120% for headlines/sub-headlines - often 100-110%. For basic paragraph text, I generally prefer more than 120% for line-spacing - often 140-150%. But these are merely my personal preferences.
For the web... this all translates to roughly a 1.0 to 1.1 line-height property for headlines and a 1.3 to 1.5 line-height property for body copy.
You can't simply apply the same rule everywhere. It varies with your context for example :-
(1) Type of text components : Making the line height in the range of 110-125% makes large lines of text more readable inside main article or body. But same is not applicable for "disclaimer text in the footer" and "terms and conditions" as it shouldn't take much of your document or page space.
(2) Typeface or font used :- Fonts with glyphs cause the line height to increase. So you may observe different spacing with same line height value when using different fonts.
(3) Type of document :- Web documents are read on screen and slightly more line height increases readability, but same is not true for news paper or books.
Above are just few examples of many other factors which affects the document readability.
For block text readability I personally use this simple rule:
Vertical space between the lines (put in lowercase) should be ca. 180% of the height of the lowercase letters.
Other criteria (than readability) may exist, like e.g. economy of paper/space, so those can come in play for certain applications.
To illustrate the rule, I copy paste the letter "o" and fit it between the lines:
So the optimal spacing is when I can fit slightly less than two whole letters between the lines.
However, the only universal approach here is to simply adjust the value that is provided by your software, and look at the text to achieve this spacing. Otherwise it may be not possible to just tell the number because different software use different units and approaches to define this spacing. Usually those approaches are not even related to the optical line height (lowercase height), so it is better just to look at the result.