Line length affects whether a block of text actually gets read more than any other typographic decision. And nobody sets it on purpose, because the default is whatever width the placeholder happened to be.
How do you work out characters per line?
Measure is counted in characters, so you have to derive it from inches. For a typical sans face the mean advance width across running text is about 0.5 em, with common faces falling between 0.45 and 0.55. That gives a rough, serviceable conversion:
characters per line = width in points / (0.5 × point size)
Applied to the spans this grid produces, at three type sizes:
| Span | Width (in) | 14 pt | 18 pt | 24 pt |
|---|---|---|---|---|
| 4 columns | 3.9778 | 40.9 | 31.8 | 23.9 |
| 6 columns | 6.0667 | 62.4 | 48.5 | 36.4 |
| 8 columns | 8.1556 | 83.9 | 65.2 | 48.9 |
| 9 columns | 9.2000 | 94.6 | 73.6 | 55.2 |
| 12 columns | 12.3333 | 126.9 | 98.7 | 74.0 |
Robert Bringhurst, in The Elements of Typographic Style, gives 45 to 75 characters as the range for a single column of continuous text. Read the table against that range and several layout questions settle at once. Body text at 18 point across six columns runs 48.5 characters and works. Across eight columns it runs 65.2 and works. Across the full twelve columns it runs 98.7 characters, a third longer than the upper bound. That failure is a specific one. The eye loses its place on the return sweep and re-reads or skips a line.
Turn it around. At 18 point, 66 characters need 8.25 inches of measure. The nearest span on this grid is eight columns at 8.1556 inches, which is 0.0944 inches short. So eight columns is the body-text span, and you make that decision once for the whole deck.
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What is leading and why does it matter?
Leading is the distance from one baseline to the next, quoted as a multiple of the type size. The choice compounds down a block:
| Leading | Line box at 18 pt | Five lines (in) |
|---|---|---|
| 1.00 em | 18.00 pt | 1.2500 |
| 1.15 em | 20.70 pt | 1.4375 |
| 1.20 em | 21.60 pt | 1.5000 |
| 1.35 em | 24.30 pt | 1.6875 |
| 1.45 em | 26.10 pt | 1.8125 |
| 1.60 em | 28.80 pt | 2.0000 |
Five lines at 1.20 against the same five lines at 1.45 differ by 0.3125 inches. That's more than a whole line, and it decides whether a text block clears the chart below it. Tight leading suits short lines and display sizes. A longer measure needs more leading, because leading is what the eye uses to find the start of the next line. At 1.35 leading, 18 point lines fit the 6.5 inch live height 19.26 times. That's the theoretical maximum text capacity of a slide, and roughly ten times more than any slide should use.
How do you size type for a room?
Then there's the room, which changes everything. Project a slide onto a 10 foot screen and it's magnified 9.0 times, because 10 feet is 120 inches against the slide's 13.3333. A capital letter takes up about 0.7 of the em box in a typical sans face, so an 18 point capital is 0.175 inches on the slide and 1.575 inches on the screen. For the viewer, the number that counts is the angle that capital subtends at their eye, measured in arcminutes:
| Size (pt) | Cap on slide (in) | Cap projected (in) | 15 ft | 30 ft | 45 ft |
|---|---|---|---|---|---|
| 11 | 0.1069 | 0.9625 | 18.38 | 9.19 | 6.13 |
| 14 | 0.1361 | 1.2250 | 23.40 | 11.70 | 7.80 |
| 18 | 0.1750 | 1.5750 | 30.08 | 15.04 | 10.03 |
| 24 | 0.2333 | 2.1000 | 40.11 | 20.05 | 13.37 |
| 28 | 0.2722 | 2.4500 | 46.79 | 23.40 | 15.60 |
| 32 | 0.3111 | 2.8000 | 53.47 | 26.74 | 17.83 |
| 40 | 0.3889 | 3.5000 | 66.84 | 33.42 | 22.28 |
The threshold used here is 15 arcminutes, three times the 5 arcminute letter that defines 20/20 acuity. Why three times? 20/20 is the threshold for identifying a letter under good conditions with unlimited time. A slide is read at a glance, in a room with the lights half up, by people some of whom aren't wearing their glasses.
What is the smallest type size at each viewing distance?
Read the table against 15 arcminutes and the guidance stops being folklore. At 15 feet, every size down to 11 point clears. At 30 feet, the smallest passing size is 18 point. At 45 feet it's 28 point, so a 45 foot room can't carry body text at all, and any slide built for one is a slide of titles and numbers. The next part needs a steady hand. Run the calculation backwards. At 30 feet, 15 arcminutes needs a projected cap height of 1.5708 inches. That's 0.1745 inches on the slide, or 17.95 point of type. Eighteen point is the smallest round size that clears a 30 foot room, and it holds the threshold out to 30.08 feet. Fourteen point holds it to 23.40 feet. The ratio of those two distances is 1.2857, the same as the ratio between the two sizes, because the relationship is linear.
One more number explains why decks that were fine on the laptop fail in the room. Take a laptop screen 13 inches wide, about a 15 inch diagonal, viewed at 24 inches. It magnifies the slide 0.975 times, so an 18 point capital subtends 24.44 arcminutes. That's 1.62 times what the same letter subtends at the back of a 30 foot room. Everything looks legible while you build. The check that matters is the arithmetic above, or a printout viewed from across an office. The screen you built it on can't tell you.