Font size calculator: how big should slide text be?

The calculator turns the question into geometry. A PowerPoint slide is 13.333 by 7.5 inches, which is 960 by 540 points, so one point is the screen width divided by 960. A letter looks as large as the angle it covers at the eye, so the height it needs is the distance times that angle. Dividing by the cap height of a typical sans serif font, which we take as 70 percent of the font size, gives the size in points.

The reading target is our own rule of thumb: capital letters should cover 15 arcminutes at the farthest seat for body text and 10 for footnotes. A letter at the limit of 20/20 vision covers 5 arcminutes, and comfortable reading needs a clear reserve above the smallest letter a person can identify, so we chose three times for body text. Titles are the larger of 1.4 times the body size and 28 pt, and weaker eyesight adds 30 percent.

Three floors keep the result in line with common guidance. Body text never goes below 18 pt, which is what Microsoft's accessibility guidance for PowerPoint asks for. Footnotes never go below 12 pt and titles never below 28 pt. Guy Kawasaki's 10-20-30 rule asks for 30 pt in pitches; the calculator reaches that size for a large screen or distant seats and stays at 18 to 24 pt in a small room. Every size is rounded up to the next one in PowerPoint's font size list.

The lines and characters per slide are a rough estimate. They assume a one-line title, margins of half an inch and an average character width of half the font size, and real layouts differ with the font. Treat the numbers as a guide, and test the deck on the real screen. For a deck that people read on their own screens instead of watching it, smaller text can work, and the guide to fonts for presentations explains when.