The rule that matters
The smallest readable character height is roughly 1/200 of the viewing distance for comfortable reading, and 1/150 for a lecture where students take notes. If the back row is 14 metres away, the smallest character on screen must be 70 mm tall.
From character to screen size
If a slide has 20 lines of text, the smallest character is about 1/25 of the screen height. So a 70 mm character means a screen height of 1.75 metres, which is a 126-inch 16:9 panel. That is much larger than most halls install.
The two ways halls get it wrong
- Size by wall. They buy a 98-inch because it fits, and the back row is at 16 metres. The smallest character ends up at 45 mm, which is unreadable for notes.
- Size by budget. They buy two 86-inch screens side by side. Now there are two images and the lecturer has to choose which one to point at.
The table we use
| Back row distance | Min character | Screen height needed | 16:9 diagonal |
|---|---|---|---|
| 8 m | 40 mm | 1.0 m | 75" |
| 11 m | 55 mm | 1.4 m | 100" |
| 14 m | 70 mm | 1.75 m | 126" |
| 18 m | 90 mm | 2.25 m | 165" |
The alternative that is often better
In a hall deeper than 15 metres, a single large screen at the front is worse for the back row than two screens at the sides, because the viewing angle to the centre becomes too acute. Two 98-inch screens angled inward at 15 degrees each usually beat one 150-inch screen, and they cost less.
The thing nobody checks
Ceiling height and sight lines. A 150-inch screen needs its bottom edge at least 1.1 metres above the floor so the front row does not block it, which means the top edge is at 3 metres. In a hall with a 3.2 metre ceiling and a projector housing, that does not fit.
What to take to the meeting
The distance from the screen to the last row, the number of lines on a typical slide, and the ceiling height. Those three numbers settle the size before anyone argues about brands.