| Visual angle | Character height (mm) | Character height (px) |
|---|---|---|
| 10 arcminutes | – | – |
| 15 arcminutes | – | – |
| 20 arcminutes | – | – |
Calculations are based on visual angle and viewing distance.
Use this tool to help calculate the minimum caption font height (in mm) based on the viewing distance from the caption screen.
Enter the seating distance in metres using the slider, or type a precise value into the number field. Seating distance should be measured from the furthest seat intended for caption users to the caption surface (for example, the last row of the auditorium).
(Optional) You can also enter the physical width of the caption screen (in mm) and the horizontal resolution (in pixels) to calculate an approximate equivalent font size in pixels.
The results are shown using different visual angles, measured in arcminutes.
Visual angle describes how large the text appears to the viewer’s eye, rather than its physical size alone.
10 arcminutes - Marginal legibility (may be difficult to read for some users)
15 arcminutes - Readable for many users in good conditions
20 arcminutes - Comfortable caption reading
These values are derived from established research in visual acuity and legibility, and are commonly used in captioning, signage, and accessibility guidance.
Results are a guide and may vary depending on font choice, typeface weight, contrast, and projection or screen quality.
Environmental factors such as lighting, screen brightness, and line length can also affect readability.
Where possible, choosing a larger size than the minimum will improve accessibility for a wider range of audiences.
Caption screen width should also be considered. Very wide screens can produce long lines that are harder to read, while very narrow screens can cause excessive line breaks.
As a general guide, based on subtitle and captioning practice:
Aim for no more than 35 - 45 characters per line (including spaces)
This is typically 6 - 9 words per line