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About this tool
Work out the minimum cap height and point size text needs to stay legible at a viewing distance, using visual-angle standards.
The Text Legibility Distance Calculator returns the minimum capital-letter height text needs in order to stay legible at a given viewing distance, in millimetres, inches, points and screen pixels. It works from visual angle rather than a rule of thumb: cap height = 2 × distance × tan(angle ÷ 2), using the character heights human-factors standards call for — roughly 20 to 22 minutes of arc for comfortable reading, about 16 arcminutes as the floor, and near 30 arcminutes for signage read at a glance. Useful for wayfinding, exhibition graphics, posters, packaging and presentation slides.
Open Text Legibility Distance Calculator on AltFTool — it loads instantly in your browser.
Enter the Viewing distance and its Distance unit (millimetres, centimetres, metres, inches or feet), then pick a Legibility target: Signage and wayfinding, Comfortable reading, Standard recommended, Absolute minimum, or Custom angle.
Set the Typeface class so the cap-height ratio matches — Grotesque sans at 0.716 em down to Transitional serif at 0.662 — and tick 'Also convert to pixels for a slide, screen or artboard' for a pixel size.
Read Minimum cap height in mm with the inch and point equivalents, plus Em box height, Typical x-height, Target visual angle and the 1 in / 10 ft rule of thumb, then press 'Copy result'.
Sizes come from the visual angle recommendations in ISO 9241-303 and ANSI/HFES 100.
Converts between the two using the real cap-height-to-em ratio of your typeface class.
Turns the physical requirement into a pixel size for a slide or artboard of known dimensions.
About 1 inch (26 mm) of capital height, which is where the familiar sign-maker rule of 1 inch per 10 feet comes from — it corresponds to a visual angle of roughly 30 arcminutes. For sustained reading rather than a glance you can go smaller, down to about 0.7 inch (18 mm) at that distance.
Around 16 minutes of arc of character height is the accepted floor for accurate reading by someone with normal vision. Below that, reading speed and accuracy fall off quickly, so standards put the recommended range higher, at about 20 to 22 arcminutes.
No. Font size is the em box; cap height is the height of an uppercase letter inside it, and it is typically 0.66 to 0.72 of the em. Helvetica sits at about 0.716 em and Times at about 0.662, so the same 100 pt setting produces visibly different letter heights.
Work backwards from the back row. For a 1.5 m tall projection viewed from 8 m, the 22-arcminute comfortable target needs about 51 mm of cap height, which works out at roughly 52 px of font size on a 1080-pixel-tall slide. As a safety net, keep the smallest text on any slide above 24 pt.
Add the Text Legibility Distance Calculator widget to your blog or website — free, responsive, no signup. Just keep the “Widget by AltFTool” credit link visible.
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