Signage Letter Height Calculator

This calculator starts from the US Sign Council's published Legibility Index rule of thumb — an average of 1 inch of letter height for every 30 feet of viewing distance, drawn from USSC's on-premise sign research (its full Standard Legibility Index table ranges 24-38 depending on illumination, letter style, and color, with 30 cited as the simplified average) — and layers on adjustments for the conditions that actually govern whether a sign gets read in time. Viewer speed matters because a driver has far less time to notice, read, and react than a pedestrian: the calculator scales the base height up by a factor of (1 + speed/80), so a car doing 65 mph needs noticeably taller letters than someone standing still at the same distance. Contrast and illumination move the number in opposite directions: low-contrast letter/background combinations multiply the height by 1.5 since weak contrast is harder to resolve at range, while an illuminated sign gets a 15% reduction because added light improves legibility. Serif letterforms get a 10% bump over sans-serif, reflecting the general finding that simpler, more uniform strokes hold up better at distance.

From the resulting minimum height, the calculator derives a stroke width (one-sixth of letter height, within the commonly cited 1/5–1/7 range for legible signage) and works the distance formula in reverse to estimate how many seconds a moving viewer actually has to read the sign, then converts that into a rough word-count budget at about 2.5 words per second. This is a distance-and-speed rule-of-thumb model, not a substitute for your local sign code: it's built for on-premise business signage in the USSC's own research context (a driver approaching and reading a sign in normal traffic). ADA guidance covers a different, much shorter-distance regime — pedestrian/interior wayfinding signage read from single-digit to low-tens-of-feet away — and the UK Sign Design Guide uses its own ratios for UK safety and wayfinding signage; neither is the source of the 30ft/inch base figure here, and real-world legibility standards vary by jurisdiction and application. Factors this model doesn't capture — letter spacing, sign mounting height, ambient glare, driver distraction — can matter as much as the ones it does.