Enter a viewing distance, a traffic speed, or a letter height you already have. Get the cap height and point size for all three legibility thresholds — not the single contradictory rule of thumb every other chart quotes. Built by the team at Digital Polo for designers, sign shops and brand teams.
Decodable if the viewer stops and looks. A floor, never a target.
Effortless for a stationary viewer. Interior and close range.
Read in under a second, in motion, in poor light. Use for exterior signage.
All heights are cap height — baseline to the top of a capital — not point size. Point size is shown for Helvetica / Neue Haas, whose cap height is 0.717 of the em; a different face at the same point size gives a different letter. Figures assume 20/20 vision, high contrast and a clean sans serif. For exterior signage, design against the read at a glance column.
The furthest a viewer with 20/20 vision can decode the word if they stop, look and concentrate. Useful as an absolute floor. Never use it as a design target.
Effortless reading by a stationary viewer. The right threshold for interior wayfinding, directories, menu boards and exhibition graphics.
Read in under a second, in motion, in rain, at dusk, by an eye that is no longer twenty. The threshold essentially all exterior business signage should meet.
Legibility is angular. A three-inch letter at 30 feet and a six-inch letter at 60 feet are optically identical — same angle at the eye, same difficulty, same result. That angle is measured in arcminutes, and every published sign chart is quoting the same relationship at a different angle without saying which.
For a viewing distance D in feet and a target angle θ in arcminutes, the required cap height in inches is D × θ ÷ 286.5. At 11.5 arcminutes that gives 25 feet of distance per inch of height; at 28.6 arcminutes it gives exactly 10, which is the familiar rule of thumb. Both are correct. They answer different questions.
Every figure here is cap height, not point size. 72 pt is one inch of em box, and the capital inside it is only about 0.72 inches tall — roughly 28% short. The point-size output above corrects for this using the cap ratio of the typeface you select, but the number you hand a fabricator should always be a cap height in inches or millimetres.
What this tool does not do: it does not check ADA minimums, local sign ordinances, or the sign-area limits in your lease. It does not account for stroke weight, letter spacing, case or contrast — all of which can sink a correctly sized letter. And it assumes good conditions. For the full treatment of all of those, read the signage spec guide on letter height by viewing distance.
Related reading: the print-ready file checklist, vector vs raster explained, and kerning, leading and tracking. If you produce signage artwork commercially, we also run white-label design for sign and banner shops.
It depends which of three thresholds you are designing for. For a letter to be merely decodable by someone who stops and looks, allow 1 inch of cap height per 25 feet of viewing distance. For comfortable reading by a stationary viewer, allow 1 inch per 15 feet. For a sign that must be read on a single glance — by a driver, in poor light, or by an older eye — allow 1 inch per 10 feet. Most exterior business signage should be specified against the 10-foot figure.
Because each one quotes a different legibility threshold without saying so. All of them are describing the angle a letter subtends at the eye, measured in arcminutes. The 1-inch-per-25-feet charts use roughly 11.5 arcminutes, the point at which a letter can still be decoded. The 1-inch-per-10-feet rule uses roughly 28.6 arcminutes, the angle at which a letter is read instantly. Neither is wrong; they answer different questions.
No. 72 points is one inch of em box, not one inch of letter. The cap height inside that em box is typically 0.70 to 0.72 of the em, so 72 pt Helvetica gives a capital about 0.72 inches tall — around 28% short of an inch. A true 1-inch cap height needs roughly 100 pt. The shortcut: multiply cap height in inches by 100 to get the point size.
It converts traffic speed to feet per second by multiplying mph by 1.47, then multiplies by the time a driver needs to see, read, decide and act. That gives the distance at which the sign must already be legible, which then converts to cap height at each threshold. Four seconds suits a simple business sign where the driver may turn in; six seconds is more realistic where a lane change is required.
Cap height is the distance from the baseline to the top of a capital letter. It is the dimension every signage legibility chart is really quoting, and the dimension a fabricator works to. It is not the same as point size, which measures the em box the letter sits inside. Always specify signage type in inches or millimetres of cap height, never in points.
No — this calculator returns legibility figures, not code minimums. The 2010 ADA Standards set separate legal floors for visual characters based on mounting height, starting at 5/8 inch and rising to 3 inches, plus requirements for stroke thickness, character spacing and contrast. Check the calculator result against the applicable ADA minimum and your local sign ordinance, and use whichever is larger.
Yes. Switch the units selector to metres and millimetres and the same ratios apply — the underlying relationship is angular, so it is unit-independent. The imperial ratios of 1 inch per 25, 15 and 10 feet become roughly 1 mm per 0.30, 0.18 and 0.12 metres.
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