Character height on a sign: calculating from reading distance

Where the ratio comes from

The calculation rests on angular size: the eye resolves a character reliably once it subtends a sufficient angle. That gives a linear relationship — twice the distance, twice the size. The 3–5 mm per metre range covers the spread of conditions: the lower bound assumes ideal circumstances, the upper bound real service.

One important caveat: what is measured is capital height, not point size. Point size includes ascenders, descenders and internal spacing, and at identical point size different typefaces differ noticeably in capital height. A specification quoting point size instead of character height reads ambiguously in fabrication.

Choosing the distance

The usual mistake is to measure from the sign to the nearest point where it becomes visible. It should be measured from the point where the decision is made: the junction, the lift lobby exit, the place where the corridor splits. That is where the sign has to be read, not once someone is standing beneath it.

A practical constraint follows. If the calculated character height does not fit a sign the interior can accommodate, the answer is not a smaller typeface but moving the sign closer to the decision point or splitting the information across two signs.

• Ceiling-mounted corridor sign: distance from the previous junction
• Wall sign by the lifts: distance from the lift doors
• Door sign: arm's length, 1–1.5 metres
• Floor plan panel: comfortable standing reading distance, roughly 0.5–1 metre
• External approach sign: distance from where a driver picks a lane

What else affects legibility

Contrast between character and background matters more than size: dark type on a light ground reads from further away than light on dark at the same size. Reversed-out type visually thins the stroke, so it is set one weight heavier.

Then come typeface and setting. Wayfinding needs faces with open apertures and characters that stay distinct in isolation — where lowercase l, capital I and the figure 1 do not merge. Letter spacing runs slightly wider than for text: a sign is read whole, at a glance, not line by line. Setting in all capitals reads worse than mixed case, contrary to common belief: the word loses the silhouette by which it is recognised.

Checking before fabrication

The only reliable check is to print a section at full scale, fix it at the intended height, and look at it from the design distance under the lighting that will actually be there. It takes under an hour and regularly reveals what a screen cannot: glare from a luminaire, a name that does not fit, insufficient contrast against the real wall.

Test the longest name on the list of zones, not an average one. The modular grid is sized for the worst case — otherwise a single name breaks the whole typology after artwork has been approved.