Readable LED Sign Tips: Letter Size, Distance and Speed

A phone held up in a crowd or at an arrivals gate is a small sign. Whether anyone can read it depends on letter height, distance, contrast and how long each letter stays in view. This guide from the LED Scroller team applies sign-industry and road-sign rules to phone, tablet and laptop screens, shows the calculation, and turns the results into settings for LED Scroller.

Those rules were written for printed, lit and electronic signs, not phones, so the results are estimates. Where advice is our judgement rather than a measured result, we say so.

Sign rules of thumb: distance per inch of letter height

Sign makers use a legibility index: feet of viewing distance per inch of letter height. Divide the distance in feet by the index to get the letter height in inches. An index of 25 ft/in is about 3 m of distance per centimetre of letter height.

The United States Sign Council (USSC) table of index values, as reproduced by the trade magazine Signs of the Times, runs from 20 to 38 depending on lighting, typeface and colours. Externally lit black Helvetica on white scores 29 in mixed case and 25 in all capitals; white neon on black scores 38 and 32, red neon on black only 29 and 25. All are for static signs and normal vision.

The US Access Board's review of electronic road-sign research reports about 35 ft/in for average observers, but to cover 85% of viewers the index fell to 22 ft/in for younger groups and 17 ft/in for older ones (Garvey and Mace). The ADA Standards for wall signs are stricter: capitals at least 5/8 inch (16 mm) tall plus 1/8 inch (3.2 mm) per foot beyond 72 inches, roughly 8 ft/in.

Below, the ADA formula gives a comfortable distance for nearly everyone, 17 ft/in a limit for a mixed crowd, and 25 ft/in a best case.

How tall the letters get on a phone, tablet and laptop

LED Scroller draws your message on a grid of LED dots spanning the full banner height. In fullscreen, capitals fill roughly two-thirds to three-quarters of the screen height (we use 70%), so letter height depends on the screen's short side in landscape, not on the font size slider.

Screens. A 6.1-inch iPhone 16 has 1179 pixels on its short side at 460 pixels per inch: 1179 ÷ 460 = 2.56 in (6.5 cm). A 6.7-inch phone with a 19.5:9 shape gives 6.7 × 9 ÷ √(19.5² + 9²) = 2.81 in (7.1 cm). We assume 2.7 in (6.9 cm). An 11-inch iPad Air gives 1640 ÷ 264 = 6.2 in (15.8 cm); a 13 to 14-inch 16:10 laptop about 7.2 in (18 cm).

Letters and distance. 70% of 2.7 in is about 1.9 in (4.8 cm). For the phone, the ADA formula gives 6 ft + (1.9 − 0.625) × 8 ≈ 16 ft (4.9 m); 1.9 × 17 ≈ 32 ft (9.8 m); 1.9 × 25 ≈ 48 ft (14.5 m).

Our reading (judgement): a phone works well to about 5 m (16 ft) and is a stretch beyond 10 m (32 ft); a tablet or laptop roughly doubles both. Plan with the left-hand columns, and for longer distances see when to use an LED scroller.

Two or three letters at a time: keep messages short

Big letters mean fewer of them. With the spacing LED Scroller adds, each capital is about 0.8 screen heights wide. A phone in landscape is about 2.2 times wider than tall, so it shows about 3 capitals at once; an 11-inch tablet (about 1.45:1) and a 16:10 laptop show about 2.

That window is narrow. In a study summarised by the Access Board, Peli and Fine found readers with normal vision did well with streaming text when about four to five letters were visible; readers with vision impairments needed six to seven. A phone shows fewer.

Someone who looks up mid-message waits for the start to come round, so loop length matters. LED Scroller leaves one blank screen between repeats, about three letter widths on a phone. We aim for a loop of about 10 seconds (judgement; US highway guidance keeps a two-phase message cycle within 8 seconds). At 1.7 letters per second, (14 + 3) ÷ 1.7 ≈ 10 seconds, so keep phone messages to about 14 characters including spaces: WELCOME HOME fits, WELCOME HOME GRANDMA ROSE does not.

One road-sign researcher cited by the Access Board (Dudek) recommends capitals because readable lower case needs a 7 × 9 dot matrix, and in LED Scroller lower-case letters are only about 72% as tall as capitals, so use capitals for names and short words. See our LED banner message ideas.

A scroll speed people can read

Kang and Muter (1989), as summarised by the Access Board, found smooth scrolling read as well as static text, and readers preferred it. But Bentzen and Easton found static messages clearly better for readers with vision loss, and Harris and Whitney advised leaving scrolled text on screen for at least twice the normal reading time.

The 2009 US Manual on Uniform Traffic Control Devices (MUTCD) bans scrolling text on highway message signs and recommends about 1 second per word, never under 2 seconds per phase. Drivers only glance, but moving text costs everyone reading time.

LED Scroller's speed slider runs from 1 to 20. The text advances by a fraction of a dot column per frame, in proportion to the number, so the same number looks faster at high font sizes, where dots are bigger. By our calculation from the tool's settings, speed 10 at a font size near 400 px moves a capital across a phone screen in about 2 to 3 seconds: a good start for short messages.

Colour, contrast, bold and glow

Legibility depends on luminance contrast more than on hue. WCAG, the web accessibility guideline, computes it as (L1 + 0.05) ÷ (L2 + 0.05) from the relative luminance of the lighter and darker colours, giving 1:1 to 21:1, and asks for at least 4.5:1 for normal text. Its formula weights green about ten times more than blue, so equally vivid colours are not equally readable.

The USSC table agrees: white neon on black scores 38, red neon 29, about a quarter less distance. Keep the background black: light-on-dark messages gave a 29% longer night-time legibility distance than dark-on-light in work by Garvey and Mace cited by the Access Board. Multicolour suits a party marquee, but at a distance its blue and purple letters fade first (judgement).

Font size sets the dots, not the letter height

The font size slider sets how many rows of dots the banner uses: low values give many small dots, high values a few large ones. Road-sign studies cited by the Access Board recommend a 5 × 7 dot matrix, so capitals 7 dots tall. In LED Scroller, capitals are about 12 dots tall at 400 px, 8 at 450 px and only 4 at the 500 px maximum, where letters lose their shape. So use the largest setting that keeps capitals at least 7 dots tall, roughly 380 to 450 px. See how LED dot-matrix displays work.

Bold and glow

In a sans-serif such as Arial, the stem of a capital I is about 13% of the letter height in regular weight and 20% in bold. The ADA Standards require 10% to 30% for wall signs; bold sits mid-range and survives the dot grid better. LED glow puts a halo around each dot, which helps dots merge into strokes from afar (our judgement); if letters look smeared up close, switch it off.

Blink: rarely, slowly, one word

Flashing content can trigger seizures in people with photosensitive epilepsy. WCAG success criterion 2.3.1 says web pages should not contain anything that flashes more than three times in any one second, unless the flashes are below the general and red flash thresholds. A flash is a pair of opposing changes in relative luminance. The area thresholds assume roughly a 10-degree field of view, which a nearby fullscreen phone exceeds, so the rate counts.

LED Scroller's blink switches the dots every half second: one on-off cycle per second, under the limit even counting each change separately, and the rate is fixed. WCAG is a web standard, not a sign rule; we use it as a sensible bar.

Blinking also halves reading time, and on a scrolling banner some letters pass while dark. Keep it for one short word that fits on screen, such as HI or GO, ideally paused, and leave it off if someone nearby is known to be photosensitive.

Brightness, battery and heat

The Access Board review cites about 1,000 cd/m² (nits) for electronic signs in bright daylight and about 30 cd/m² at night. Apple lists the iPhone 16 at 1,000 nits typical maximum and 2,000 nits outdoor peak, and the 11-inch iPad Air at 500 nits, so in sunshine a phone can outshine a bigger tablet. In a dark venue, if close viewers see smeared letters, drop brightness a step (judgement).

Set auto-lock to never or its longest option, because LED Scroller does not stop your phone from locking, and bring a power bank. On OLED screens dark pixels draw less power: Purdue University researchers found dark mode at 100% brightness saved an average of 39% to 47% of battery power across six apps, but only 3% to 9% at 30% to 50% brightness. So keep the background black.

Apple rates iPhone and iPad for 0 to 35 °C (32 to 95 °F) and says that when a device gets too warm its display dims or goes black. Between uses, keep the phone in the shade.

Ambient light and how to hold the sign

Glossy screens reflect the sky and sun, which lightens the black background and lowers contrast (the ADA Standards require a non-glare finish on wall signs for this reason). Tilt the screen so it does not mirror a bright sky toward readers, and keep it in shade when you can.

Always use landscape. In portrait, letters grow to fill the tall banner and less than one capital fits on screen. Turn off rotation lock, rotate, then go fullscreen.

Face readers squarely. Turned 45 degrees away, a screen looks about 30% narrower (cos 45° ≈ 0.71), and many LCDs dim at an angle.

Hold it high and still. In a crowd, raise it above head height in both hands, by the corners, elbows braced: shake blurs moving text. Raise it in short spells so people behind can see; see our concert banner page. At arrivals, hold it at chest height facing the exit doors, name first, as on our airport pickup sign page.

Settings checklist

1. Orientation: landscape, fullscreen.
2. Text: 2 to 5 words up close, 1 or 2 far away, capitals for names.
3. Colour: white, yellow, neon green or cyan on black.
4. Font size: the largest that keeps letters well formed, about 380 to 450 px.
5. Bold and glow: on.
6. Speed: about 10, checked with the stopwatch test.
7. Blink: off, except for one short word.
8. Phone: brightness at maximum, auto-lock and Low Power Mode off.
9. Holding: still, square to readers, above heads in a crowd.

You can also download the banner as a PNG or animated GIF. LED Scroller is free, needs no signup, adds no watermark and runs in any modern browser; the free Android app on Google Play works offline.

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