Teaching · Special Education · Low vision and displays

Text big enough
for this reader,
on this screen.

Type the reader’s acuity and distance and the screen’s size and resolution. The calculator turns visual angle into millimetres, points and pixels, and gives the display scaling that makes ordinary text readable.

Size the textWhat you get

The display scaling calculator

The worked example is a made-up seventh grader with 20/70 acuity on a 13.3-inch laptop. Nothing you type leaves this browser; use a code, not a name.

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What you get

Teaching · Special EducationFree

What you finish with
The smallest and the comfortable x-height, print size in points and M units, screen font size in pixels, and the scaling for 16-pixel text, with a table across acuities. It prints as a setup note and downloads as a spreadsheet.
Who it is for
Teachers of students with low vision, special education teams, school technology staff, and students and families setting up a screen.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The Special Education space in Caveat Teaching.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Take the corrected distance acuity from the most recent eye report.
  2. Measure the usual distance from eyes to screen.
  3. Choose a reading reserve: 2 × for fluent reading.
  4. Type the screen’s diagonal, resolution and current scaling.
  5. Set the scaling or font size, then check it with the reader.

Size the text

How the text size is found

Visual angle, then millimetres, then pixels.

The worked example: a 20/70 reader on a 13.3-inch laptopVisual angle17.5′5′ × 70 ÷ 20Smallest x-height2.07 mmat 16 inWith reserve4.14 mm2 × for fluent readingOn this screen42 pxat 125% scaling, 166 ppiThe worked example: a 20/70 reader on a 13.3-inch laptopVisual angle17.5′5′ × 70 ÷ 20Smallest x-height2.07 mmat 16 inWith reserve4.14 mm2 × for fluent readingOn this screen42 pxat 125% scaling, 166 ppi
Text size is an angle at the eye; distance and pixel size turn it into millimetres and pixels.

Visual angle

Acuity is an angle. A 20/20 eye resolves a letter 5 arcminutes tall; a 20/70 eye needs 70 ÷ 20 = 3.5 times that, 17.5 arcminutes. The same angle is a bigger letter far away and a smaller one up close, so distance matters as much as acuity, and moving a screen nearer is often the cheapest fix.

Reserve and fonts

The smallest readable letter is too small to read quickly. Low-vision reading research finds reading speed levels off at about twice the threshold size (a 2:1 reserve), and sustained reading is easier at about three times. Fonts differ in x-height, the height of a lowercase x, so the same point size looks larger in Verdana than in Times.

Sources

  1. Whittaker, S. G., and Lovie-Kitchin, J. (1993). Visual requirements for reading. Optometry and Vision Science, 70(1), 54–65.
  2. Legge, G. E. (2007). Psychophysics of Reading in Normal and Low Vision. Lawrence Erlbaum. Critical print size and acuity reserve.
  3. National Eye Institute. Low vision (nei.nih.gov).
  4. W3C. Web Content Accessibility Guidelines 2.2, Success Criterion 1.4.4 Resize Text, and the CSS reference pixel (w3.org).

The worked example is made up. This is a starting point for a trial with the reader, not an eye-care recommendation; follow the reader’s eye care provider and vision specialist. Your entries stay in this browser. Support: team@caveatacademic.com.