Written and maintained by CASRAI Editorial Board
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General product and reference information only; it is not medical advice and does not replace an eye examination.
A Snellen chart is a chart of block letters or other symbols in graduated sizes used to test visual acuity; the best-known result, 20/20 (6/6 in metres), is the Snellen fraction for logMAR 0.0. The table below puts the four notations people meet side by side.
Snellen chart conversion table
Values are from Michael Bach’s visual acuity reference (the Freiburg Acuity Test author). Each row is one standard line step of 0.1 logMAR. Metre values are rounded as published.
| logMAR | Decimal | Snellen (feet) | Snellen (metres) |
|---|---|---|---|
| -0.1 | 1.26 | 20/16 | 6/4.8 |
| 0.0 | 1.0 | 20/20 | 6/6 |
| 0.1 | 0.8 | 20/25 | 6/7.5 |
| 0.2 | 0.63 | 20/30 | 6/9.5 |
| 0.3 | 0.5 | 20/40 | 6/12 |
| 0.4 | 0.4 | 20/50 | 6/15 |
| 0.5 | 0.32 | 20/60 | 6/19 |
| 0.6 | 0.25 | 20/80 | 6/24 |
| 0.7 | 0.2 | 20/100 | 6/30 |
| 0.8 | 0.16 | 20/125 | 6/38 |
| 0.9 | 0.13 | 20/160 | 6/48 |
| 1.0 | 0.10 | 20/200 | 6/60 |
Check your product’s labelling for the test distance and the notation it uses before comparing readings against this table.
How to read a Snellen result
A Snellen result is a fraction. The top number is the testing distance and the bottom number is the distance at which a reference eye could read that line: in 20/40 the 20 is 20 feet, in 6/12 the 6 is 6 metres. Per Bach, the 20 and the 6 correspond to testing distances of 20 ft and 6 m, so 20/20 and 6/6 are the same result written in two units. The Snellen ratio and the decimal figure are identical once you do the division (20/40 = 0.5).
Chart design and testing distance
The American Academy of Ophthalmology’s EyeWiki describes the Snellen design this way: each element of a letter subtends 1 arcminute at the designated distance, and letters are drawn on a 5 by 5 grid, so a whole letter subtends 5 arcminutes. The designated distance is 20 feet (6 metres). EyeWiki also lists the limits of the design: lines carry unequal numbers of letters, and letter spacing and crowding vary, which it says makes a chart without standardisation within itself unreliable for statistical analysis.
Snellen vs logMAR vs ETDRS
Snellen vs logMAR
LogMAR is a logarithmic scale where lower is better. The Royal College of Ophthalmologists notes that poorer vision is recorded as a higher number: 6/60 Snellen is 1.00 logMAR and 6/6 Snellen is 0.00 logMAR. The College describes LogMAR charts as more accurate than other acuity charts and cautions that there is no direct correlation between logMAR and Snellen scoring. Bach gives the scoring rules: one line is 0.1 logMAR, one letter on a five-letter line is 0.02 logMAR, and logMAR values can be averaged arithmetically, which Snellen fractions cannot.
The logMAR chart and ETDRS
Bach states that charts following ISO 8596 should have equidistant, progressive acuity steps of 0.1 logMAR. EyeWiki describes the ETDRS chart, developed in 1982 for research, as having five letters per row with equal spacing on a logarithmic scale, tested from 4 metres, with logMAR computed as 1.1 minus (0.02 times letters correct).
Types of Snellen charts
US regulation does not list sub-types. 21 CFR 886.1150 identifies a visual acuity chart as a chart, such as a Snellen chart with block letters or other symbols in graduated sizes, intended to test visual acuity, and classifies it as Class I (general controls). In the openFDA classification database the product code is HOX, Chart, Visual Acuity (Ophthalmic). A separate Class I code under the same regulation, QTO, covers a Digital Visual Acuity Test, defined as an aid in the assessment of visual acuity in patients.
Common mix-ups
- Higher logMAR is worse. 0.0 is 20/20 and 1.0 is 20/200, so a larger logMAR number records poorer vision.
- 6/ and 20/ differ only in distance. They are the same scale written in metres or feet.
- A Snellen chart is not an ETDRS chart. ETDRS is tested from 4 m with five letters per row; Snellen is designated for 6 m or 20 ft with varying letters per line.
- Averaging. Bach notes logMAR values can be averaged in the normal arithmetic way.
FAQ
What does 20/20 mean on a Snellen chart?
It means the result matches the reference line at the 20 ft testing distance, which is logMAR 0.0 and a decimal acuity of 1.0 (6/6 in metres).
How do I convert Snellen to logMAR?
Look the row up in the table above. One line is 0.1 logMAR, so 20/40 is 0.3, and the RCOphth gives 6/60 as 1.00.
Is a visual acuity chart a regulated device?
Yes. 21 CFR 886.1150 classifies it as Class I, under general controls.
For related supply references, see the optometry clinic supply checklist, the pre-employment physical exam supply checklist, the corporate health fair supply checklist, reading glasses strength chart and the overview what is ophthalmology.
Sources
- 21 CFR 886.1150, Visual acuity chart, https://www.law.cornell.edu/cfr/text/21/886.1150 (checked 2026-10-30).
- openFDA device classification, regulation 886.1150, https://api.fda.gov/device/classification.json?search=regulation_number:886.1150 (checked 2026-10-30).
- Royal College of Ophthalmologists, Snellen and LogMAR acuity testing, https://curriculum.rcophth.ac.uk/logmar-vs-snellen (checked 2026-10-30).
- Michael Bach, Visual acuity, https://michaelbach.de/sci/acuity.html (checked 2026-10-30).
- American Academy of Ophthalmology EyeWiki, Visual Acuity Testing in Adults, https://eyewiki.org/Visual_Acuity_Testing_in_Adults (checked 2026-10-30).
