About the Visual Acuity Converter
Eye charts report the same measurement in several notations. In the US, Snellen acuity is written with a 20-foot test distance (20/20, 20/40); the UK, Europe and Australia use 6 metres (6/6, 6/12); much of continental Europe and many research papers use decimal acuity (1.0, 0.5); and clinical trials use logMAR, the logarithm of the minimum angle of resolution, where 0.0 is normal and higher numbers mean worse vision.
Enter a value in any notation and this converter returns all the others, plus the approximate ETDRS letter score, the minimum angle of resolution in arc-minutes and the WHO distance vision impairment category it falls into. A reference table lists the standard chart lines so you can see where your result sits.
Conversions are exact mathematical relationships, but real charts only print certain lines, so a converted value may fall between two lines. WHO categories are defined for presenting vision in the better eye; a single-eye score or a value with an uncorrected refractive error does not by itself mean a person has a vision impairment.
With the default inputs, the logmar is 0.3. Change any value above to recalculate instantly.
How to use the visual acuity converter
- 1Choose the notation you already have: 20/x, 6/x, decimal or logMAR.
- 2Enter the value (for 20/40 type 40; for 6/12 type 12).
- 3Read the equivalent value in every other notation.
- 4Check the reference table to see the nearest standard chart line.
Formula and method
A Snellen fraction compares the test distance (numerator) with the distance at which a person with standard vision could read the same letters (denominator). The ratio denominator ÷ numerator is the minimum angle of resolution (MAR) in arc-minutes: 20/40 and 6/12 both mean a MAR of 2. Decimal acuity is the reciprocal of MAR (0.5), and logMAR is its base-10 logarithm (0.30).
On an ETDRS chart each line is 0.1 logMAR and holds five letters worth 0.02 logMAR each, so a letter score of 85 corresponds to logMAR 0.0 (20/20) and every line lost subtracts five letters. WHO categories (ICD-11) use presenting acuity in the better eye: worse than 6/12 is mild, worse than 6/18 moderate, worse than 6/60 severe and worse than 3/60 blindness.
- N
- Snellen numerator (test distance: 20 ft or 6 m)
- D
- Snellen denominator
- MAR
- Minimum angle of resolution, arc-minutes
- logMAR
- Base-10 logarithm of MAR
Worked examples
20/40 to the other notations
20/40 means a MAR of 40 ÷ 20 = 2 arc-minutes. Decimal acuity is 1 ÷ 2 = 0.5, logMAR is log10(2) = 0.30, and the metric equivalent is 6 × 2 = 6/12. The ETDRS score is 85 − 50 × 0.301 ≈ 70 letters.
logMAR 1.0
A logMAR of 1.0 means MAR = 10, so the Snellen equivalents are 20/200 and 6/60, and decimal acuity is 0.1. In the better eye this is the threshold of US legal blindness, and in WHO terms it is the boundary of moderate impairment.
Metric 6/9 to decimal and feet
6/9 is a MAR of 1.5, giving decimal 0.67 and logMAR 0.18. Multiplying 20 by 1.5 gives the US equivalent of 20/30. The approximate letter score is 85 − 50 × 0.176 = 76.
Frequently asked questions
What is 20/20 vision in logMAR?+
20/20 (6/6) equals a logMAR of 0.0 and a decimal acuity of 1.0. Negative logMAR values such as −0.1 (20/16) mean better-than-standard acuity, which many healthy young adults have.
How do I convert 6/6 to 20/20?+
Multiply the denominator ratio: 6/x in metres equals 20/(x × 20 ÷ 6) in feet. So 6/6 = 20/20, 6/9 = 20/30, 6/12 = 20/40, 6/18 = 20/60 and 6/60 = 20/200.
What does 20/40 vision mean?+
You need to be 20 feet away to read letters a person with standard vision can read from 40 feet. It equals 6/12 and decimal 0.5, and is the minimum corrected acuity for a standard driving licence in many US states and the UK.
Why do researchers use logMAR?+
logMAR charts have the same number of letters on every line and equal steps between lines, so scores can be averaged and compared statistically. Each line is 0.1 logMAR and each letter 0.02, giving finer measurement than Snellen.
What visual acuity is legally blind?+
In the US, statutory blindness is central acuity of 20/200 or worse in the better eye with the best correction, or a visual field of 20 degrees or less. The WHO blindness category is presenting acuity worse than 3/60 (20/400) in the better eye.
Is decimal acuity the same as a percentage?+
No. Decimal 0.5 does not mean 50% of useful vision; it means you need letters twice the standard size. Functional vision loss is better described by logMAR or letter scores.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.