acuity / Vision
Visual acuity notation converter
The same acuity, written another way.
Answers now
Written as
20/40Snellen · 20 ft
- 0.5
- Decimal (1.0)
- 6/12
- Snellen · 6 m
- 0.301
- logMAR
Decimal = 1 ÷ minimum angle of resolution. Three scales for one quantity, so conversion is arithmetic.
Step by step
All three notations describe the same thing: the smallest angle the eye can resolve. Decimal is its reciprocal. Snellen 20/40 means “at 20 feet, reads the line a normal eye reads at 40”, so the decimal is 20÷40 = 0.5. logMAR is the base-10 logarithm of that angle. Nothing is looked up, which is why this page works today.
What this number cannot tell you
Rewriting the notation does not make two tests comparable. Charts differ in spacing, optotype (tumbling E, Landolt C, Snellen letters), lighting and test distance, and the same child can land a line apart on two of them. This page does not convert acuity to a spectacle prescription, does not decide whether a child is short-sighted, and does not grade anyone for referral — each of those rests on evidence of its own. The Chinese five-point scale (4.0–5.3) is deliberately absent: its baseline differs from logMAR, and folding it in would mislead.
An example
0.5 on the report is 20/40 in Snellen (6/12 in metres) and 0.30 in logMAR.
Common questions
- Will this tell me whether my child is normal?
- No. What happens here is arithmetic, and “normal” needs an age-based reference table before it means anything — there is none behind these figures. The section above spells out what stays out of reach.
- Do the numbers I type reach your server?
- No. Everything is worked out in the browser on this device, and the page has no channel for sending data out — the site’s security policy closes that route outright, so it is not a promise you have to take on trust. Reload and the numbers are gone; to keep them, copy them.
Next
The others in the same group.