logMAR / Snellen calculator

A simple visual acuity calculator for converting Snellen notation to exact and letter-adjusted logMAR, decimal acuity, and an exact Snellen ratio with the letter modifier removed. © AAOO/Dr Grant Hannaford 2026

Input

Enter either a compact visual acuity string such as 6/6-2 or complete the manual fields below.

Visual acuity string

Accepted forms: 6/6, 6/6-2, 6/9+1, 20/20-1.

Snellen numerator

Snellen denominator

Letters +

Extra letters read beyond the line.

Letters -

Missed letters on the line.

Displayed Snellen denominator rounding

Output

The exact Snellen output removes the letter modifier by converting the adjusted letter score back into a ratio.

Exact Snellen ratio without + / - letters

6/6.6

Base logMAR

0.00

Adjusted logMAR

0.04

Decimal acuity

0.91

Letter adjustment

-2 letters

Parsed / standardised entry

6/6-2

Example: 6/6-2 gives an adjusted logMAR of 0.04, decimal acuity of approximately 0.91, and an exact Snellen ratio of 6/6.6.