One visual acuity in every notation: Snellen in metres and feet, decimal, logMAR and ETDRS letters, and the letter size for the test distance of your room.
Convert an acuityNothing you enter is sent or stored.
How the notations relate
All notations describe the same thing: the smallest detail the eye resolves, expressed as the minimum angle of resolution (MAR) in minutes of arc. The decimal acuity is 1 / MAR, and logMAR is the logarithm of the MAR. So 6/12 = 20/40 = 0.5 decimal = logMAR 0.30.
- Snellen: test distance / distance at which the letter subtends 5 minutes of arc (6/… in metres, 20/… in feet).
- logMAR: −log10 of the decimal acuity; each line on a logMAR chart is 0.1.
- ETDRS letters: 85 − 50 × logMAR; each letter counts 0.02 logMAR, five letters make one line.
Letter size and test distance
On the 6/6 line each letter subtends 5 minutes of arc: 8.73 mm tall at 6 m. A chart printed for 6 m shows the wrong acuity when it hangs closer: read at 4 m, its 6/6 line tests 0.67 instead of 1.0. The calculator gives the letter height of each line for your room, direct or via a mirror.
The distance test needs at least 4 m. In a shorter room a mirror on the wall opposite the patient, with the chart behind the patient, nearly doubles the distance.
Planning a refraction room?
Our refraction room plan selects the vision chart, the instruments and the set-up for your room size. For the refraction itself: phoropters, trial lens sets and autorefractors.
Frequently asked questions
What is 6/12 in decimal and logMAR?
6/12 is 20/40, decimal 0.5 and logMAR 0.30, or 70 ETDRS letters.
What is 20/20 in metres?
20/20 is 6/6: decimal 1.0, logMAR 0.00, 85 ETDRS letters.
Why does my chart give a different result in a small room?
A chart is calibrated for one distance. Closer, every letter looks bigger, so the patient reads a line further than the true acuity. Use a chart or projector set to the actual distance.
Is anything I enter stored?
No. The calculator runs in your browser; nothing you enter is sent to us or saved.