How to use the visual acuity notation converter
- Choose the notation you have: Snellen in feet (20/x), Snellen in meters (6/x), decimal acuity, logMAR or MAR in arcminutes.
- Enter the value. For Snellen, type only the bottom number, for example 40 for 20/40.
- Read the same acuity in the four other notations.
- Use the table of common chart lines to see each line in every notation.
Formula
decimal = top / bottom (20/40 = 6/12 = 0.5)
MAR = 1 / decimal (in arcminutes)
logMAR = log10(MAR) = -log10(decimal)
Snellen feet bottom number = 20 / decimal; Snellen meters bottom number = 6 / decimal
The meter form is the foot form times 6/20 = 0.3, so 20/200 is 6/60. A decimal acuity of 1.0 is 20/20, 6/6, MAR 1 and logMAR 0.00.
20/40 in the other notations
An acuity of 20/40 has a decimal value of 20 / 40 = 0.500. Its MAR is 2.000 arcminutes and its logMAR is log10(2) = +0.301. In meters it is 6/12.
20/20 is 6/6, 1.000 and +0.000. 20/200 is 6/60, 0.100 and +1.000, and 20/10 is 6/3, 2.000 and -0.301. Going the other way, a logMAR of 0.3 corresponds to a decimal acuity of 10 to the power of -0.3, which is 0.501, or about 20/39.9.
Why there are several notations
Snellen fractions come from the distance at which a chart is read: 20 feet in the United States and 6 meters in much of the rest of the world. The top number is the test distance and the bottom number is the distance at which a person who resolves a detail of the standard size would read the same line. Decimal acuity is the same fraction written as one number, and it is common in Europe.
MAR and logMAR come from the size of the smallest detail a person can resolve. Because logMAR is a logarithm, equal steps on a logMAR chart are equal ratios in size, which makes it convenient for research and for averaging results. Each line of a logMAR chart is 0.1 apart.
What this page does not do
It converts notation only. It is not an eye test and it does not say whether a result is normal, good or poor. Printed charts round their lines to convenient numbers, so a converted value may differ slightly from the line on a given chart. Questions about a result belong with an eye care professional.
For other conversions, see the Unit Converter, for lens work the Thin Lens Calculator, and for another body measurement, the Waist to Height Ratio Calculator.
Assumptions and limits
- Snellen values use a top number of 20 for feet and 6 for meters.
- The bottom number must be above 0 and at most 2,000, decimal acuity between 0.001 and 10, logMAR from -1 to 3, and MAR above 0.1 and at most 1,000.
- Results are rounded to 0.1 for Snellen bottom numbers and to 3 decimals for the others.
- This tool converts notation and gives no medical interpretation.
- Nothing typed here leaves your browser.
Frequently asked questions
Is 20/20 perfect vision?
No. It is a reference line on a chart: the line a person with average-sized detail resolution reads at 20 feet. Many people read smaller lines, such as 20/15 or 20/10.
Why is logMAR negative for better acuity?
logMAR is the logarithm of the minimum angle of resolution. A smaller angle means better acuity, and a MAR below 1 arcminute has a negative logarithm.
Does 6/12 equal 20/40?
Yes. Both mean the same decimal acuity of 0.5. The meter form is the foot form multiplied by 0.3, so 20/40 is 6/12, and 20/70 is 6/21.
Why does a converted value not match my chart line exactly?
Charts round their lines. A logMAR of 0.3 is exactly 20/39.9 here, but a chart prints it as 20/40.