Pennsylvania College of Optometry

Snellen Chart Scoring Aid

Instructions to Download, Customize & Use the Effective Snellen Chart.

Those interested in using the Effective Snellen Fraction Tool:

Download instructions - PDF 

Snellen 4 Clinicians EXCEL File

Those interested in the details of how and why this tool was developed, and how it is structured are referred to:

Army Research Laboratory Technical Report 4756 - PDF.

Suggestions for improving either the tool or the downloading instructions should be addressed to:

How to Download a Local Master File

  • Download a copy of the EXCEL™ file: “Snellen 4 Clinicians”
  • Save this file as your local master file.
  • Open file.
  • There are four sheets in this file. Concentrate first on the two sheets labeled “Snellen”.
  • One sheet is entitled “Snellen0,” and is initialized with all zeroes (i.e., no correct answers). Another sheet is entitled: “Snellen1,” and is initialized with all ones (i.e., all correct answers).
  • Select the Snellen sheet that most closely matches your customary usage. This sheet is your local master sheet.

How to Customize Your Local Master Sheet(s)

  • Your local master sheet shows that the testing distance (column AB) and Snellen denominators (column AC) are shaded in grey because these values need only be entered one time for each clinical setting.
  • The downloaded local master sheet assumes that the distance used in your visual acuity test is 10 feet. If necessary, change this distance, measured in feet, to match the one used in your clinic. If more than one distance is used, make an additional master sheet for each distance.
  • If necessary, change the Snellen denominators (usually listed on the Snellen chart) in column AC under the heading: “Enter Snellen
    Denominators for Test Distance Used.”
  • Save these files, now customized for your clinical testing distance(s).
  • The format of your local master Snellen chart is a matrix with 10 lines and 11 possible letter-entries per line. Thus, up to 11 letters per line may be accommodated.
  • Create copies of your master sheet to accommodate both your testing distance(s) (discussed above), and the number of different Snellen charts used in your clinic. [# Copies = # testing distances * # Snellen charts.]
  • On each one of these local masters, overwrite the downloaded Snellen letters with those Snellen letters from the charts actually used in your clinic. Where necessary, use the “delete” key to eliminate an unwanted letter that may be present; this will provide a blank in its place.
  • At this point, your clinic has local master sheets that represent the letters of each Snellen chart and the test distance(s) used in your clinic.
  • Save these master files, now customized for your clinical Snellen charts.
  • You will need to re-apply the above set of instructions only if you change your testing distance(s) or your [Snellen] chart(s).

How to Use Your Effective Snellen Chart(s) in Clinic.

  • To test a patient’s visual acuity, make a computer copy of the appropriate Snellen-chart-and-testing-distance local master sheet. One such chart is needed for each patient.
  • In each computer file, space is provided to enter: the patient’s name, the patient’s identification (e.g., a number or code), the date, and the test administrator’s name or code. The line labeled “Other” allows for an entry or other identification required by the test or by the clinic.
  • Enter the patient responses directly into this computer file. The examiner moves the cursor from response-box to response-box (located under each Snellen letter), and, depending on the initialization, enters either a 0 [= an incorrect response], or a 1[= a correct response]. Any entry other than a 0 or a 1 will place a red-speckled background in the response-box as a warning of an unacceptable entry. To correct, simply overwrite the response-box.
  • At the end of the measurement, the patient’s effective Snellen fraction, in “20/…” notation, is immediately supplied to the examiner at the bottom right, shaded box of the patient’s sheet (i.e., row 27, column AB).
  • Save a file for each patient. Future retesting responses may be efficiently saved as an additional sheet within each patient’s file.


  • The downloaded master Snellen file can be altered to accommodate true logMAR charts, such as the Early Treatment of Diabetic Retinopathy Study (ETDRS) charts.
  • Examples of how this is accomplished are given in the sheets labeled “ETDRS0” and “ETDRS1” in the downloaded master file.
  • These sheets contain a portion of the ETDRS Chart 1 from Ferris, et al. Am J Ophthalmol. 94:91-96, 1982. The lines of letters in these sheets correspond to the Snellen range: 20/20 to 20/160.
  • Each sheet is initialized with either a 0 or a 1; letters can be overwritten.
  • Other charts may also be formed using the downloaded file. For example, were suitable letters available for use in the EXCEL file, a
    “Landolt C” chart could be constructed.
  • The master Snellen chart has two locations of hidden columns in which computations are performed.
  • To locate these hidden sections in Excel™, be sure that the file is registered so that the left-most column of your file appears as column A.
  • The first set of hidden columns lies between columns M and AA. This section performs the bookkeeping for the matrix of Snellen letters and of the patient’s responses (i.e., 0 or 1).
  • The second set of hidden columns lies between columns AC and AI. The logMAR transformations and other computations are performed in this section.

Suggestions for improving either the tool or the downloading instructions should be addressed to: