The authors describe an online tool which has the purpose of converting count finger measurements of visual acuity into Snellen equivalents. The purpose of this is to allow the re-interpretation of retrospectively collected data that provide visual acuity in this way. Retrospective review of visual acuity measurements pose challenges for authors, as the data are often not in a form that will allow mathematical interpretation (as in the form of finger counting). There are some published guidelines for the conversion of counting fingers to decimal acuity, however, they do not take the testing distance into consideration and are therefore inaccurate. The authors describe an online count fingers Snellen calculator (www.countfingers.com) which makes it possible to convert count fingers measurements into Snellen equivalent, taking the test distance into account. The calculator utilises the width of the examiners’ digits and the inter-digit distances when the examiner is holding their hand in front of a patient. The authors recognise the limitations of the calculator, particularly when used for retrospective data. Authors suggest that the calculator still offers a reasonable quantification of retrospective data, provided the person who measured the vision originally is available to have their hands measured.