Health Tools

Visual Acuity Test


Visual Acuity Testing (Snellen Chart)


This calculator is intended as a convenient screener for visual acuity to be used on a mobile device and should not be used as a replacement for in-office testing. Evidence for smartphone apps for Snellen visual acuity is limited, and currently no app has been found to be accurate to within at least one line of formal visual acuity testing (and further validation is required). Variation in screen resolution and text size may impact accuracy.


- Ensure proper room lighting and set phone brightness to 100%.
- Ensure proper room lighting and set phone brightness to 100%.
- Hold the screen 4 feet (1.2 m) from the patient (approximately the end of a standard hospital bed if patient is sitting upright).
- Test each eye independently. The patient should completely cover the opposite eye.
- See Pearls/Pitfalls for further instructions.

- To assess binocular and monocular visual acuity in cooperative patients.

- Select lowest line that patient can read correctly.


- Visual acuity worse than 20/25 should be evaluated by a licensed eye professional to determine whether corrective lenses or other treatments may be necessary.
- Visual acuity is not a measure of a patient’s prescription. This is a separate value that needs to be measured in an office setting.


- Any patient with sudden changes in visual acuity from baseline or new obscurations in vision requires immediate referral to an ophthalmologist for dilated fundus examination and further testing.

Last Update : 11 February 2023 11:41 AM
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