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Converts a Snellen visual acuity fraction (e.g. 20/40) between notation systems using the standard defined relationships: decimal visual acuity = numerator/denominator, and the metric (6 m) Snellen denominator = imperial denominator x (6/20). This is pure notation conversion — it does not perform an eye exam or diagnose any vision condition. For general reference and educational purposes only. Consult an optometrist or ophthalmologist for an actual eye examination.
How it works
Enter your Snellen fraction (numerator, usually 20, and denominator). The tool converts it to decimal visual acuity and to metric (6/x) notation, with a plain-language explanation of what it means.
- Enter snellen numerator (feet, usually 20).
- Enter snellen denominator (feet).
- Click Calculate to see your results.
Examples
20/40
Decimal acuity = 20/40 = 0.5 (50%). Metric = 6/12. Meaning: at 20 feet, this person sees what someone with normal vision sees at 40 feet.
Who should use it
- Understanding an eye exam result.
- Converting between US (feet) and international (metric) vision notation.
Industry applications
- Optometry/ophthalmology patient education
- Vision science education
Advantages
- Instant conversion between imperial, metric, and decimal notation.
- Plain-language explanation included.
Limitations
- Does not perform an actual vision test.
- Snellen notation alone does not capture all aspects of visual function (e.g. contrast sensitivity, peripheral vision).
Common mistakes to avoid
- Assuming a Snellen fraction alone reveals the cause of reduced acuity — it does not indicate why vision is reduced, only how it compares to a reference standard.
Best practices
- Use your most recent eye exam's Snellen result for the most relevant conversion.
Tips
- Snellen notation reflects acuity at a fixed testing distance under standard conditions — real-world vision (e.g. in dim light) can differ.