Visual Acuity Converter
Convert visual acuity values among Snellen 20/x, Snellen 6/x, decimal acuity, MAR, and logMAR notation for educational comparison.
Enter only the denominator from a 20-foot Snellen fraction.
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Visual acuity is the familiar chart-based number used to describe how small a detail someone can resolve at a standardized distance. In many everyday settings it is written as a Snellen fraction such as 20/20, 20/40, 6/6, or 6/12. Research papers, international reports, and some clinical systems may instead use decimal acuity, MAR, or logMAR. The visual acuity converter puts those notations into a common format so you can compare them directly.
The converter accepts one value at a time. If you select Snellen 20/x, enter the denominator from a 20-foot fraction. If you select Snellen 6/x, enter the denominator from a 6-meter fraction. If you select decimal acuity, MAR, or logMAR, enter that numeric value directly. The calculator then displays equivalent 20-foot Snellen, 6-meter Snellen, decimal acuity, MAR, and logMAR values, plus a plain-language comparison with the 20/20 reference point.
This tool is for educational use. It converts notation; it does not measure your eyes, diagnose a condition, determine whether you can drive, or recommend glasses, contacts, surgery, medication, or any other treatment. A visual acuity score is only one piece of an eye exam. Symptoms, eye health, refraction, chart design, lighting, testing distance, contrast, and whether the measurement was made with or without correction can all change how the number should be understood.
All five outputs come from a small set of relationships. A Snellen fraction is converted to decimal acuity by dividing the numerator by the denominator. For 20/x notation, decimal acuity equals 20 divided by x. For 6/x notation, decimal acuity equals 6 divided by x. Once decimal acuity is known, MAR is the reciprocal: MAR = 1 / decimal. LogMAR is the base-10 logarithm of MAR: logMAR = log10(MAR).
These relationships also work in reverse. If decimal acuity is 0.5, the 20-foot Snellen denominator is 20 / 0.5 = 40, so the equivalent is 20/40. The 6-meter denominator is 6 / 0.5 = 12, so the equivalent is 6/12. MAR is 1 / 0.5 = 2, and logMAR is log10(2), which is about 0.301. The values look different, but they describe the same mathematical acuity relationship.
The direction of the scale changes depending on the notation. Higher decimal acuity means sharper chart acuity. Lower MAR means sharper chart acuity. Lower logMAR also means sharper chart acuity, and logMAR can be negative when decimal acuity is greater than 1.0. This is one reason logMAR is popular in research: equal steps on a logMAR chart are easier to analyze statistically than traditional Snellen fractions.
Snellen notation is the most recognizable format. The first number is the testing distance, and the second number is the comparison distance for the chart detail. In a 20/40 result, the person tested at 20 feet resolves detail that the reference observer would be expected to resolve at 40 feet. In a 6/12 result, the same idea is expressed in meters. Because 20 feet is approximately 6 meters, 20/40 and 6/12 are equivalent for practical notation conversion.
Decimal acuity is more compact. It turns the Snellen fraction into a single number: 20/20 and 6/6 become 1.0, 20/40 and 6/12 become 0.5, and 20/10 and 6/3 become 2.0. Decimal acuity is common in many countries because it is easy to compare and simple to use in calculations. The tradeoff is that people who are used to Snellen fractions may not immediately know what a value such as 0.63 means.
MAR stands for minimum angle of resolution. It is the reciprocal of decimal acuity, so 20/20 has MAR 1, 20/40 has MAR 2, and 20/10 has MAR 0.5. LogMAR is the logarithm of MAR. A logMAR value of 0 corresponds to 20/20, about 0.3 corresponds to 20/40, and about -0.3 corresponds to 20/10. In logMAR, a lower number represents sharper acuity, which is the opposite direction from decimal acuity.
Best for quick everyday reading of a chart result. Higher decimal values and smaller Snellen denominators indicate sharper chart acuity.
Best for research-style comparisons. Lower MAR and lower logMAR values indicate sharper chart acuity.
A few common values make the scales easier to remember. The 20/20 reference point is 6/6 in metric Snellen, 1.0 in decimal acuity, 1 MAR, and 0 logMAR. A 20/40 result is 6/12, 0.5 decimal acuity, 2 MAR, and about 0.301 logMAR. A 20/10 result is 6/3, 2.0 decimal acuity, 0.5 MAR, and about -0.301 logMAR.
| Snellen 20/x | Snellen 6/x | Decimal | MAR | logMAR |
|---|---|---|---|---|
| 20/10 | 6/3 | 2.0 | 0.5 | -0.301 |
| 20/20 | 6/6 | 1.0 | 1.0 | 0 |
| 20/40 | 6/12 | 0.5 | 2.0 | 0.301 |
| 20/200 | 6/60 | 0.1 | 10 | 1.0 |
Real exam records may not line up perfectly with these examples. Eye charts use specific line sizes, and different charts can have different spacing, contrast, and scoring rules. A mathematical conversion might produce 20/66.7, while a chart report might list a nearby printed line such as 20/60 or 20/70. That difference is normal when converting a continuous formula to a chart with fixed line choices.
A visual acuity conversion is not the same thing as a complete eye assessment. It does not reveal whether a person has nearsightedness, farsightedness, astigmatism, cataract, glaucoma, macular disease, dry eye, eye teaming issues, or any other condition. It also does not show whether a number was measured with glasses, without glasses, through a pinhole, with contact lenses, after dilation, or under a particular lighting setup. Those details are essential when interpreting an actual exam record.
The calculator also cannot decide whether a result is acceptable for driving, work, school, sports, or legal documentation. Rules vary by location and context, and many standards include more than one measurement. Some situations consider visual field, one-eye versus two-eye acuity, best-corrected acuity, contrast sensitivity, or medical history. A converted number can help you read notation, but it should not be treated as a certification.
If an acuity number is tied to symptoms, sudden changes, pain, flashes, floaters, double vision, a curtain-like shadow, or a new difference between eyes, a calculator is not the right tool for interpretation. Keep the original result and context, then discuss the concern with a qualified eye-care professional. The converter intentionally avoids diagnosis and treatment advice because the same acuity value can occur in many different situations.
When comparing records, preserve the original notation first. Write down whether the measurement was for the right eye, left eye, or both eyes together; whether it was unaided, aided, or best-corrected; and which chart or testing distance was used if that information is available. Then use the converter as a translation layer. This helps prevent common mistakes, such as comparing 20/40 unaided in one record with 20/20 corrected in another and assuming the eyes themselves changed by that amount.
Conversions are especially helpful when sources mix systems. A study may report logMAR, an international clinic may provide decimal acuity, and an older record may list Snellen notation. By converting everything to one scale, you can see the approximate relationship among the numbers. For example, decimal 0.5, MAR 2, logMAR 0.301, 20/40, and 6/12 all describe the same underlying acuity ratio.
Rounding should be handled gently. For a quick conversation, 20/40 and logMAR 0.3 are close enough. For research, audit work, or a clinical record, use the original source value and the rounding rules required by that setting. The calculator shows transparent formula-based equivalents so you can understand the relationship, but it does not replace the chart score, examiner notes, or professional interpretation attached to the original measurement.
This converter translates the same acuity relationship among Snellen 20/x, Snellen 6/x, decimal acuity, MAR, and logMAR notation. It uses the standard math relationships decimal = numerator / denominator, MAR = 1 / decimal, and logMAR = log10(MAR). The result is an educational notation conversion only, not a diagnosis, prescription, or complete description of eye health.
Choose the matching Snellen input type, then enter only the denominator. For example, select Snellen 20/x and enter 40 for 20/40, or select Snellen 6/x and enter 12 for 6/12. The numerator is fixed by the selected notation, so the calculator can convert the fraction into decimal acuity, MAR, and logMAR.
LogMAR is the base-10 logarithm of MAR, and MAR is 1 divided by decimal acuity. If decimal acuity is greater than 1.0, MAR is less than 1.0, and the logarithm becomes negative. That is why values such as -0.1 can represent acuity numerically better than 20/20, while positive logMAR values represent acuity numerically worse than 20/20.
20/20 is a common reference point for distance visual acuity, meaning the person can resolve a standard chart detail at 20 feet that the reference observer is expected to resolve at 20 feet. It is not the same as perfect vision and does not measure contrast sensitivity, peripheral vision, eye alignment, color vision, or eye health. A notation conversion should not be used to decide whether vision is medically normal.
Snellen charts usually use standard line values, but the formulas can produce values between those chart lines. For example, decimal acuity of 0.3 converts mathematically to about 20/66.7, even though a real chart might report the nearest available line such as 20/60 or 20/70. Keeping a decimal denominator makes the conversion transparent and avoids pretending the result came from a specific chart design.
No. Visual acuity notation does not determine a glasses prescription, contact lens fit, eye disease status, or treatment plan. The calculator can help interpret numbers from a chart or report, but sudden vision changes, eye pain, flashes, floaters, or concerns about your sight should be discussed with a qualified eye-care professional.
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