Why does LogMAR go down as vision improves, when Snellen denominators also go down?

LogMAR and the Snellen denominator do move in the same direction (both decrease as vision improves), but this is easy to confuse with decimal VA, which moves in the opposite direction (increases as vision improves). Always check which of the three scales — Snellen, LogMAR or decimal — is being referenced.

Is 6/6 always 'perfect' vision?

6/6 (or 20/20) represents average, statistically normal visual acuity at the standard test distance — not the theoretical best possible human vision, which can be somewhat better (e.g. 6/5 or 6/4) in some individuals. It's a reference benchmark, not an absolute ceiling.

Why do some charts and studies prefer LogMAR over Snellen?

LogMAR has equal step sizes between chart lines (unlike traditional Snellen charts, where step sizes vary), which makes it statistically more robust for research and for tracking small, meaningful changes in vision over time in clinical practice.

Visual Acuity Converter (Snellen/LogMAR) — Free Allied Health Calculator

🎯

A referral letter from an overseas clinic lists vision in imperial Snellen notation, but your clinical notes and outcome tracking use metric and LogMAR — before charting it, you need a reliable conversion.

Visual Acuity Converter (Snellen/LogMAR)

Vision

Snellen numerator

Snellen denominator

LogMAR = log10(Denominator / Numerator)
Decimal VA = Numerator / Denominator

Use the Metric/Imperial toggle at the top of the page to set whether your Snellen fraction is metric (6/x, tested at 6 metres) or imperial (20/x, tested at 20 feet) — both represent the same clinical distances, just different units.

Reference: National Vision Research Institute — LogMAR chart methodology; Bailey-Lovie visual acuity notation

⚠️ Screening estimate only — not a diagnostic or clinical assessment. Verify with a qualified allied health professional. Meets AHPRA/ACSQHC standards.

1 What this calculator does

Converts a Snellen visual acuity fraction (e.g. 6/12 or 20/40) to LogMAR and decimal notation, and shows the equivalent value in the other Snellen unit system (metric 6/x or imperial 20/x). Useful when comparing results across charts, clinics or countries that use different acuity notation systems.

2 Formula & professional reasoning

LogMAR = log10(Denominator / Numerator)
Decimal VA = Numerator / Denominator
Equivalent Snellen (other system) = (Denominator/Numerator) x other reference distance

Snellen notation expresses the test distance over the distance at which a person with 'normal' vision could read the same line — 6/12 (metric) and 20/40 (imperial) represent the same acuity, just measured at different standard distances (6 metres vs 20 feet). LogMAR (logarithm of the Minimum Angle of Resolution) converts this ratio to a continuous logarithmic scale, which has more even step sizes between chart lines and is preferred in research and many clinical settings for its statistical properties. Decimal VA is simply the Snellen ratio as a single number, commonly used in some countries and for quick comparison.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Normal acuity

Given: Snellen 6/6 (metric)

Working: LogMAR = log10(6/6) = log10(1) = 0 | Decimal = 6/6 = 1.0

Answer: LogMAR: 0.00 | Decimal VA: 1.00 | Equivalent: 20/20

💡 LogMAR 0.00 represents normal reference acuity — the benchmark against which other results are compared.

Standard

Mild vision reduction

Given: Snellen 6/12 (metric)

Working: LogMAR = log10(12/6) = log10(2) = 0.30 | Decimal = 6/12 = 0.50

Answer: LogMAR: 0.30 | Decimal VA: 0.50 | Equivalent: 20/40

💡 This is a common threshold referenced in driving licence vision standards in several jurisdictions — always confirm current local requirements separately.

Advanced

Significant vision impairment

Given: Snellen 6/60 (metric)

Working: LogMAR = log10(60/6) = log10(10) = 1.00 | Decimal = 6/60 = 0.10

Answer: LogMAR: 1.00 | Decimal VA: 0.10 | Equivalent: 20/200

💡 6/60 (20/200) is a commonly referenced threshold in legal blindness definitions in some countries — definitions vary by jurisdiction and should be confirmed against current local criteria.

4 Sanity check

Normal vision reference

6/6 (metric) = 20/20 (imperial) = LogMAR 0.00 = Decimal 1.0

This is the calibration reference point every conversion should be checked against

LogMAR direction

Lower LogMAR = better vision (0.00 is best/normal) — this is the opposite direction to decimal VA, where higher is better

A common source of confusion — always double check which scale direction is being reported

Common chart line equivalents

6/6=LogMAR 0.0 | 6/9=LogMAR 0.18 | 6/12=LogMAR 0.30 | 6/18=LogMAR 0.48 | 6/60=LogMAR 1.0

Useful for a quick sense-check against standard LogMAR chart line values

Near vs distance acuity

This calculator is for distance visual acuity notation only — near vision uses separate notation systems (e.g. N-point, Jaeger)

Do not apply this conversion to near vision test results

5 Common errors

Error	Cause	Consequence	Fix
Mixing metric and imperial numerators/denominators	Entering a metric numerator (6) with an imperial-style denominator scaled for 20ft testing, or vice versa	Produces a meaningless or incorrect LogMAR/decimal result	Use the Metric/Imperial toggle to match the notation system the original test result was recorded in, and keep numerator/denominator consistent with that system
Confusing LogMAR direction with decimal VA direction	Assuming a higher LogMAR number means better vision, by analogy with decimal VA where higher is better	Leads to exactly backwards interpretation of vision changes over time	Remember: LogMAR 0.00 is best (normal); LogMAR increases as vision worsens — decimal VA is the opposite (1.0 is best, decreases as vision worsens)
Applying distance VA conversion to near vision results	Using this calculator for reading/near-vision test results (e.g. N8, Jaeger J2)	Near vision uses entirely different notation systems that aren't interchangeable with distance Snellen/LogMAR	Use dedicated near-vision notation references — do not convert near vision scores using this distance-VA tool
Rounding intermediate values before final calculation	Rounding the Snellen fraction or decimal VA before computing LogMAR	Introduces small but avoidable errors, especially noticeable when tracking change over time	Calculate LogMAR directly from the original Snellen fraction rather than from an already-rounded decimal VA