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Acuity
Converter.
Convert visual acuity between Snellen (20/x and 6/x), LogMAR, decimal, and MAR — instantly, with the formulas shown and the full reference table below.
MAR = denominator ÷ numerator · LogMAR = log₁₀(MAR) · decimal = numerator ÷ denominatorFull conversion table
| Snellen (ft) | Snellen (m) | LogMAR | Decimal | MAR (′) |
|---|---|---|---|---|
| 20/10 | 6/3 | -0.30 | 2.00 | 0.50 |
| 20/12.5 | 6/3.8 | -0.20 | 1.60 | 0.63 |
| 20/15 | 6/4.5 | -0.12 | 1.33 | 0.75 |
| 20/16 | 6/4.8 | -0.10 | 1.25 | 0.80 |
| 20/20 | 6/6 | 0.00 | 1.00 | 1.00 |
| 20/25 | 6/7.5 | 0.10 | 0.80 | 1.25 |
| 20/30 | 6/9 | 0.18 | 0.67 | 1.50 |
| 20/32 | 6/9.6 | 0.20 | 0.63 | 1.60 |
| 20/40 | 6/12 | 0.30 | 0.50 | 2.00 |
| 20/50 | 6/15 | 0.40 | 0.40 | 2.50 |
| 20/60 | 6/18 | 0.48 | 0.33 | 3.00 |
| 20/63 | 6/18.9 | 0.50 | 0.32 | 3.15 |
| 20/70 | 6/21 | 0.54 | 0.29 | 3.50 |
| 20/80 | 6/24 | 0.60 | 0.25 | 4.00 |
| 20/100 | 6/30 | 0.70 | 0.20 | 5.00 |
| 20/125 | 6/37.5 | 0.80 | 0.16 | 6.25 |
| 20/160 | 6/48 | 0.90 | 0.13 | 8.00 |
| 20/200 | 6/60 | 1.00 | 0.10 | 10.00 |
| 20/400 | 6/120 | 1.30 | 0.05 | 20.00 |
Snellen to LogMAR conversion
Divide the denominator by the numerator to get MAR, then take the base-10 log: 20/40 → MAR = 2 → LogMAR = log₁₀(2) = 0.30. LogMAR runs opposite to intuition — lower is better, 0.00 is the 20/20 standard, and each 0.1 step is exactly one row of an ETDRS chart.
LogMAR to Snellen conversion
Reverse it: denominator = 20 × 10^LogMAR. So LogMAR 0.30 → 20 × 10⁰·³ ≈ 20/40, LogMAR 1.0 → 20/200, and negative values beat 20/20 (LogMAR −0.12 ≈ 20/15). The calculator above handles the rounding to standard chart lines.
Snellen to ETDRS letter scores
ETDRS studies often record acuity as a letter score: every letter read is worth 0.02 LogMAR, so approximate letter score = 85 − 50 × LogMAR. 20/20 (LogMAR 0.0) ≈ 85 letters, 20/40 ≈ 70 letters, 20/200 ≈ 35 letters. The full story of how the two chart formats differ is in our ETDRS vs Snellen guide.
Decimal notation, common in Europe and Asia, is simply the Snellen fraction evaluated. Need the physical letter sizes behind these values? See the visual acuity chart reference or print a correctly-sized chart with the free eye chart generator.
Scoring like this,
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Ocular Edge scores every exam in Snellen and LogMAR automatically — per letter, per eye — on calibrated charts running on any TV.