How to use it
- Type the acuity. A fraction such as 20/40, 6/12 or 10/20 is understood in any mode.
- For a single number, choose what it means: the denominator over 20 ft, the denominator over 6 m, a decimal acuity or a logMAR value.
- Read the line in every notation and its ICD-11 category.
How it works
All notations describe the same thing, the smallest letter detail you can see:
- Decimal = test distance ÷ letter distance, so 20/40 = 6/12 = 0.5
- MAR = 1 ÷ decimal (minutes of arc), 2′ for 20/40
- logMAR = log10(MAR), 0.30 for 20/40
- Snellen in feet = 20/(20 × MAR), in metres = 6/(6 × MAR)
| Feet | Metres | Decimal | logMAR |
|---|---|---|---|
| 20/20 | 6/6 | 1.0 | 0.00 |
| 20/25 | 6/7.5 | 0.8 | 0.10 |
| 20/40 | 6/12 | 0.5 | 0.30 |
| 20/60 | 6/18 | 0.33 | 0.48 |
| 20/200 | 6/60 | 0.1 | 1.00 |
Worked example
Type 0.8 and choose decimal acuity: MAR = 1 ÷ 0.8 = 1.25, logMAR = log10(1.25) = 0.10, in feet 20 × 1.25 = 20/25 and in metres 6 × 1.25 = 6/7.5. That is no impairment.
Tips
- Denominators are rounded: below 10 to one decimal (6/7.5), above that to whole numbers, so a logMAR of 0.3 shows as 20/40 although the exact value is 39.9.
- Charts in different countries use slightly different line steps; the conversion is exact, the printed lines may not be.
- Glasses or contact lenses matter: note whether the acuity was measured with or without them.
- The converter only changes notation; an eye test needs an optometrist or ophthalmologist.