Convert diopters into estimated 20/xx clarity quickly today. Add cylinder and age for refined estimates. See if correction may return vision near 20/20 levels.
These are example estimates using the defocus model: x ≈ 20 × 10^(0.3 × |D|). Real outcomes vary.
| Defocus |D| (D) | Estimated 20/x | Nearest common 20/x | Metric 6/y |
|---|---|---|---|
| 0.00 | 20/20 | 20/20 | 6/6 |
| 0.25 | 20/24 | 20/25 | 6/8 |
| 0.50 | 20/28 | 20/30 | 6/9 |
| 1.00 | 20/40 | 20/40 | 6/12 |
| 1.50 | 20/56 | 20/60 | 6/18 |
| 2.00 | 20/80 | 20/80 | 6/24 |
| 3.00 | 20/159 | 20/160 | 6/48 |
| 4.00 | 20/317 | 20/320 | 6/96 |
This calculator supports two estimation approaches. Both are simplified and intended for educational planning only.
First compute the spherical equivalent (SE) from your prescription:
SE = Sphere + (Cylinder ÷ 2)
Then estimate the Snellen denominator x in 20/x using the magnitude of defocus:
x ≈ 20 × 10^(0.3 × |SE|)
This uses a published screening conversion chart that maps refractive state to an estimated visual acuity. Hyperopia estimates depend strongly on age group.
This calculator estimates distance visual acuity from refractive error, expressed in diopters (D). It reports an approximate Snellen result (20/x), a metric equivalent (6/y), decimal acuity, and logMAR. These values help you compare “clarity” across different formats used in clinics and research. It also helps document changes before and after updated correction.
Enter sphere and, if available, cylinder and axis from a prescription. Sphere describes myopia (negative) or hyperopia (positive). Cylinder represents astigmatism strength, and axis describes its orientation. Axis is shown for completeness, but the estimate mainly uses the combined strength of sphere and cylinder.
The tool converts your numbers into spherical equivalent (SE): SE = Sphere + (Cylinder ÷ 2). SE summarizes refractive power into one value. For example, −2.00 sphere with −1.00 cylinder becomes SE = −2.50 D. Larger |SE| generally means blur increases and Snellen denominator rises.
Choose the defocus approximation for a broad, quick estimate, or the age-based screening chart for a more structured reference. The defocus model uses x ≈ 20 × 10^(0.3 × |SE|). The chart method interpolates in logMAR space using typical screening conversion points.
Snellen 20/x is familiar but unevenly spaced. Decimal acuity is 20 ÷ x, so 20/40 equals 0.50. logMAR is log10(x ÷ 20), where lower numbers are better. For reference: 20/20 ≈ 1.00 decimal and 0.00 logMAR, 20/40 ≈ 0.50 decimal and 0.30 logMAR, and 20/200 ≈ 0.10 decimal and 1.00 logMAR. On many charts, a 0.10 logMAR change is roughly one line. The calculator snaps to common chart lines to match real test steps.
Hyperopia can look “better” in younger eyes because accommodation can partially compensate. That is why age group is used in the screening chart option. If you select an older group, the same plus diopters may produce a worse estimated acuity, reflecting reduced focusing reserve.
The “near 20/20” indicator is a simple rule-of-thumb. It does not confirm normal vision. Lighting, pupil size, contrast, dry eye, cataract, and retinal issues can change real performance. Use the estimate for planning and comparison, not diagnosis.
Diopters do not translate perfectly into acuity because eyes differ and charts differ. Astigmatism, higher-order aberrations, and unequal correction between eyes can reduce clarity beyond SE. For best results, enter current prescription values, compare both methods, and confirm with professional testing.
No. It is an estimate based on simplified models and screening references. Real acuity depends on chart type, lighting, pupil size, eye health, and testing conditions. Use it for education and planning only.
The tool uses spherical equivalent, so cylinder changes SE by half its value. Astigmatism can reduce acuity more than SE suggests, especially if uncorrected. Treat the output as a baseline, not a guarantee.
Snellen charts typically use standard steps like 20/25, 20/40, and 20/100. Snapping rounds the estimate to these common denominators so the result aligns with how vision is usually reported on real charts.
Use the defocus model for a quick approximation across many values. Use the age-based screening chart when you want a reference-style estimate, especially for hyperopia where accommodation and age can change expected performance.
Younger people may accommodate to overcome some hyperopia at distance, making measured acuity appear better than expected. This is variable and can cause eye strain. The chart method adjusts estimates by age group.
logMAR is a standardized scale used in research and many clinics. It spaces vision changes more evenly than Snellen. Lower logMAR means better acuity. The calculator derives logMAR from the snapped Snellen denominator.
This page targets distance acuity (20 feet). Near vision depends on working distance, accommodation, and add power. If you need reading estimates, use a dedicated near-vision or add-power tool and measure with near charts.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.