Estimate your Snellen visual acuity based on your diopter sphere (SPH) measurements.
Nearsighted (Myopia -)
Farsighted (Hyperopia +)
Estimated Visual Acuity
20/–
Understanding the Conversion
In the world of optometry, "20/20" is a measure of visual acuity (how sharp you see at a distance), while diopters (the numbers on your prescription like -2.00) measure the corrective power of a lens. While there is no perfect mathematical formula that works for everyone—because factors like pupil size and retinal health vary—clinical approximations allow us to estimate your "un-corrected" vision.
Common Approximations for Myopia:
Diopters (SPH)
Estimated Snellen (US)
Metric Scale (6m)
-0.50
20/25 to 20/30
6/7.5
-1.00
20/40 to 20/60
6/12
-2.00
20/150 to 20/200
6/60
-3.00
20/300+
6/90
How this Calculator Works
The calculator uses a standard clinical logarithmic approximation. For nearsightedness (myopia), every 0.25 diopters of error typically moves you further down the Snellen chart. For example, a -1.00 diopter SPH generally correlates to 20/50 vision, meaning you see at 20 feet what a person with normal vision sees at 50 feet.
Important Considerations
Astigmatism: This calculator primarily uses the Sphere (SPH) value. If you have significant Cylinder (CYL) values, your actual visual acuity may be lower than estimated here.
Hyperopia (Farsightedness): Farsighted individuals can often "accommodate" (flex their eye muscles) to see clearly at a distance, making the 20/20 conversion less linear than it is for myopia.
Legal Blindness: In the United States, a person is considered legally blind if their best-corrected vision is 20/200 or worse in their better eye.
function calculateVisualAcuity() {
var sph = parseFloat(document.getElementById('sphValue').value);
var type = document.getElementById('eyeType').value;
var resultBox = document.getElementById('eyeResultBox');
var output = document.getElementById('snellenOutput');
var detail = document.getElementById('acuityDetail');
if (isNaN(sph)) {
alert("Please enter a valid numeric Sphere value (e.g., -1.25 or 0.75).");
return;
}
var absSph = Math.abs(sph);
var denominator = 20;
if (absSph === 0) {
denominator = 20;
} else if (type === "myopia") {
// Myopia calculation: standard clinical approximation
// Every 0.25 diopters roughly doubles the visual angle or adds 10-20 to denominator
// Using a refined lookup logic for standard points
if (absSph <= 0.25) denominator = 25;
else if (absSph <= 0.50) denominator = 30;
else if (absSph <= 0.75) denominator = 40;
else if (absSph <= 1.00) denominator = 50;
else if (absSph <= 1.25) denominator = 70;
else if (absSph <= 1.50) denominator = 100;
else if (absSph <= 1.75) denominator = 150;
else if (absSph <= 2.00) denominator = 200;
else if (absSph <= 2.50) denominator = 300;
else if (absSph <= 3.00) denominator = 400;
else denominator = Math.round(absSph * 150);
} else {
// Hyperopia is harder to predict due to accommodation
// Generally, low hyperopia still results in 20/20 for young people
if (absSph <= 1.00) denominator = 20;
else if (absSph <= 2.00) denominator = 30;
else if (absSph <= 3.00) denominator = 50;
else denominator = Math.round(absSph * 25);
}
resultBox.style.display = "block";
output.innerHTML = "20 / " + denominator;
var category = "";
if (denominator <= 20) category = "Normal Vision";
else if (denominator <= 40) category = "Mild Visual Impairment";
else if (denominator < 200) category = "Moderate Visual Impairment";
else category = "Severe Visual Impairment / Legal Blindness Range";
detail.innerHTML = "Estimated Scale: " + category + " (Uncorrected)";
// Smooth scroll to result
resultBox.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}