Enter your Sphere (SPH) and Cylinder (CYL) values for each eye from your optical prescription to get an estimated severity of your refractive error. This tool provides an interpretation of your prescription strength, not a direct calculation of your visual acuity (e.g., 20/20).
Diopters
Diopters
Diopters
Diopters
Estimated Severity:
function calculateVisionSeverity() {
var sphOD = parseFloat(document.getElementById("sphOD").value);
var cylOD = parseFloat(document.getElementById("cylOD").value);
var sphOS = parseFloat(document.getElementById("sphOS").value);
var cylOS = parseFloat(document.getElementById("cylOS").value);
// Validate inputs
if (isNaN(sphOD) || isNaN(cylOD) || isNaN(sphOS) || isNaN(cylOS)) {
document.getElementById("resultOD").innerHTML = "Please enter valid numerical values for all fields.";
document.getElementById("resultOS").innerHTML = "";
document.getElementById("resultOverall").innerHTML = "";
return;
}
var resultOD = interpretPrescription(sphOD, cylOD, "Right Eye (OD)");
var resultOS = interpretPrescription(sphOS, cylOS, "Left Eye (OS)");
document.getElementById("resultOD").innerHTML = resultOD.message;
document.getElementById("resultOS").innerHTML = resultOS.message;
// Determine overall severity
var overallSeverityLevel = Math.max(resultOD.severityLevel, resultOS.severityLevel);
var overallMessage = "";
if (overallSeverityLevel === 0) {
overallMessage = "Overall: Very mild or no significant refractive error detected.";
} else if (overallSeverityLevel === 1) {
overallMessage = "Overall: Mild refractive error.";
} else if (overallSeverityLevel === 2) {
overallMessage = "Overall: Moderate refractive error.";
} else if (overallSeverityLevel === 3) {
overallMessage = "Overall: Severe refractive error.";
} else {
overallMessage = "Overall: Significant refractive error requiring professional attention.";
}
document.getElementById("resultOverall").innerHTML = "" + overallMessage + "";
}
function interpretPrescription(sphere, cylinder, eyeLabel) {
var sphAbs = Math.abs(sphere);
var cylAbs = Math.abs(cylinder);
var sphType = sphere 0 ? "Farsightedness" : "No Spherical Error");
var sphSeverity = "";
var cylSeverity = "";
var overallSeverityLevel = 0; // 0: None/Very Mild, 1: Mild, 2: Moderate, 3: Severe
// Interpret Sphere
if (sphAbs <= 0.25) {
sphSeverity = "Very mild spherical error";
overallSeverityLevel = Math.max(overallSeverityLevel, 0);
} else if (sphAbs <= 2.00) {
sphSeverity = "Mild " + sphType;
overallSeverityLevel = Math.max(overallSeverityLevel, 1);
} else if (sphAbs <= 5.00) {
sphSeverity = "Moderate " + sphType;
overallSeverityLevel = Math.max(overallSeverityLevel, 2);
} else {
sphSeverity = "Severe " + sphType;
overallSeverityLevel = Math.max(overallSeverityLevel, 3);
}
// Interpret Cylinder
if (cylAbs <= 0.25) {
cylSeverity = "Very mild or no astigmatism";
overallSeverityLevel = Math.max(overallSeverityLevel, 0);
} else if (cylAbs <= 1.00) {
cylSeverity = "Mild astigmatism";
overallSeverityLevel = Math.max(overallSeverityLevel, 1);
} else if (cylAbs <= 2.50) {
cylSeverity = "Moderate astigmatism";
overallSeverityLevel = Math.max(overallSeverityLevel, 2);
} else {
cylSeverity = "Severe astigmatism";
overallSeverityLevel = Math.max(overallSeverityLevel, 3);
}
var message = "" + eyeLabel + ": " + sphSeverity;
if (cylAbs > 0.25) { // Only add astigmatism detail if it's significant
message += " with " + cylSeverity + ".";
} else {
message += ".";
}
return { message: message, severityLevel: overallSeverityLevel };
}
Understanding Your Vision Prescription: What Do the Numbers Mean?
Many people receive an optical prescription after an eye exam but aren't entirely sure what the various numbers and abbreviations signify. While this calculator cannot determine your exact visual acuity (like 20/20 vision), it can help you understand the estimated severity of your refractive error based on the values provided by your eye care professional.
What is a Vision Prescription?
A vision prescription is a document from an optometrist or ophthalmologist that specifies the lens power required to correct your vision. It's crucial for ordering eyeglasses or contact lenses. Here are the common abbreviations and what they mean:
OD (Oculus Dexter): Refers to your Right Eye.
OS (Oculus Sinister): Refers to your Left Eye.
OU (Oculus Uterque): Refers to Both Eyes (less common on prescriptions for specific lens powers).
SPH (Sphere): This indicates the main lens power, measured in diopters, needed to correct nearsightedness (myopia) or farsightedness (hyperopia).
A minus sign (-) indicates nearsightedness (difficulty seeing distant objects).
A plus sign (+) indicates farsightedness (difficulty seeing near objects).
The higher the number (regardless of sign), the stronger the prescription.
CYL (Cylinder): This indicates the lens power needed to correct astigmatism. Astigmatism is an imperfection in the curvature of your eye's cornea or lens, leading to blurred vision at all distances.
It's almost always a minus sign (-) on modern prescriptions.
A higher number indicates a greater degree of astigmatism.
AXIS: This number, ranging from 1 to 180 degrees, indicates the orientation or angle of your astigmatism. It's always present when a CYL value is present. While important for lens manufacturing, it doesn't directly contribute to the "strength" of the refractive error in the same way SPH and CYL do.
ADD (Add Power): This is the additional magnifying power applied to the bottom part of multifocal lenses (bifocals, trifocals, progressive lenses) for reading or close-up work. It's typically a positive (+) number and is usually the same for both eyes. This corrects presbyopia, the age-related loss of near focusing ability.
PRISM: Less common, prism power is used to correct eye alignment problems (strabismus) or double vision.
How to Interpret Prescription Strength
While there's no single formula to convert your prescription directly into a "20/X" visual acuity score, the numbers themselves give a strong indication of the degree of correction required. Generally:
Mild Refractive Error: Sphere values between +/-0.25 and +/-2.00 diopters, and Cylinder values between +/-0.25 and +/-1.00 diopters. People with mild errors might only need glasses for specific tasks like driving or reading.
Moderate Refractive Error: Sphere values between +/-2.25 and +/-5.00 diopters, and Cylinder values between +/-1.25 and +/-2.50 diopters. Correction is usually needed for most daily activities.
Severe Refractive Error: Sphere values greater than +/-5.00 diopters, and Cylinder values greater than +/-2.75 diopters. These indicate significant vision impairment without correction.
This calculator uses these general guidelines to provide an estimated severity level for your prescription.
Limitations of This Estimator
It's crucial to understand that this tool provides an estimation of refractive error severity based on common optical guidelines. It does NOT:
Calculate your exact visual acuity (e.g., 20/20, 20/40). Visual acuity is measured in an eye exam using an eye chart, not calculated from prescription numbers alone.
Diagnose any eye conditions. Only a qualified eye care professional can diagnose and treat eye health issues.
Account for other factors affecting vision. Your overall vision quality is influenced by many factors beyond just refractive error, such as eye health, age, and other medical conditions.
When to See an Eye Care Professional
Regular eye exams are vital for maintaining good eye health and ensuring your prescription is up-to-date. If you experience any changes in vision, discomfort, or have concerns about your eye health, always consult an optometrist or ophthalmologist.