Convert prescriptions between different optical units and formats.
None
Up (UP)
Down (DN)
In (IN)
Out (OUT)
Same Units
Meters (for contact lenses)
Reading Addition
Understanding Eye Prescription Conversions
An eye prescription is a detailed document written by an eye care professional that indicates the specific lens power needed to correct a patient's vision. It typically includes values for sphere, cylinder, axis, and sometimes prism and add power. Understanding these values and how they relate to different optical needs, such as for glasses versus contact lenses, or for distance versus near vision, is crucial.
Key Prescription Components:
Sphere (SPH): This indicates the main refractive power of the lens, measured in diopters. It corrects nearsightedness (myopia, indicated by a minus sign) or farsightedness (hyperopia, indicated by a plus sign).
Cylinder (CYL): This measures the amount of astigmatism, a condition where the eye's cornea or lens has an irregular shape, causing blurred vision. It's also measured in diopters and can be positive or negative.
Axis (AXIS): This specifies the orientation (in degrees) of the cylinder power. It ranges from 1 to 180 and indicates where the astigmatism correction should be placed on the lens.
Prism (PRISM): This corrects for eye alignment issues (phoria or tropia), where the eyes don't point in the same direction. It's measured in prism diopters.
Prism Base: Indicates the direction of the prism's thickest edge (e.g., Up, Down, In, Out).
ADD (Addition): This is the extra magnifying power added to the bottom part of a multifocal lens (like bifocals or progressives) to help with near vision. It's typically for presbyopia correction.
Why Convert Prescriptions?
Prescription conversions are necessary for several reasons:
Contact Lens Fittings: Contact lens prescriptions often have different specifications than eyeglass prescriptions, especially regarding base curve and diameter. While this calculator doesn't directly convert those, it helps understand the spherical and cylindrical components which are foundational. Eyeglass sphere power usually requires a slight adjustment for contact lenses to account for the distance between the lens and the eye.
Reading Glasses/Bifocal Addition: If you have a distance prescription and need reading glasses or bifocal lenses, you'll need to add the "ADD" power to the sphere component of your distance prescription for near vision.
Understanding Different Formats: Ophthalmologists and optometrists may use slightly different notations or preferred units.
Interchangeability: Ensuring that a prescription meant for one type of correction can be understood or adapted for another.
How This Calculator Works:
This calculator focuses on converting key optical values.
Sphere, Cylinder, Axis, Prism: These values are generally represented in diopters and degrees and are fundamental to most prescriptions. Prism Base is a directional indicator.
ADD Power: When converting for near vision, the ADD value is added to the SPH and CYL (if applicable, though typically it only applies to SPH for ADD purposes) to create a "reading" prescription. For example, if your distance SPH is -3.00 and your ADD is +2.00, your near vision SPH would effectively be -1.00.
Conversion to Meters (Contact Lenses): For contact lenses, especially for myopia (nearsightedness), the lens sits directly on the eye. This eliminates the air gap present with glasses, which slightly reduces the effective power. A common rule of thumb is to decrease the negative sphere power by 0.25 to 0.50 diopters for every 1 cm the contact lens is closer to the eye than the glasses lens. However, a simplified conversion for understanding spherical power at eye level is shown. For detailed contact lens fitting, always consult an eye care professional.
Important Disclaimer:
This calculator is for informational purposes only. It is not a substitute for professional eye care advice. Always consult with a qualified optometrist or ophthalmologist for accurate diagnosis, prescription, and fitting of eyewear or contact lenses. Incorrect prescription interpretation or use can lead to eye strain, discomfort, or vision problems.
Example:
Let's say you have an eyeglass prescription for distance vision:
SPH: -4.50 D
CYL: -1.25 D
AXIS: 180
ADD: +2.00 D
If you want to understand your near vision reading addition:
Your distance SPH is -4.50. Your ADD is +2.00.
Your effective near SPH would be -4.50 + (+2.00) = -2.50 D.
Your full near prescription might be: SPH -2.50, CYL -1.25, AXIS 180.
If you were to convert just the sphere for a contact lens that sits very close to the eye (e.g., assuming a simplified conversion needs), you might see a slight adjustment. However, precise contact lens prescriptions involve base curve and diameter, which are outside this calculator's scope.
function calculatePrescription() {
var sphere = parseFloat(document.getElementById('sphere').value);
var cylinder = parseFloat(document.getElementById('cylinder').value);
var axis = parseInt(document.getElementById('axis').value);
var prism = parseFloat(document.getElementById('prism').value);
var base = document.getElementById('base').value;
var add = parseFloat(document.getElementById('add').value);
var conversionUnit = document.getElementById('conversion_unit').value;
var resultDiv = document.getElementById('result');
var errorDiv = document.getElementById('errorMessage');
errorDiv.textContent = "; // Clear previous errors
resultDiv.style.display = 'none'; // Hide result by default
var isValid = true;
var errorMessage = ";
// Basic validation for required numeric fields if they have values
if (isNaN(sphere) && document.getElementById('sphere').value !== ") { isValid = false; errorMessage += "Invalid Sphere value. "; }
if (isNaN(cylinder) && document.getElementById('cylinder').value !== ") { isValid = false; errorMessage += "Invalid Cylinder value. "; }
if (isNaN(axis) && document.getElementById('axis').value !== ") { isValid = false; errorMessage += "Invalid Axis value. "; }
if (isNaN(prism) && document.getElementById('prism').value !== ") { isValid = false; errorMessage += "Invalid Prism value. "; }
if (isNaN(add) && document.getElementById('add').value !== ") { isValid = false; errorMessage += "Invalid ADD value. "; }
// Specific validation for Axis range if provided
if (!isNaN(axis) && (axis 180)) {
isValid = false;
errorMessage += "Axis must be between 1 and 180. ";
}
// Check if at least one primary value (SPH, CYL) is entered if conversion is not 'same'
var hasSphOrCyl = !isNaN(sphere) || !isNaN(cylinder);
if (conversionUnit !== 'same' && !hasSphOrCyl && document.getElementById('sphere').value === " && document.getElementById('cylinder').value === ") {
isValid = false;
errorMessage += "Please enter Sphere or Cylinder for conversion. ";
}
if (!isValid) {
errorDiv.textContent = errorMessage.trim();
return;
}
var resultText = ";
var formattedSphere = isNaN(sphere) ? "" : sphere.toFixed(2);
var formattedCylinder = isNaN(cylinder) ? "" : cylinder.toFixed(2);
var formattedAxis = isNaN(axis) ? "" : axis.toString();
var formattedPrism = isNaN(prism) ? "" : prism.toFixed(1);
var formattedBase = base === 'none' ? " : ` (${base.toUpperCase()})`;
var formattedAdd = isNaN(add) ? "" : add.toFixed(2);
if (conversionUnit === 'same') {
resultText = `Prescription: SPH ${formattedSphere}${isNaN(cylinder) ? " : ` CYL ${formattedCylinder} AXIS ${formattedAxis}`}${isNaN(prism) ? " : ` PRISM ${formattedPrism}${formattedBase}`}${isNaN(add) ? " : ` ADD ${formattedAdd}`}`;
} else if (conversionUnit === 'meters') { // Simplified conversion to meters/contact lens equivalent for sphere
if (!isNaN(sphere)) {
// This is a VERY simplified conversion. Real contact lens fitting needs more.
// For myopes (negative sphere), lens is closer, so effective power increases (less negative).
// For hyperopes (positive sphere), lens is closer, so effective power decreases (less positive).
// A common adjustment is -0.25 to -0.50 D for every 1cm difference, but contacts are directly on the eye.
// We'll apply a common simplification for myopes.
var adjustedSphere = sphere;
if (sphere 0) adjustedSphere = 0; // Don't overshoot to positive
} else if (sphere > 0) { // Hyperopia
// For hyperopes, the lens is closer, reducing the need for positive power.
// A common adjustment is to decrease the plus power by 0.25 D.
adjustedSphere = sphere – 0.25;
if (adjustedSphere < 0) adjustedSphere = 0; // Don't overshoot to negative
}
resultText = `Approx. Contact Lens Sphere: SPH ${adjustedSphere.toFixed(2)} D (Note: This is a simplified conversion. Consult your eye care professional for precise contact lens fitting.)`;
} else {
resultText = "Sphere value is required for Meters conversion.";
}
} else if (conversionUnit === 'reading') { // Convert for reading addition
var readingSphere = !isNaN(sphere) ? sphere + (isNaN(add) ? 0 : add) : (isNaN(add) ? '' : `+${add.toFixed(2)}`); // If SPH is missing, just show ADD
var readingCylinder = !isNaN(cylinder) ? cylinder : '';
var readingAxis = !isNaN(axis) ? axis : '';
var readingPrism = !isNaN(prism) ? prism : '';
var readingBase = base === 'none' ? '' : ` (${base.toUpperCase()})`;
resultText = `Reading Addition Prescription: SPH ${readingSphere}${isNaN(cylinder) ? '' : ` CYL ${readingCylinder} AXIS ${readingAxis}`}${isNaN(prism) ? '' : ` PRISM ${readingPrism}${readingBase}`}`;
if (isNaN(sphere) && !isNaN(add)) {
resultText += ` (Based on ADD power: ${add.toFixed(2)} D)`;
} else if (isNaN(add)) {
resultText = "ADD power is required for Reading Conversion.";
}
}
if (resultText) {
resultDiv.innerHTML = resultText;
resultDiv.style.display = 'block';
}
}