Many people who wear framed glasses may also wear contact lenses. During a regular eye exam, their eye doctor may fill out both prescriptions for glasses and contacts. But are contact lens prescriptions the same as glasses? If you only have a prescription for glasses, how can you convert it to a contact lens prescription?

  Before buying contact lenses, it's crucial to determine if you're suitable for them. Contact lenses sit directly on your eyes, so if you have any eye health problems or have had eye surgery, it's important to consult your optometrist first to assess your eye condition. Additionally, different contact lens brands, sizes, and materials can significantly affect eye comfort and safety.

  After you confirm all the above, you may ask your eye doctor for a contact lens prescription. It’s important to know that prescriptions for glasses and contacts are different. A glasses prescription cannot be directly switched into a contact lens prescription but can help you get an approximate power range. Here’s a step-by-step guide to help you understand how to convert your glasses prescription to a contact lens prescription.

Step 1: Know about Your Glasses Prescription

• Sphere (SPH): Indicates the lens power, measured in diopters (D), needed to correct your nearsightedness (-) or farsightedness (+).
• Cylinder (CYL): Indicates the lens power needed to correct astigmatism. If there's no number, you either have no astigmatism or it's not significant enough to warrant correction.
• Axis: Specifies the orientation of astigmatism correction in degrees.
• Add: The additional magnifying power applied to the bottom part of multifocal lenses to correct presbyopia (usually only in glasses prescriptions).

Step 2: Consider the Vertex Distance

Vertex distance is the space between the back of the glasses lens and the front of the cornea. Contact lenses sit directly on the eye, so this distance needs to be accounted for. This conversion is more critical for higher prescriptions (usually above ±4.00D).

Here's a general conversion for high prescriptions:

• For a glasses prescription of -4.00D or more, the contact lens power will be slightly less.
• For a glasses prescription of +4.00D or more, the contact lens power will be slightly more.

Step 3: Convert the Prescription

Use an online calculator or follow these simplified rules for an approximate conversion:

• For a spherical (SPH) power between 0.00 and ±4.00D, the contact lens power remains the same.
• For powers between ±4.25 and ±8.00D, use the following formula:
  • Glasses Power / (1 - (Vertex Distance * Glasses Power)) = Contact Lens Power
  • Assume vertex distance is 12mm.

Step 4: Add Cylinder and Axis

Contact lenses often have limited options for astigmatism correction. Soft toric lenses correct astigmatism but might not match every glasses prescription perfectly. Your eye care professional will help you find the closest match.

Step 5: Include Base Curve and Diameter

Contact lens prescriptions include additional parameters not found in glasses prescriptions:

• Base Curve (BC): Indicates the curvature of the contact lens that fits your eye.
• Diameter (DIA): The overall size of the lens.

These values are crucial for comfort and proper fit and are determined by your eye care professional.

Step 6: Get a Professional Fitting

Even with all the calculations, the final step is to see an eye care professional. They will:

• Ensure the lenses fit properly.
• Confirm the prescription provides the best vision correction.
• Check for comfort and eye health.

  While understanding the basics of converting a glasses prescription to contact lenses is helpful, it’s essential to consult with an eye care professional for an accurate and safe prescription. Proper fitting and professional guidance are crucial for maintaining eye health and achieving the best vision correction.