Can You Use Your Glasses Prescription for Contacts?
No. While they look similar, glasses and contacts measure vision correction differently because of the distance from your eye. Using glasses numbers for contacts often leads to blurry vision and eye strain. In this guide, we’ll show you why they differ and how to accurately convert your prescription.
Understanding the Difference Between Glasses and Contacts
The main difference between these two corrective tools is simple. It comes down to position.
Eyeglasses sit on your nose. The lenses rest a short distance away from your eyes. This gap is usually about 12 millimeters.
Contact lenses sit directly on the tear film of your eye. There is no gap.
This difference in position changes how light focuses on your retina. A lens that sits 12 millimeters away requires a different power than one that touches the eye.
The Role of Vertex Distance in Conversion
The distance between the back of a lens and the front of your eye is called the vertex distance.
This measurement is the key to converting a prescription.
Physics dictates that the adequate power of a lens changes as you move it closer to the eye.
If you have a mild prescription, this change is minimal. It might not be very important.
If you have a higher prescription, the change is significant. The general rule is that prescriptions over 4.00 diopters (either positive or negative) require a conversion.
The Scientific Principles Behind Prescription Conversion
Light bends when it passes through a corrective lens. The goal is to make that light land ideally on your retina.
When you move a lens closer to your eye, the focal point shifts.
For nearsighted people who wear minus lenses, moving the lens closer increases its effective power. The lens acts strongly when it touches the eye. Therefore, you need a lower number for contacts than for glasses.
For farsighted people who wear plus lenses, moving the lens closer decreases its effective power. The lens acts weaker when it touches the eye. Therefore, you need a higher number for contacts than for glasses.
Basics of Prescription Conversion
You can estimate the change in power if you know the rules.
Remember that this only applies if your prescription is greater than +/- 4.00.
For Minus Prescriptions (Nearsightedness):
You will need less power for your contacts. If your glasses say -5.00, your contacts might be -4.75.
For Plus Prescriptions (Farsightedness):
You will need more power for your contacts. If your glasses say +5.00, your contacts might be +5.25.
If your prescription is -1.50 or +2.00, the numbers likely stay the same. The vertex distance does not impact low numbers enough to matter.
How to Read a Contact Lens Prescription
Your contact lens prescription has extra information that you will not find on your glasses script.
Here are the main abbreviations you need to know:
- OD: This stands for oculus dexter. It means your right eye.
- OS: This stands for Oculus sinister. It means your left eye.
- PWR / SPH: This is the power or sphere. It indicates the strength of correction.
- BC: This stands for Base Curve. It measures the curvature of the lens.
- DIA: This stands for Diameter. It measures the width of the lens.
The Base Curve and Diameter are critical for comfort. We will discuss them in detail later.
How to Use Prescription Conversion Tables
Eye doctors often use charts to make quick conversions. You can view the logic below to understand how the numbers shift.
Minus Power Conversion Example
Glasses Prescription
Estimated Contact Prescription
-4.00 to -4.25 -4.00
-4.50 to -4.75 -4.25 to -4.50
-5.00 to -5.50 -4.75 to -5.00
-6.00 to -6.25 -5.50 to -5.75
-8.00 -7.00 to -7.25
Plus Power Conversion Example
Glasses Prescription
Estimated Contact Prescription
+4.00 to +4.25 +4.00 to +4.25
+4.50 to +4.75 +4.75 to +5.00
+5.00 to +5.25 +5.25 to +5.50
+6.00 +6.50
Note that these are estimates. The exact number depends on the precise vertex distance used during your eye exam.
The Process of Calculating Contact Lens Power
If you want to understand the math, there is a formula used in optics.
The formula is: Fc = F / (1 - xF)
- Fc is the contact lens power.
- F is the glasses lens power.
- x is the vertex distance in meters (usually 0.012 for 12mm).
Let us try a high minus example.
Your glasses are -10.00.
We calculate -10 divided by (1 minus (0.012 times -10)).
The result is roughly -8.9.
Your doctor would likely prescribe a -9.00 contact lens.
As you can see, a -10.00 glasses prescription becomes a -9.00 contact lens. That is a full 1.00 diopter difference. This proves why you cannot simply guess.
Key Measurements: Base Curve and Diameter
This is the most important section for your safety.
You cannot calculate Base Curve (BC) or Diameter (DIA) from a glasses prescription. These numbers do not exist on your glasses script.
Base Curve
This measures how steep or flat your cornea is. A contact lens must match the shape of your eye.
If the base curve is too tight, it will starve your eye of oxygen. It acts like a suction cup. This can damage your cornea.
If the base curve is too loose, the lens will slide around. It might fall out when you blink. It will also be very uncomfortable.
Diameter
This is the size of the lens. It determines how much of your eye is covered. If the Diameter is wrong, the lens will irritate your eyelids or affect your vision.
Only an eye doctor can measure your Base Curve and Diameter. They use a machine called a keratometer or a corneal topographer to get these numbers.
Astigmatism Considerations and Toric Lenses
Astigmatism adds another layer of complexity.
If you have astigmatism, your glasses prescription has a Cylinder (CYL) and an Axis value.
For very low amounts of astigmatism (under 0.75), doctors often use a "spherical equivalent." They drop the cylinder part and adjust the sphere power slightly to compensate.
For higher amounts of astigmatism, you need Toric lenses. These lenses are weighted. They must stay in a specific position in your eye to work.
If you rotate a spherical lens, the vision is the same. If you rotate a toric lens, the vision gets blurry.
Converting a glasses prescription to a toric contact lens requires professional expertise. You must ensure the lens does not rotate excessively when you blink.
Understanding Contact Lens Fitting Sessions
A fitting session is a specialized appointment. It is different from a routine eye exam.
During a fitting, the doctor will place a trial lens on your eye. They will use a slit lamp microscope to watch the lens moves.
They look for three things:
- Centration: Does the lens stay in the center of the eye?
- Movement: Does the lens move slightly when you blink? It needs to move to allow tears to flow underneath.
- Rotation: For astigmatism, does the lens stay at the correct angle?
You cannot do this at home. This step confirms that the math we discussed earlier actually works for your specific eye shape.
Common Mistakes to Avoid
Many people try to bypass the doctor to save time or money. This often leads to wasted money on lenses that do not fit.
Guessing the Base Curve
You might see a default base curve like 8.6 on a website. You assume it fits everyone. It does not. An 8.6 might be too loose for someone who needs an 8.4.
Ignoring Brand Differences
Not all lenses are the same. A -4.00 in one brand might feel different than a -4.00 in another brand. The materials allow different amounts of oxygen to pass through.
Self-Prescribing Toric Lenses
Trying to convert your own astigmatism numbers usually fails. The axis of a contact lens can shift slightly compared to glasses.
Potential Adjustments Post-Conversion
Even with perfect math, the first lens might not be the winner.
The brain works in mysterious ways. Sometimes a patient sees 20/20 on the chart but feels dizzy.
The doctor might need to adjust the power. They might lower the prescription to improve comfort. They might change the material if your eyes feel dry.
This is why trial lenses are so important. They let you test the conversion in the real world before you buy a year's supply.
Ensuring Comfort and Fit with Contacts
Comfort is king. You should not feel a contact lens once it settles.
If you feel scratching or irritation, the fit is likely wrong. The lens might be too flat or too steep.
Proper fit also protects your long-term eye health. A tight lens can cause neovascularization. This is when new blood vessels grow into your cornea because it is gasping for oxygen. This can threaten your vision.
Importance of Professional Eye Exams
You might feel confident with the math now. You understand vertex distance and base curves.
However, you still need a doctor to sign off on the prescription. In the United States and many other countries, it is illegal to sell contact lenses without a valid prescription.
Contact lenses are classified as medical devices by the FDA. This classification exists to protect consumers from eye infections and corneal ulcers.
Final Thoughts on Maintaining Eye Health
Converting a glasses prescription to contacts involves science, math, and anatomy.
The math for the power conversion is straightforward. The fitting of the physical lens is complex.
Your eyes are irreplaceable. While it is helpful to understand how your prescription works, you should always trust a professional to finalize the numbers. A proper fit ensures you see clearly and keep your eyes healthy for years to come.
Frequently Asked Questions
Can You Convert a Glasses Prescription to Contacts?
You can estimate the values using math, but you cannot create a valid prescription yourself. You need a doctor to specify the Base Curve and Diameter for a safe fit.
Are Glasses Prescriptions and Contact Lens Prescriptions the Same?
No. They measure correction differently because glasses sit away from the eyes and contacts sit directly on the eyes. They also contain different measurements for fit.
What Is Vertex Distance?
Vertex distance is the space between the back of your corrective lens and the front of your cornea. It is usually about 12 to 14 millimeters for glasses and zero for contacts.
Can I use my glasses prescription for contacts?
No. Using glasses numbers for contacts will often result in the wrong power. It also lacks the necessary fitting information, like the base curve.
Can I Convert My Prescription Without Seeing an Eye Doctor?
No. You can do the math to guess the power, but you cannot measure the shape of your eye at home. You need an exam to get the Base Curve and Diameter.
How Often Should Contact Lens Prescriptions Be Updated?
Most contact lens prescriptions are valid for one year. You should see your doctor annually to check the health of your corneas and ensure the fit is still correct.
Why do I need an exam for glasses if I just had one for contact lenses?
A contact lens exam checks the fit of the lens on the eye. A glasses exam checks the refraction at a distance. Two different procedures verify different aspects of your vision health.
