Realizing that the numbers in your glasses prescription are not always the same numbers that are needed for your contact lenses is an important first step in being successful with contacts. Ordering the wrong power in your contact lenses can result in blurry vision, eyestrain, headaches, double vision, etc.
When it comes to converting a glasses prescription to contact lenses, there are different ways of going about it for different people. This article will describe 3 methods of converting a glasses prescription to contact lenses and who should be using each one.
1. Plan a Visit to Your Eye Doctor’s Office
This is the method that most people are familiar with. If you need glasses or contact lenses, just head over to your eye doctor’s office and hope for the best when it comes to your wallet.
Your eye doctor is trained to assess the fit of contact lenses on your eyes and to convert the numbers on your glasses prescription to make them appropriate for contact lenses. At the end of a contact lens assessment, your eye doctor will provide you with a contact lens prescription that you can use to order contact lenses anywhere you please.
The Contact Lens Rule
Eye doctors are required by law to provide you with a glasses prescription after an eye exam and a contact lens prescription after a contact lens assessment. They cannot withhold your prescription in an attempt to force you to purchase your eye-wear from them. If your optometrist doesn’t give you a prescription, they are in violation of the law. If your eye doctor refuses to comply with the law, you can help by alerting the FTC of their unjust practices. You can do so here. To learn more about issuing a complaint, click here.
In the United States, online contact lens vendors are required to request your contact lens prescription information before they can sell contact lenses to you. The information you submit is sent to your eye doctor where he/she will have the option of reviewing it.
The law is written in such a way that upon receiving a prescription verification request from an online vendor, your eye doctor has 8 hours to respond to the request. This is where your eye doctor can intervene if they disapprove of what you’re attempting to order. However, failure of your eye doctor to give a response with 8 hours results in a defacto approval of your order.
Does every optometrist comply?
Unfortunately, just as with almost all jobs and professions, there are some optometrists who like to cut corners in order for them to save time, see more patients, and collect more fees. I have heard many accounts of optometrists charging for a “contact lens assessment”, while never doing more than a simple conversion of your glasses prescription on paper which can be performed in under 15 seconds (by anyone).
It sounds rather obvious when I type this out, but a contact lens assessment actually requires your eye doctor to assess your contact lenses. This means he/she must do a vision check for each eye wearing a contact lens to make sure it’s acceptable. They must also evaluate the centration and curvature of the contact lenses on your eyes. And if you’re lucky, they might even ask you how you feel in the contact lenses.
2. Using the Back Vertex Formula
In the United States, you cannot use methods 2 and 3 in this article to order your contact lenses. These methods are reserved for opticians, optometrists, and any eye-care professional in training for the purposes of practice.
The ‘Back Vertex Formula’ is a mathematical equation which is what eye doctors use to produce a theoretical power for contact lenses from a glasses prescription.
The Back Vertex Formula is:
As you can see, this is a fairly simple equation with only 2 variables.
Fc represents the theoretical power of contact lenses
F represents the power in the glasses
X represents the vertex distance
The vertex distance refers to the distance (in meters) between your eye and the back of your lenses on a correctly fitting pair of glasses. Generally, this distance is the standard testing distance (12-14mm) that optometrists use when measuring your glasses prescription. However, your optometrist may choose to use a different testing distance if he/she is so inclined.
So let’s do a sample calculation using the back vertex formula.
Let’s use the following parameters:
F = -5.00
x = 0.013 (remember that x is in meters)
Fc = -5.00 / (1 – [0.013 * -5.00] )
Fc = -5.00 / (1 – [ -0.065] )
Fc = -5.00 / 1.065
Fc = -4.70
Of course, a glasses prescription contains much more than 1 number, so which number do you have to use for ‘F’?. The answer is, it depends.
If there is a ‘Sphere’ number and no ‘Cylinder’ number, you use the ‘Sphere’ number as F, and you only perform the calculation once (per eye).
If there is a ‘Sphere’ number and a ‘Cylinder’ number, then you have to do the calculation twice. Once with the ‘Sphere’ number as ‘F’ and once with the ‘Sphere’ + ‘Cylinder’ number as ‘F’.
If you are working with a prescription that has both ‘Sphere’ and ‘Cylinder’, you may want to look into performing a power cross diagram. You can learn more about power cross diagrams in this tutorial.
If you think that doing all these calculations and diagrams is too annoying and troublesome, don’t worry you’re not alone. Even optometrists don’t perform these calculations. All of the answers have already been calculated and organized into easy to use charts called Back Vertex Charts. And this brings us to Method 3.
3. Using Back Vertex Charts
Again, these types of charts are designed to assist eye-care professionals in performing on-the-fly conversions to save them time in clinical practice. They are not meant to bypass getting a professional contact lens assessment.
In a busy optometry clinic or optical shop, optometrists and opticians don’t have time to do back vertex calculations using the formula I provided above. Instead, they use reference charts that list the solutions to all the possible prescriptions they might encounter on a day to day basis.
Here’s an example of how a Vertex Conversion Chart is used.
Right: Sphere -5.50 Cylinder none Axis none
Left: Sphere -6.25 Cylinder none Axis none
They would find the answer by looking up -5.50 and -6.25 in a chart such as this one:
In the example above, the converted numbers would be:
Right: Sphere -5.12 Cylinder none Axis none
Left: Sphere -6.75 Cylinder none Axis none
However, the issue with these charts is that they only convert one number at a time. If you have any astigmatism in your prescription (any numbers in the ‘Cylinder’ column), then you have 2 numbers that need to be converted.
But hold on…
The numbers in the ‘Cylinder’ column cannot be converted in the same way as the numbers in the ‘Sphere’ column.
It’s because the ‘Cylinder’ does not actually represent a power found in your glasses or contacts. It denotes the difference in power between your primary and secondary powers.
So what to do if a glasses prescription has both ‘Sphere’ and ‘Cylinder’ in it? Well, this is where things get a little complicated. Eye care professionals use a technique called a ‘power cross’ to deconstruct the numbers in the glasses prescription, they then use vertex conversion charts to convert each power, and then they reconstruct the traditional sphere/cylinder prescription format.
You can learn more about power cross diagrams here.
Confused? Don’t give up just yet!
Thanks to Contacts Advice, there is now an easier solution than using the power cross.
The Easy Method
If one performed all the calculations necessary to convert every single possible Sphere and Cylinder combinations that one might see on a glasses prescription, they could develop charts that bypass the need to do the calculations each time.
And it just so happens that I devoted the time to do this and created Contacts Advice’s Vertex Conversion Charts for Sphere and Cylinder Combinations.
If you’re dealing with ‘Sphere’ number that is between 0 and -8.00 and a ‘Cylinder’ number that is between 0 and -3.50, then no matter what the prescription is, you’ll find it on my charts as well as the calculated back vertex powers.
Here is an example prescription with both Sphere and Cylinder:
Right: Sphere -6.00 Cylinder -1.25 Axis 180
Left: Sphere -6.00 Cylinder -2.00 Axis 170
Simply find each of those combinations in the correct chart. In this example, you’d use the one below.
The results of the example above would be:
Right: Sphere -5.62 Cylinder -1.00 Axis 180
Left: Sphere -5.62 Cylinder -1.63 Axis 170
It’s as easy as that! No messy calculations or power cross diagrams required. You just look up the desired ‘Sphere’ and ‘Cylinder’ and you get the back vertex conversion for both instantly.
The chart above is only for ‘Sphere’ and ‘Cylinder’ combinations where the ‘Sphere’ is -6.00. If the ‘Sphere’ you are looking for isn’t -6.00, you will have to use a different chart.
The cylinder numbers for both your eyes are pretty high. Most eye doctors would probably say that you’re not a very good candidate for contact lenses. If you just go to any website that sells contact lenses and look up some contact lens brands for astigmatism, you’ll see that cylinder values that are available usually max out at -2.25. Your cylinder is almost double that. That means that even with the strongest cylinder powers available, you still wouldn’t see very well with contact lenses.
The practical cylinder upper limit for contact lenses is -3.25, but even at that level it’s very difficult to successfully wear contact lenses. The higher your cylinder, the more challenging contact lens wear becomes.
I’m curious, have you worn contact lenses before? If so, which kind (soft/hard) and brand?
Thanks for the question :)
Well, I cannot give you a contact lens prescription, but I can discuss the numbers you presented. Your glasses prescription has -0.50 cylinder in each eye. Contact lenses are never made with cylinders of -0.50. When this case presents itself, most eye doctors would take the spherical equivalent of your glasses prescription as a starting point for your contact lenses.
The spherical equivalent of your glasses numbers is:
Right: Sph -1.00
Left: Sph +0.25
The prescription in your left eye is very low, especially if you consider your spherical equivalent. +0.25 is literally the smallest power available in contact lenses. In fact, some brands don’t offer +0.25 because it is so low. However, if you look at monthly contact lens brands, you should be able to find it.
Please keep in mind that I am not a doctor and this is not meant to be medical advice. You are not being prescribed contact lenses.
Hello, I was wondering why sphere numbers of -1.50 and -1.75 are not on the chart.
Spheres of -1.50 and -1.75 behave the same optically in glasses and in contact lenses. Since the mathematical formula that governs the conversions doesn’t affect those numbers, that is why they don’t appear in the conversion charts.
This information is provided as an information resource only, and is not to be used or relied on for any diagnostic or treatment purposes. This information does not create any patient-doctor relationship, and should not be used as a substitute for professional diagnosis and treatment.
Can you please help tell me what my contact lense prescription would be?
All my doctor wrote on my GLASSes script is
Right = -5.25 cylinder sphere
Left -4.75 cylinder sphere
Hi Nicole, since your glasses script only contains sphere numbers, the place to start for you would be to perform the back-vertex calculations. Using back-vertex charts, we would get:
This is the starting point your eye doctor would most likely use for your contact lens trials. Depending on how well you see with these numbers, he/she would either prescribe those numbers or refine them as he/she sees necessary.
Remember, I am not a doctor, I am not prescribing you contact lenses, and this is not a replacement for professional medical advice :)
I just want to start by making clear that I can not give you a contact lens prescription. I can discuss your prescription with you, but I am not a doctor, and only a qualified doctor can prescribe you contact lenses.
So it seems like your glasses prescription numbers are:
R: Sphere -2.50 Cylinder None Axis None
L: Sphere -3.00 Cylinder -1.00 Axis 170
Can you elaborate on why you think your doctor made an error? Usually, when numbers on a glasses prescription are less than -4.00, doctors don’t need to adjust the numbers due to differences in the vertex distance (the distance between lenses and the eye) between glasses and contact lenses.
Since your numbers are not higher than -4.00, in a typical situation, your doctor would probably start with the numbers on your glasses prescription for your first contact lens trial. He/she would then assess whether any adjustments need to be made to those numbers based on how well you’re seeing, and whether or not they can improve your vision by adjusting the numbers.
Now, of course, contact lenses aren’t available with Cylinders of -1.00, so he/she will probably have selected a -0.75 or -1.25 for your left eye instead of -1.00.
Hope this helped.
My contacts are as follows
Left. D -2.25 DIA 14.0 BC 8.4
Right (astigmatism). D -3.50 DIA 14.5 BC 8.6 CYL -1.25 AXIS 010
I am wearing Acuvue Oasys with Hydraclear Plus.
What would be the correct eye glasses prescription?
Although it is possible to convert a glasses prescription to contact lenses, doing the reverse is not possible.
It’s a little long to explain but I’ve written an full-length article explaining the reasons in depth.
==> Can You Convert a Contact Lens Prescription To Glasses?
Hi, I’m doing some exercises. If I’m given these glasses numbers:
Right : sph -3.00 / cyl -1.50 / axis 5
Left : sph -2.75 / cyl -1.25 / axis 174
Would they change to this:
Right : sph -3.00 / cyl -1.25 / axis 180
Left : sph -2.75 / cyl -1.25 / axis 170
Thanks for your help ;)
Hi Allina, theoretically that would be a good place to start.
For the right eye:
Since -1.50 cylinder is not available in contact lenses, it would generally be brought down to -1.25. An axis of 5 is also not available in contact lenses so it would have to be rounded down to 0 (180) or up to 10. The only way to determine if 180 or 10 is better would be assess the fit of the contact on the patient’s eye.
For the left eye:
Everything would stay the same except the axis which gets rounded to the nearest 10, which would be 170 in this case. However, it is possible that a sub-optimal fit would cause this to have to be adjusted further.
Hope this helps!
Thank you Julie, it helps very much ^_^
What a wonderful effort you have done !
I have question about the axis
My eyeglass prescription is
Sph -6.50 cyl -1.75 axis 15
Sph -6.25 cyl -1.5 axis 170
As you know contacts don’t come with a axis of 15 also cyl 1.38 (after conversion) is not an option ?
That is correct. When eye doctors are faced with these numbers they are forced to round up to down to the nearest available value. A glasses axis of 15 usually gets rounded to 10 or 20, and 1.38 gets rounded to 1.25 or 1.50. Doctors will use your overall situation (contact lens fit, visual demands, etc) to choose the one they think would work best and then refine it from there if necessary.
Hello! My glasses prescription is as follows:
Would my contacts be:
OD Sphere: -3.25
OD Cyl: NaN
OS Sphere: -3.00
OS Cyl: NaN
A contact lens fitter would probably start with:
OD Sphere: -3.50
OS Sphere: -3.00
After seeing how the vision is with that combination, he/she would make any necessary adjustments. Hope this helps!
Hello Julie ,
My doctor gave me this prescription and I wanted to know if I can wear contact lenses.
OD (right) sphere +25 cylinder -75 axis 30
OS(left) Sphere p10 cylinder -50 axis 20
That is a very odd glasses prescription for your left eye sphere. The doctor must have written very sloppily for it to look like p10. Sphere values always start with either a + or a – and are followed by a number ending with .00, 0.25, 0.50 or 0.75.
Alternatively, when the sphere value is zero, the doctor will write ‘pl’ or ‘plano’. That would be my guess.
All things considered, your prescription is very light and most people with numbers like these would see no real value in wearing contact lenses as it would make little difference in vision.