Convert astigmatism into normal contact lenses

Many people who want to wear contacts wonder if they can wear normal contact lenses with astigmatism. The answer is yes, but there is a simple math formula involved to convert your astigmatism prescription into a normal prescription equivalent. If you haven’t read why you CAN wear normal contacts with astigmatism, then I recommend going through this article first. If you’re all set, then Click here to jump to that conversion formula, and have your optometrist provided prescription ready.

Wanting to wear contact lenses could be for any of the following reasons:

  • Looking a certain way for an occasion
  • Wearing wear non prescription sunglasses
  • Playing high intensity sports where your glasses risk falling off your face or breaking from getting hit
  • Just to try them

Whatever your reason is, it’s important you wear contact lenses that help you see. This means they have to be of the right power to match your eye prescription. I am assuming you have your prescription with you from the last time you went to the optometrist.

Buying contact lenses online

These days, buying contact lenses is as easy as going to some website, selecting a product, quantity, and entering the prescription number for each eye.

On these websites though, you don’t actually enter the prescription. Instead you have to pick the number from a long drop down list that matches your prescription. For standard, normal contact lenses, there is only one number to pick, the spherical value.

What astigmatism people need to know about buying contact lenses online

If you have astigmatism, then your prescription should have a number for cylinder and axis as well, but you will notice that you can’t pick these values for standard contact lenses. Your optometrist would have also told you that if you want contact lenses, you need Toric Lenses. Learn more about wearing contacts with astigmatism here. Some websites sell them too with the option to select your cylinder and axis values, but they are so expensive compared to standard lenses that you don’t even want them anymore.

Wearing Contact Lenses with Astigmatism
contact lenses

So now how can you experiment with contacts without paying so much for toric lenses?

Here is a temporary solution:

Convert your astigmatism prescription into standard contact lens equivalents.

It may not be 100% crystal clear, and I certainly don’t recommend long term usage, but it will get you through a night, or a football match, or a fight.

Whatever you do, don’t ignore the cylinder value and go for the lens matching your spherical. You wont see anything with that.

With some basic math, you will be able to see clearly enough with a standard contact lenses, even if you have astigmatism. All you have to do this use this formula:

Standard Contact Lens = Your spherical + (0.5 x your cylinder)

Be wary of the signs. Use a calculator if you need to.

Note: If your cylinder value is 0.5 or less, you dont need to do this conversion. You have little astigmatism and can go ahead wearing a normal contact lens with the spherical value on your prescription. Lets do an example.

If your eye prescription is:

Spherical: -3.25

Cylinder: -1.00

Axis: 15

Then your normal contact lens equivalent is

-3.25 + 0.5 x (-1.00) = -3.75

If your eye is

Spherical: -3.5

Cylinder: 0.5

Axis: 155

You may simply use a -3.5 contact lens since your astigmatism is not severe.

When dealing with positive prescriptions, the same formula applies. For example

spherical: 2.5

Cylinder: 1.00

axis: whatever

Your standard equivalents is

2.5 + 0.5 x 1.00 = 3.00.

The websites usually list the lens powers in divisions of 0.25. Pick the one closest to what the formula gave you.

This is a workable starting point for experimenting with contacts without having to pay for expensive toric lenses. These websites also offer smaller quantities in case it doesn’t work out for you. If contacts in general are going to be for occasional use, why pay so much for something that is used so infrequently?

If you find that you enjoy wearing contacts and its going to become a regular thing, get proper toric lenses for yourself if you have astigmatism. Go to your optometrist, get a legit prescription for contact lenses, and pay the price.

The use of standard lenses is a temporary thing. You will always see better with the right product for your eyes compared to this makeshift method for rare occasions or trial purposes.

Bonus Material

If you have any interest in conversion factors for physics are chemistry, here‘s a page about that.

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