A Spherical Equivalent is a set of two numbers, one value for each eye, that gives you an estimate of your eyes' refractive error.  It is not the most accurate number for most people as it does not include the astigmatism component.  The Spherical Equivalent is calculated as follows:

how to calculate spherical equivalent

The axis is removed

  • The axis is not part of the calculation for spherical equivalent at all. It completely disappears.

The cylinder is divided by 2

  • The cylinder power is only present in a particular direction. In other words, it is only present in half the lens. When attempting to combine the cylinder with the sphere, your doctor must take this into account by only taking half of the cylinder.

  • Because cylinder powers only come in steps of -0.25, it is possible that when it is divided by 2, that the number does not end in a multiple of -0.25.

    That is where your eye doctor’s professional judgement comes into play to decide which closest multiple of -0.25 will be the most appropriate for you based on many different considerations.

The sphere and the 1/2 cylinder are combined

  • The sphere and the cylinder are then added together to give you the equivalent sphere.

An example of this would be:

Glasses Prescription:






Right (OD)




Left (OS)




Theoretical Spherical equivalent:

Spherical Equivalent






Calculating the spherical equivalent has many practical uses. Eye doctors are skilled at performing these calculations for many different things, including:

1. Order spherical contact lenses for you

Whether its for color contact lenses or clear ones, contact lenses for astigmatism are more expensive and can create fluctuations in vision if the contact lenses are not stable on your eyes.

If you have a small amount of astigmatism, your eye doctor may take the spherical equivalent of prescription to order you regular (spherical) contact lenses.

2. If your ‘Cylinder’ number is too high for contact lenses

In this case, your doctor would not be taking the true spherical equivalent, he/she you would be applying the same principles in order to drop your ‘Cylinder’ number down to the highest available ‘Cylinder’ power for contact lenses for astigmatism. 

In example, if your cylinder is -3.25 but your doctor wants to put you in a contact lens that has a maximum cylinder correction of -2.75, he/she will have to apply the principles of the spherical equivalent in order to do so.

3. To help you adjust to your new glasses prescription

If your eye doctor detects a large change in your cylinder, it is not uncommon for them to reduce the cylinder using the principles of the spherical equivalent in order to help you adjust to your new glasses. New glasses containing a large change in cylinder can cause symptoms of eyestrain, dizziness and headaches.


Remember that the higher your ‘Cylinder’ number is in the original prescription, the worse your vision will be through the Spherical Equivalent.

Generally speaking, if your ‘Cylinder’ is higher than -1.00, your eye doctor will most likely advise against using the Spherical Equivalent to order contact lenses, as the vision will be too greatly affected. 

Also remember that using your spherical equivalent with any amount of ‘Cylinder’ in your glasses prescription will result in some amount of blurriness. If you’re that person who cannot tolerate any blur in his/her vision, do not allow your Spherical Equivalent to be used for your contact lenses. You will not be happy with the outcome.

If you have astigmatism and you’re interested in ordering color contact lenses, speak with your eye doctor about what he/she can do with the numbers in your prescription in order to help you out.


Once an eye doctor has issued you a valid contact lens prescription, you can then use it to order contact lenses in stores or online. Depending on the website you chose to buy your contact lenses from, you may or may not have to provide the website with your prescription (depending on the regulations governing the site).

Regardless of whether you have to provide your contact lens prescription or not, it is always extremely important to only use a valid contact lens prescription to order contact lenses. 

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