Eyewear and Vision
Spherical equivalent.
Enter the sphere and cylinder from a prescription to get the single-number spherical equivalent. It works with plus or minus cylinder. Runs in your browser; nothing is sent or stored.
Your prescription (one eye)
Spherical equivalent
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The short answer
Spherical equivalent = sphere + half the cylinder. For -2.00 -1.50 x 090 it is -2.00 + (-0.75) = -2.75 D.
Worked example
A prescription of -2.00 -1.50 x 090: half the cylinder is -0.75, so the spherical equivalent is -2.00 + (-0.75) = -2.75 D. The two principal meridians are -2.00 D and -3.50 D, and the equivalent sits halfway between them.
A farsighted case: +1.25 -0.75 x 180 gives +1.25 + (-0.375) = +0.88 D, which is +1.00 D to the nearest quarter.
How it works
Spherical equivalent (SE) = sphere + cylinder / 2. It is the single spherical lens power that puts the circle of least confusion on the retina, so it summarises an astigmatic prescription as one number. The axis does not enter the sum. The formula is the same whichever sign convention the cylinder is written in; just keep the sign as printed.
SE is used in studies and for comparing prescriptions. It is a summary, not a lens you can wear in place of the full prescription: it ignores the astigmatism.
Sources (opened 2026-10-08): Wavefront Automated Refraction Comparison of Three Different IOLs, Vision (Basel) 2026;10(1):6, doi:10.3390/vision10010006 (PubMed Central full text); American Academy of Ophthalmology, How to Read an Eyeglasses Prescription (general background on prescription notation only; it does not discuss spherical equivalent).
FAQ
How do you calculate spherical equivalent?
Add half of the cylinder to the sphere. For a prescription of -2.00 -1.50 x 090, half the cylinder is -0.75, so the spherical equivalent is -2.75 D.
Does the axis matter for spherical equivalent?
No. The axis tells you where the astigmatism lies but it does not change the spherical equivalent.
Does it work with plus cylinder notation?
Yes. Use the sphere and cylinder with their signs as written. The result is the same for the transposed form of the same prescription.
Is spherical equivalent the same as my prescription?
No. It is a one-number summary that ignores astigmatism. Glasses and contacts are made from the full sphere, cylinder and axis.
Why do studies report spherical equivalent?
Because it reduces sphere and cylinder to one comparable number. In published work it is typically defined as the sphere plus half the cylinder.
Can I use this to pick contact lenses?
No. Contact lenses need a fitting by an eye-care professional. A spherical equivalent is a summary and is not a contact lens prescription.
Disclaimer
Not medical advice. This tool does simple arithmetic on numbers you type in. It does not replace an eye exam, a prescription or the advice of an optometrist or ophthalmologist, and it is not a way to order or change a prescription.