Glasses Prescription Calculator
Read an eyeglass prescription: compute the spherical equivalent from sphere and cylinder, and see whether you are nearsighted, farsighted and astigmatic.
How it works
Spherical equivalent = SPH + (CYL ÷ 2) · Sign of SPH: − = nearsighted, + = farsighted
A glasses prescription lists three numbers per eye: sphere (the main lens power in dioptres), cylinder (the extra power that corrects astigmatism) and axis (the direction that cylinder is oriented). The spherical equivalent collapses sphere and cylinder into one figure by adding half the cylinder to the sphere, which is handy for comparing prescriptions or estimating contact-lens power. A negative sphere means light focuses in front of the retina (nearsighted); a positive sphere means it focuses behind (farsighted).
Worked example
For SPH −2.00, CYL −0.75, axis 90, the spherical equivalent is −2.00 + (−0.75 ÷ 2) = −2.375 D. The negative value means nearsighted, the magnitude under 3 D means mild, and a −0.75 cylinder means mild astigmatism oriented at 90°.
Frequently asked questions
What does the spherical equivalent tell me?
It is a single dioptre value that blends your sphere and half your cylinder. Doctors use it to summarise a prescription, compare eyes, and estimate a starting power for contact lenses, which often are not made in every cylinder value.
Is a higher number worse?
A larger magnitude (further from zero, in either direction) means stronger correction is needed. Under about 3 D is generally mild, 3–6 D moderate, and above 6 D high — but this describes strength, not eye health.
Can this replace an eye exam?
No. This only interprets numbers you already have. Prescriptions change over time and must be measured by an optometrist or ophthalmologist, who also check eye health.
Related calculators
Educational estimate only, not medical advice — talk to an eye doctor, dentist, dermatologist or other qualified professional about your own care.