Eye Prescription Explainer
Understand your glasses prescription — SPH, CYL, AXIS and ADD — including whether you're short- or long-sighted, and convert between plus and minus cylinder.
For general information only — not medical advice. Results are estimates; talk to a doctor or qualified health professional about your own health.
How to use
- 1Enter SPH, CYL and AXIS for one eye (and ADD if you have it).
- 2Read the explanation and the equivalent prescription in the other cylinder notation.
How it's calculated
Transposition: new SPH = SPH + CYL, new CYL = −CYL, new AXIS = AXIS ± 90 (kept between 1 and 180). Spherical equivalent = SPH + CYL ÷ 2.
Frequently asked questions
What does a minus SPH mean?
Myopia (short-sightedness): distant objects are blurry. Plus SPH means hyperopia (long-sightedness).
What are CYL and AXIS?
They correct astigmatism — CYL is its strength and AXIS (1–180°) its orientation. No CYL means no astigmatism correction.
Can I use a glasses prescription for contacts?
No — contact lenses sit on the eye and need a separate fitting and prescription.