How to Read Your Glasses Prescription

When you visit an optician, you leave with a small piece of paper that seems to be written in code. SPH, CYL, Axis, Add — what does it all mean? Understanding your prescription is easier than you think, and it matters when ordering glasses online.

SPH (Sphere) is the main correction your eye needs. A minus number means you're short-sighted (can't see far), and a plus number means you're long-sighted (can't see close). The bigger the number, the stronger your prescription.

CYL (Cylinder) corrects astigmatism — when your eye isn't perfectly round. Not everyone has this. If your prescription shows a CYL value, you'll also see an Axis number, which tells us the direction of the astigmatism.

Axis is a number between 1 and 180 and only appears if you have astigmatism. It describes the orientation of the cylinder correction in degrees. It's not a strength measurement.

Add appears if you're over 40 or need reading glasses. It's an extra magnification added to your distance prescription for close-up work. This is used in bifocal and varifocal lenses.

PD (Pupillary Distance) isn't always on your prescription but is vital for online ordering. It's the distance between your pupils in millimetres. The average adult PD is around 63mm, but it varies. When ordering with LensZee, you can enter a single PD or separate values for each eye.

The key thing to remember is that a prescription is only valid for two years in the UK. Always use a current prescription when ordering glasses.