How to Read Your Contact Lens Prescription: SPH, CYL, AXIS, BC & DIA Explained

What Is a Contact Lens Prescription?

A contact lens prescription is different from a glasses prescription. It includes additional measurements specific to the shape and fit of your eye. Before you order online, understanding each value helps you pick the right lens and avoid discomfort.

OD and OS — Which Eye Is Which?

OD (Oculus Dexter) refers to your right eye. OS (Oculus Sinister) refers to your left eye. Always check that you are entering the correct value for each eye when ordering.

SPH — Sphere (Your Main Power)

SPH is the main correction your lens provides.

Minus (−) values: shortsightedness (myopia) — the higher the number, the stronger the correction needed.
Plus (+) values: longsightedness (hyperopia).
Plano (0.00 or PL): no power correction needed — used for purely cosmetic/coloured lenses.

SPH values go in steps of 0.25, for example: −1.00, −1.25, −1.50.

CYL — Cylinder (Astigmatism Correction)

If you have astigmatism, your prescription will include a CYL value. It is always a minus number (e.g. −0.75, −1.25). If your prescription has no CYL, your eye is round — you do not need a toric lens.

AXIS — Direction of Astigmatism

The AXIS tells the lens where to position the astigmatism correction. It is a number between 1 and 180 (degrees). AXIS only applies when there is a CYL value.

BC — Base Curve

BC is the curvature of the back surface of the lens, measured in millimetres. Common values are 8.4, 8.5, 8.6, 8.7, 8.8. A lens with the wrong BC will feel uncomfortable or may not stay centred on your eye. Your eye doctor measures this and includes it in your prescription.

DIA — Diameter

DIA is the total width of the lens in millimetres. Most soft lenses are 14.0–14.5 mm. This value is set by the lens brand and is usually fixed once you choose a product.

ADD — Addition (Multifocal Lenses)

ADD appears in multifocal or bifocal prescriptions for people who need both distance and near correction (typically over 40). It is written as Low, Medium, or High, or as a number like +1.50.

Sample Prescription Explained

Here is an example to bring it all together:

OD: SPH −2.50 / CYL −0.75 / AXIS 180 / BC 8.6 / DIA 14.2
OS: SPH −3.00 / CYL 0.00 / AXIS — / BC 8.6 / DIA 14.2

The right eye needs correction for both myopia (−2.50) and mild astigmatism (−0.75 at 180°). The left eye is myopic (−3.00) with no astigmatism.

Key Things to Remember

Always get a fresh prescription from your eye doctor — prescriptions expire (usually every 1–2 years).
Never share contact lenses with someone else.
If you are unsure of a value, call your eye doctor before ordering.