An eyeglass prescription is a short code that tells a lab how to shape your lenses. SPH (sphere) sets the main lens power, CYL (cylinder) and AXIS describe astigmatism correction, and ADD is the extra reading power used in progressive or reading lenses. OD is your right eye, OS your left, and PD is the distance between your pupils.
Once you know what each box means, ordering prescription glasses online becomes simple. This guide walks through every field line by line, with a worked example at the end.
OD, OS and OU: which eye is which
Most prescriptions use Latin abbreviations for the eyes:
- OD (oculus dexter) = right eye
- OS (oculus sinister) = left eye
- OU (oculus uterque) = both eyes
Some opticians simply write "R" and "L" instead. The top row is almost always the right eye. When you enter your values in a lens selector, double-check that you have not swapped the rows, because the two eyes often need different powers.
SPH (sphere): your main lens power
SPH is the core number of your prescription. It is measured in dioptres and usually appears in steps of 0.25.
- A minus sign (−) means short-sightedness (myopia): distant objects look blurry.
- A plus sign (+) means long-sightedness (hyperopia): close-up objects are harder to focus on.
- "PL", "Plano" or 0.00 means no spherical correction is needed for that eye.
The further the number is from zero, the stronger the lens. Higher powers are where thinner, higher-index lens materials become worth considering, as they help keep the lens edges slimmer in the frame.
CYL and AXIS: correcting astigmatism
If the CYL and AXIS boxes are empty, your lenses only need spherical power. If they are filled in, your prescription includes correction for astigmatism, where the eye focuses light differently along different directions.
- CYL (cylinder) is the amount of astigmatism correction, also in dioptres. It can be written with a minus or plus sign.
- AXIS is a number from 1 to 180 that tells the lab at what angle to place the cylinder power. It is not a strength, just an orientation in degrees.
CYL and AXIS always come as a pair. If you have a CYL value, you must also have an AXIS, and both need to be entered exactly as written.
ADD: extra power for reading
ADD (addition) is the extra magnifying power added to the lower part of the lens for close work. It is always a plus value, typically between +0.75 and +3.50, and is usually the same for both eyes.
An ADD value is common from around the mid-forties onwards, when focusing up close tends to get harder. If your prescription has an ADD, you can choose between:
- Progressive lenses, which blend distance, intermediate and near vision in one lens with no visible line.
- Reading glasses, which use only the near power for books, phones and close tasks.
Prism and PD: the less common fields
Prism is only prescribed for some people, to help the eyes work together comfortably. It comes with a base direction (BU, BD, BI or BO for up, down, in or out). If your prescription has no prism value, you can ignore it.
PD (pupillary distance) is the distance between the centres of your pupils in millimetres, often between 54 and 74 mm for adults. It makes sure the optical centre of each lens sits in front of your eye. It can be one number (e.g. 63) or two monocular values (e.g. 31.5 / 31.5). If your PD is missing, follow our guide on how to measure your pupillary distance.
Quick reference table
| Field | What it means | Typical format |
|---|---|---|
| OD / OS | Right eye / left eye | Row labels |
| SPH | Main lens power (− short-sighted, + long-sighted) | −2.50, +1.75, PL |
| CYL | Astigmatism correction amount | −0.75, −1.25 |
| AXIS | Angle of the astigmatism correction | 1–180 |
| ADD | Extra near power for reading or progressives | +1.00 to +3.50 |
| Prism | Helps the eyes align; with base direction | 1.0 BI |
| PD | Distance between pupils | 63 or 31.5 / 31.5 mm |
A worked example
Imagine your prescription reads:
- OD: SPH −2.25, CYL −0.50, AXIS 180
- OS: SPH −2.00, CYL −0.75, AXIS 170
- ADD: +1.50 | PD: 62
This person is short-sighted in both eyes (minus SPH), has mild astigmatism in both eyes (CYL with an AXIS), and has an ADD, so they could choose progressive lenses for all-day wear or single-vision distance lenses plus a separate reading pair. The PD of 62 mm is entered as a single value. For more detail on each field, see our prescription reading guide.
Shop this guide
Frequently asked questions
Is my eyeglass prescription the same as my contact lens prescription?
No. Contact lenses sit directly on the eye, so the powers and measurements can differ. Always use a prescription written specifically for glasses when ordering frames and lenses.
What does a blank CYL box mean?
A blank or "DS" (dioptre sphere) entry means no astigmatism correction is needed for that eye. Leave CYL and AXIS empty or at zero in the lens selector.
Why are my left and right eye values different?
It is completely normal for each eye to need a slightly different correction. Just enter each row exactly as written on your prescription.
How long is a prescription valid?
Validity varies by country and optician, and it is often one to two years. If your vision feels different or your prescription is old, ask your optician for an up-to-date eye test before ordering.
Ready to order your glasses?
Now that you can read every line of your prescription, browse our eyeglasses collection and pick a frame you love. Add your prescription in a few clicks with our lens selector.
