How to Read Your Eye Prescription (SPH, CYL, AXIS, ADD, PD)
Your prescription is six or seven numbers on a slip of paper. Once you know what each column does, ordering the right pair stops being guesswork.
Prescriptions look intimidating because they use abbreviations from Latin and a lot of decimal points. There are really only five things on the page.
OD and OS — which eye
OD (oculus dexter) is the right eye. OS (oculus sinister) is the left. Some clinics simply write R and L, or in Arabic يمين and يسار. OU means both eyes together.
SPH — the main power
Sphere. A minus number means you are short-sighted (myopia): distance is blurry. A plus number means you are long-sighted (hyperopia): near work is harder. The further from zero, the stronger the correction. −0.75 is mild; −6.00 is strong.
CYL and AXIS — astigmatism
These two always travel together. CYL (cylinder) is how much astigmatism you have — the cornea being shaped slightly more like a rugby ball than a football, which smears vision at all distances. AXIS is where that correction sits, an angle from 1 to 180 degrees. An axis without a cylinder is meaningless, and vice versa.
ADD — reading power
Additional power for near vision, and it appears once you reach your forties as the eye's focusing muscle stiffens. It is always a plus number and usually the same for both eyes. An ADD value is what turns a prescription into bifocals or progressives.
PD — pupillary distance
The distance in millimetres between the centres of your pupils. It tells the lab where to place the optical centre of each lens so it lines up with your eye. Most adults fall between 54 and 74 mm. It is frequently left off a clinic prescription because it is a dispensing measurement, not a medical one — but you need it to order glasses, so ask for it.
| Column | Means | Typical range |
|---|---|---|
| SPH | Short- or long-sightedness | −10.00 to +6.00 |
| CYL | Astigmatism amount | −4.00 to +4.00, or blank |
| AXIS | Astigmatism angle | 1 to 180 |
| ADD | Extra reading power | +0.75 to +3.00 |
| PD | Pupil-to-pupil distance | 54 to 74 mm |
Prism, and when to worry
A prism value with a base direction (BU, BD, BI, BO) corrects eye alignment. It is uncommon, it makes lenses more complex to produce, and it is not something to order without a current, specific prescription from your optometrist.
How long is a prescription valid?
Treat two years as the outer limit for adults, and one year for children and for anyone with diabetes or a changing prescription. An out-of-date prescription is the most common reason new glasses feel wrong.
If your prescription is over two years old, start there — we will give you the PD in writing too.
Book an eye testFrequently asked questions
What does SPH mean on a glasses prescription?
SPH stands for sphere and is the main correcting power, measured in dioptres. A minus value corrects short-sightedness (blurry distance vision) and a plus value corrects long-sightedness. The bigger the number, the stronger the lens.
Can I order glasses online without a PD?
You need it. Without the pupillary distance the lab has to assume an average, and on anything above a mild prescription that misalignment causes eye strain and headaches. Your optician must provide it on request, or you can measure it at home with a ruler and a mirror.
Is my contact lens prescription the same as my glasses prescription?
No, and they are not interchangeable. Contact lenses sit directly on the eye while glasses sit about 12 mm in front of it, so the powers differ — increasingly so on stronger prescriptions. A contact lens prescription also includes base curve and diameter, which glasses do not use.
What does a blank CYL box mean?
It means you have no measurable astigmatism, which is a good outcome. Some clinics write SPH, DS or 0.00 in the box instead of leaving it empty — all three mean the same thing.