What the numbers on your prescription mean
A prescription is a set of manufacturing instructions written in a notation nobody explains at the point they hand it to you. Here is every box, what it does to the lens, and which ones matter.
The short answer. SPH is the main correction and the number that drives how thick the lens will be. CYL and axis together describe astigmatism — a strength and a direction, meaningless apart. ADD is the extra reading power, the same in both eyes. Everything else on the card is either a measurement for the dispenser or a note for the lab.
The question this page answers: what do SPH, CYL, axis, ADD and prism mean?
The short version
If you are standing in a shop with the card in your hand, this is the whole of it. Each line is expanded underneath.
| Box | In one line |
|---|---|
| SPH | How strong the main correction is. Minus corrects short sight, plus corrects long sight. |
| CYL | An extra correction in one direction only, for astigmatism. Blank or "DS" means none. |
| Axis | Which direction that extra correction points, from 1 to 180 degrees. A bearing, not a strength. |
| ADD | Extra reading power for the lower part of a bifocal or varifocal. Usually the same in both eyes. |
| Prism | Shifts the image sideways or vertically, for eyes that do not aim at quite the same point. Uncommon. |
The three main boxes
SPH — sphere
The main correction, measured in dioptres, applied equally in every direction across the lens. A minus number corrects short sight: distance is blurred, near is clear, and the lens is thin in the middle and thick at the edge. A plus number corrects long sight, and the lens is thick in the middle and thin at the edge. "Plano", or a dash, means no spherical correction at all.
The size of this number is the single biggest factor in how thick and heavy your lenses will be. Anything under about two dioptres produces a lens nobody will notice. Past about five, lens material and frame size start to matter a great deal.
CYL — cylinder
An additional correction applied in one direction only. It exists because most corneas are not perfectly spherical — they are curved slightly more steeply across one meridian than the other, like the back of a spoon rather than a section of a ball. That is astigmatism, it is present in some degree in most eyes, and it is not a disease or a deterioration.
A blank or "DS" in this box means no astigmatic correction. Under about 0.75 dioptres it is often left uncorrected because the benefit does not justify the adaptation.
Axis
The direction the cylinder is applied in, from 1 to 180 degrees. Think of it as a compass bearing on the front of your eye: 180 is horizontal, 90 is vertical, 45 and 135 the two diagonals. It is an orientation, so a larger number is not a stronger prescription — axis 170 and axis 10 are a few degrees apart, not 160.
The axis is why a lens with cylinder has to be mounted the right way up and cannot be rotated in the frame. It is also why an astigmatic prescription that is a few degrees out feels subtly wrong in a way that is hard to describe.
The rest of the card
| Box | What it is |
|---|---|
| ADD | Extra plus power for reading, added to the lower part of a bifocal or progressive. It is the same in both eyes on nearly every prescription, and it grows with age from about +0.75 in the mid forties to about +2.50 and then stops. |
| OD and OS | Oculus dexter and oculus sinister — right eye and left eye. Some prescriptions use R and L, or RE and LE. OU means both. |
| Prism and base | A correction that shifts the image sideways or vertically, to help eyes that do not aim at quite the same point. The base direction says which way — in, out, up or down. Uncommon, and it makes the lens heavier and more expensive. |
| PD | Pupillary distance in millimetres: how far apart your pupils are. It is a measurement of your face rather than a correction, and it is frequently omitted from a prescription even though you cannot order glasses online without it. |
| BVD or vertex | Back vertex distance: how far the test lenses sat in front of your eyes, usually about 12 mm. It only matters above about 4 dioptres, where it changes the power actually needed. |
Why your prescription may look nothing like your last one
The same lens can be written two completely different ways, and both are correct. Opticians conventionally write cylinder as a negative number; many ophthalmologists and most prescriptions written after cataract surgery use positive cylinder. Converting between them is called transposition, and it changes every number on the line.
Worked example.
−2.50 −1.25 × 180 and −3.75 +1.25 × 90 are the same pair of glasses.
The rule: add the cylinder to the sphere, flip the cylinder's sign, and rotate the axis by 90 degrees — adding 90 if it is 90 or below, subtracting if above.
This catches people out constantly, because the sphere appears to have changed by more than a dioptre when nothing about their eyes has changed at all. If your new prescription looks dramatically different, transpose it before worrying — there is a good chance it is identical.
Spherical equivalent
Sometimes you need a single number to stand for a prescription with cylinder — for ordering certain contact lenses, for a rough thickness estimate, or for comparing two prescriptions. The spherical equivalent is the sphere plus half the cylinder. For our example that is −2.50 + (−1.25 ÷ 2) = −3.13.
It is a genuine average and not a replacement. A lens made to the spherical equivalent corrects neither meridian properly, and anyone with more than about a dioptre of cylinder will notice immediately.
Which number decides how thick your lenses are
Not the sphere alone. A lens with cylinder has a different power in each of two meridians, and the thickness is set by whichever is strongest in absolute terms. For negative cylinder that is sphere plus cylinder; for positive cylinder it is the sphere itself.
In our example the two meridians are −2.50 and −3.75, so the lens behaves like a −3.75 lens where it is thickest. That is the figure to use when working out whether a high-index material is worth paying for, and using the sphere alone under-estimates thickness on every prescription with meaningful cylinder.
Why you cannot order contacts with this
A spectacle lens sits about 12 mm in front of your cornea. A contact lens sits on it. That distance changes the effective power, and the stronger the prescription the more it changes it.
| Spectacle | Contact lens | Difference |
|---|---|---|
| −3.00 | −2.90 | — |
| −5.00 | −4.72 | 0.28 D |
| −8.00 | −7.30 | 0.70 D |
| −12.00 | −10.49 | 1.51 D |
| +5.00 | +5.32 | 0.32 D |
| +8.00 | +8.85 | 0.85 D |
Note the direction. A strong minus prescription needs a weaker contact lens than spectacle lens, and a strong plus needs a stronger one. Beyond the power, a contact lens prescription also specifies a base curve and a diameter matched to the shape of your cornea — numbers that come from a fitting and appear nowhere on a glasses prescription. That is why a contact lens fitting is a separate appointment rather than an administrative step.
What a prescription does not tell you
- How good your vision is. A prescription is what it takes to correct your eyes, not how well they see once corrected. Those are different measurements, and the second one is acuity.
- Whether your eyes are healthy. A sight test covers both, but only the refraction ends up on the card.
- Your pupillary distance, on many prescriptions — it is a fitting measurement rather than a clinical one, and shops are not obliged to supply it.
- How thick the lenses will be. That depends on the frame as much as the prescription, and the frame is chosen afterwards.
Read your own
The prescription reader takes the numbers off your card and explains each one, transposes it both ways, works out the spherical equivalent and the governing meridian, and flags anything that looks like a transcription error. It runs in your browser and nothing is sent anywhere.
This explains notation. It is not medical advice, it cannot tell you whether a prescription is correct for your eyes, and it is no substitute for a sight test — which also checks for things a prescription cannot show. If your vision has changed, see an optometrist.
Questions people ask
- What does SPH mean on a glasses prescription?
- Sphere: the main correction, in dioptres. A negative number corrects short sight (myopia) and a positive one corrects long sight (hyperopia). It is the same power in every direction across the lens, which is what makes it spherical.
- What does CYL mean, and is astigmatism serious?
- Cylinder is an extra correction applied in one direction only, to compensate for a cornea curved more steeply across one meridian than the other. It is extremely common and is not a disease — most eyes have some. Under about 0.75 dioptres it is often left uncorrected.
- What does the axis number mean?
- The direction, in degrees from 1 to 180, in which the cylinder correction is applied. It is an orientation, not a strength: 180 is horizontal and 90 is vertical. A prescription with a cylinder is meaningless without one, which is why the box is never blank if CYL is filled in.
- Why does my prescription look completely different from my old one?
- It may be the same prescription written the other way round. −2.50 −1.25 × 180 and −3.75 +1.25 × 90 are the same lens — opticians write cylinder in minus form and ophthalmologists often in plus. Transposing between them changes every number on the line.
- Can I use my glasses prescription to order contact lenses?
- Not directly, for two reasons. Above about 4 dioptres the power has to be adjusted because a contact lens sits on the eye rather than 12 mm in front of it. And a contact lens prescription also specifies a base curve and a diameter, which a glasses prescription does not contain and which require a fitting.