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Glasses prescription decoder (SPH, CYL, AXIS, ADD, PD)
Enter the numbers from your glasses prescription and get a plain-English explanation of SPH, CYL, AXIS, ADD and PD, plus lens material advice.
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A screening aid, not a diagnosis. Screens, lighting and distance affect results. If something looks wrong, or your vision changes suddenly, see an eye-care professional. Medical disclaimer
The parts of a glasses prescription
| Term | Meaning |
|---|---|
| SPH (sphere) | Main lens power in diopters. Minus for nearsightedness, plus for farsightedness |
| CYL (cylinder) | Extra power for astigmatism. Blank means none |
| AXIS | Direction of the astigmatism correction, 1 to 180 degrees |
| ADD | Extra magnifying power for reading, for bifocals or progressives |
| PRISM | Corrects eye alignment problems; less common |
| PD | Distance between your pupils in millimetres |
Why two prescriptions can look different but be the same
Astigmatism can be written with a minus or a plus cylinder. The decoder shows your prescription in both forms, so you can compare a prescription from an optometrist with one from an ophthalmologist.
Choosing lenses
Stronger prescriptions make thicker lenses, especially at the edges for minus lenses. High-index materials (1.60, 1.67, 1.74) keep lenses thinner and lighter. Polycarbonate or Trivex is safer for children and sports. An anti-reflective coating helps at night and on screens.
Glasses prescriptions usually expire after one or two years. Buying new glasses is a good moment to check that your eyes have not changed.
Questions people ask
What do OD and OS mean?
They come from Latin. OD (oculus dexter) is the right eye and OS (oculus sinister) is the left. OU means both eyes.
Is a higher number worse?
Larger numbers mean a stronger correction, not unhealthy eyes. A minus SPH corrects nearsightedness and a plus SPH corrects farsightedness.
Where do I find my PD?
It may be written on your prescription. If not, an optician can measure it, or some online stores measure it with your phone camera.