Glasses prescription reader: sphere, cylinder, axis and add
Type the sphere, cylinder, axis and reading add for one eye, as they appear on your prescription. You get a plain-English reading, the same lens in the other cylinder form, the spherical equivalent and the reading-only power.
Glasses prescription reader
-2.00 -1.00 x 180
Right eye (OD): nearsighted (myopia), low amount, with moderate astigmatism.
| Spherical equivalent | -2.50 |
|---|---|
| Same lens, other cylinder form | -3.00 +1.00 x 090 |
| Reading-only power (sphere + add) | 0.00 -1.00 x 180 |
| Astigmatism direction | cylinder power works at right angles to the 180 degree line |
How to use it
A glasses prescription is a small grid of numbers that nobody explains at the counter. Copy the numbers for one eye into the boxes, signs included. The usual labels:
- OD and OS are short for the Latin words for right eye and left eye. Many prescriptions write R and L instead, and OU means both eyes.
- SPH (sphere) is the main power, in diopters. Minus means nearsighted, plus farsighted. "PL" or "plano" means zero.
- CYL (cylinder) is extra power for astigmatism, acting in one direction only. A blank box, "DS" or "SPH" here means no astigmatism correction.
- AXIS, from 1 to 180 degrees, says which way the cylinder lies. It is a direction, not a strength, and only appears with a cylinder.
- ADD is extra plus power for reading, used in bifocals, progressives and reading glasses. It usually falls between +0.75 and +3.50 and is often the same for both eyes.
- PD (pupillary distance) is the gap between your pupils in millimeters, used to center the lenses. It may be one number, such as 63, or one per eye, such as 31.5/31.5.
- Prism, if present, comes with a direction (up, down, in or out) and helps the eyes work together. The tool doesn't handle prism, and lenses with prism must be made exactly as written.
Powers come in 0.25 steps. The tool accepts a sphere up to 25.00 either way, a cylinder up to 10.00 and an add up to +4.00, and asks for an axis whenever there is a cylinder.
How the numbers work
The tool opens on -2.00 -1.00 x 180 with an add of +2.00: a mildly nearsighted eye with some astigmatism and a reading add.
Spherical equivalent
An eye with astigmatism has two main powers at right angles. The spherical equivalent (SE) is the single power that averages them: SE = sphere + (cylinder ÷ 2). Here, -2.00 + (-1.00 ÷ 2) = -2.50. Eye doctors use it to compare prescriptions over time and to classify nearsightedness. It does not replace the cylinder in your lenses.
Plus and minus cylinder
Some prescribers write the cylinder as a minus number, others as a plus number, and both describe the same lens. To switch forms, add the cylinder to the sphere to get the new sphere, change the cylinder's sign, and turn the axis by 90 degrees, keeping it between 1 and 180. So -2.00 -1.00 x 180 becomes -3.00 +1.00 x 90. The spherical equivalent stays the same (-3.00 + 0.50 = -2.50), which makes a handy check.
Reading-only power
For lenses used only up close, the add joins the sphere while the cylinder and axis stay as they are: -2.00 + 2.00 = 0.00, so the reading-only lens is plano -1.00 x 180. That is why some mildly nearsighted people in their fifties read comfortably with their glasses off: the add more or less cancels the sphere, though the astigmatism part still helps. Bifocals and progressives combine both powers in one lens, and the column on reading glasses and progressives compares the options.
The labels
| Label | Rule the tool uses |
|---|---|
| Nearsighted | SE of -0.50 or lower |
| Farsighted | SE of +0.50 or higher |
| Close to zero | SE between those two |
| Low, moderate, high | SE size under 3.00, 3.00 to under 6.00, 6.00 or more |
| Mild, moderate, marked astigmatism | cylinder under 1.00, 1.00 to 2.00, over 2.00 |
The nearsighted line (SE of -0.50 or lower) and high myopia (SE of -6.00 or lower) follow the International Myopia Institute definitions. Everything else, including the farsighted sizes and all the astigmatism wording, is a descriptive label for this page, not a clinical grade. The default reads as nearsighted, low, with moderate astigmatism and an SE of -2.50.
What this tool can't tell you
A prescription is a snapshot. The tool can't say whether the numbers are still right for you, and they do change, especially in children who are becoming more nearsighted and in adults over 40. Nor does it say anything about eye health: the same numbers fit a healthy eye and one with early glaucoma or a developing cataract.
A bigger number is not a disease. High myopia, though, is linked to a greater risk of some problems later in life, including retinal detachment and glaucoma, so regular eye exams matter more. The column on floaters and retinal detachment describes the warning signs.
See an eye doctor if your glasses no longer seem sharp, if they give you headaches or eye strain, or if a child squints, sits very close to screens or holds books near their face. The American Academy of Ophthalmology advises a baseline eye exam at 40 for adults without symptoms, and earlier for people with diabetes, high blood pressure or a family history of eye disease.
Some symptoms need help fast:
- New flashes of light, a sudden shower of floaters, or a shadow or curtain across part of your vision need same-day eye care.
- Sudden loss of vision in one eye, even briefly, or severe eye pain with blurred vision, halos, headache or nausea is an emergency. Go to an emergency department or call your local emergency number (such as 911, 999, 112, or 119 in Japan).
Questions people ask
Why is my axis 180 when nothing about my eye is tilted?
The axis is an angle, read like a protractor laid over the lens: 180 (the same as 0) is horizontal and 90 is vertical. It tells the lab which way to turn the cylinder. A larger number is not a stronger lens.
My new prescription looks completely different. Is one of them wrong?
Check the cylinder signs first. If one uses plus cylinder and the other minus, transpose one with the tool and compare again. Transposing never changes the spherical equivalent, so that is a quick first check. Real changes happen too, and the person who examined you can explain them.
Can I use these numbers to buy contact lenses?
No. A contact lens sits on the eye, so the power often differs, and a contact lens prescription also includes base curve, diameter and lens type from a fitting. The glasses to contacts calculator shows the power side of that difference.
Why don't ready-made readers match my add?
Ready-made readers have the same plus power in both lenses and no cylinder, while your add sits on top of your own sphere and astigmatism correction. The reading glasses strength tool explains what off-the-shelf readers can and can't do.
What does "DS" in the cylinder box mean?
It stands for diopters sphere: no astigmatism correction for that eye. The lens has the same power in every direction, and the axis box stays empty.
Sources
- International Myopia Institute, Defining and Classifying Myopia (2019)
- Standard optical conventions for plus and minus cylinder notation and transposition
- American Academy of Ophthalmology patient pages (aao.org/eye-health)