Read your glasses prescription and calculate your eye power
This glasses prescription reader turns the shorthand on your optician’s slip into plain language. Type in the SPH, CYL, AXIS and ADD for each eye, optionally a prism and your PD, and the tool explains each number, rewrites the prescription in the other cylinder form (transposition), and works out the spherical equivalent and the near power with your add.
It doubles as an eye power calculator: people often ask for “eye power” meaning the sphere and cylinder numbers in dioptres. It only explains and converts numbers you already have. It cannot test your eyes, diagnose anything or replace a sight test.
How to use the Glasses Prescription Reader
- Copy the numbers for the right eye (OD) and left eye (OS) from your prescription. Use a minus sign for minus powers, and write plano as 0 or leave it blank.
- Add the AXIS whenever there is a CYL value, and the ADD if you have varifocal, bifocal or reading lenses.
- Optional: enter prism with its base direction and your PD in millimetres.
- Read the transposed prescription, spherical equivalent and near power in the table, then the plain-language notes under it.
- Take the same numbers to a lens seller or use the contact lens power calculator if you want a contact lens estimate.
Glasses Prescription Reader features
Plain-language notes
Each number is explained in words for each eye, including what plus, minus and axis mean.
Cylinder transposition
Switches plus cylinder to minus cylinder and back, with the axis moved by 90 degrees and kept between 1 and 180.
Spherical equivalent
Sphere plus half the cylinder, a single-number summary used when a contact lens or trial frame has no cylinder.
Near power
Adds the ADD to the sphere to show the power for close work.
PD and prism
Accepts a single or split PD and optional prism with base direction.
What SPH, CYL, AXIS, ADD, PD and prism mean
- SPH (sphere): the main lens power in dioptres (D). A minus sign means a diverging lens, often used for short-sightedness; a plus sign means a converging lens, often used for long-sightedness and reading.
- CYL (cylinder): extra power to correct astigmatism, when the front of the eye is more curved in one direction.
- AXIS: the direction, from 1 to 180 degrees, of the cylinder. It is the meridian with no cylinder power. If CYL is blank, AXIS is not used.
- ADD: added magnification for near vision, written once or per eye, normally from +0.75 to +3.50.
- PD (pupillary distance): the distance between your pupils in millimetres, needed to centre the lenses. Measure it with the pupillary distance calculator.
- Prism: measured in prism dioptres (Δ) with a base direction (up, down, in or out), used for eye alignment problems.
- OD / OS: Latin abbreviations for right eye (oculus dexter) and left eye (oculus sinister). OU means both eyes.
The transposition formula
A cylinder prescription can be written two ways without changing the lens. To transpose: new sphere = sphere + cylinder, new cylinder = −cylinder, and new axis = axis ± 90° (subtract 90 if the axis is above 90, otherwise add 90, so it stays within 1 to 180).
Worked example: −2.50 −1.00 × 180 becomes (−2.50 + −1.00) = −3.50, cylinder +1.00, axis 180 − 90 = 90, that is −3.50 +1.00 × 090. Optometrists in the UK and Australia usually write minus cylinder; some ophthalmologists in the US write plus cylinder. Both are the same lens.
| As written | Transposed |
|---|---|
| −2.50 −1.00 × 180 | −3.50 +1.00 × 090 |
| +1.00 +0.50 × 090 | +1.50 −0.50 × 180 |
| −0.75 +0.75 × 045 | 0.00 −0.75 × 135 |
Spherical equivalent and eye power
The spherical equivalent (SE) is sphere + cylinder ÷ 2. For −2.50 −1.00 × 180, SE = −2.50 + (−0.50) = −3.00 D. It is the single-number “eye power” people usually mean, and it is how some contact-lens fitting and refractive-error surveys summarise a prescription. It is only a summary: the cylinder still matters for sharp vision.
Things to check before ordering
- Prescriptions are normally in steps of 0.25 D; a number like −2.37 suggests a typing error.
- Check the date: sight tests are repeated every one to two years in most countries, and a prescription can expire.
- Contact-lens prescriptions are different from glasses, with base curve and diameter. Do not use one for the other.
Glasses Prescription Reader FAQ
How do I calculate my eye power?
The usual single figure is the spherical equivalent: sphere plus half the cylinder. For −2.50 SPH and −1.00 CYL that is −3.00 D. The exact numbers only come from a sight test.
What does SPH, CYL and AXIS mean?
SPH is the main lens power, CYL is the extra power for astigmatism, and AXIS (1 to 180) says in which direction the cylinder sits.
How do I change plus cylinder to minus cylinder?
Add the cylinder to the sphere, change the sign of the cylinder, and move the axis by 90 degrees. This tool does it for you and shows the result.
Is −3.50 +1.00 × 90 the same as −2.50 −1.00 × 180?
Yes. They are the same lens written in two cylinder forms.
What if my CYL box is empty?
It means no astigmatism correction was written, so the lens is spherical. There is nothing to transpose.
Can this tell me if I need glasses?
No. It only explains the numbers on a prescription. See an optometrist or ophthalmologist for an eye test and any diagnosis.