Prescription Explainer
Enter SPH, CYL, AXIS, ADD and PD to understand your glasses prescription in plain language.
A glasses prescription can look like a string of cryptic numbers and abbreviations. This tool turns it into plain language. You enter the values for each eye, usually labelled OD or R for the right eye and OS or L for the left: sphere (SPH), cylinder (CYL), axis, addition (ADD) and pupillary distance (PD). The tool then explains what each value means.
From the numbers it estimates the type of refractive error: a minus sphere points to myopia (nearsightedness), a plus sphere to hyperopia (farsightedness), a cylinder value to astigmatism, and an ADD value to near correction, typically for presbyopia. It also transposes the cylinder between the minus form used by most optometrists and the plus form used by many ophthalmologists, and it calculates the spherical equivalent (sphere plus half the cylinder), a single number that roughly summarises the overall power.
The tool helps you understand your prescription; it does not check whether it is correct or still up to date, and it cannot convert a glasses prescription into a contact lens prescription, which needs a separate fitting. For a detailed guide, see how to read your glasses prescription and our page on pupillary distance.
How to use
- Have your written prescription in front of you and identify the rows for the right (OD/R) and left (OS/L) eye.
- Enter the SPH value with its sign (+ or -); enter 0 or leave it empty if it says plano, PL or the symbol for infinity.
- Enter CYL with its sign and the AXIS (a number from 0 to 180) if they are present; they always go together.
- Enter ADD if your prescription includes it (often the same for both eyes) and PD, either as one number or per eye.
- Check the values carefully; a missing minus sign changes the meaning completely.
- Read the explanation, error type, transposed form and spherical equivalent for each eye.
Reading your result
Values close to zero mean a weak correction. As a rough guide, spheres up to about ±3.00 D are often called low, ±3.00 to ±6.00 D moderate and beyond ±6.00 D high; cylinders under about 1.00 D are considered mild. High myopia is associated with a higher risk of conditions such as retinal detachment and glaucoma, so regular eye examinations are particularly important. ADD values usually range from about +0.75 to +3.00 D and tend to increase with age.
Transposed prescriptions describe exactly the same lens, just written differently. If your prescription changes quickly, differs a lot between the eyes, or your vision is not clear with new glasses, contact your optometrist or ophthalmologist.
Caution: For understanding only. It does not create, verify or replace a prescription and cannot be used to order contact lenses. Always rely on your eye-care professional.
Frequently asked questions
What does 'D' mean on a prescription?
D stands for dioptre, the unit of lens power. The higher the number, ignoring the sign, the stronger the lens.
Why are there plus and minus cylinder forms?
Both describe the same correction. Optometrists usually write minus cylinder and many ophthalmologists use plus cylinder; transposition converts one into the other.
Can I use my glasses prescription for contact lenses?
No. Contact lenses sit on the eye, so the power may differ, and they also need curvature and diameter measured during a fitting.
What is the spherical equivalent used for?
It gives a single number that summarises overall focusing power. Clinicians use it to compare prescriptions or track myopia over time.