Free tool
Prescription Decoder.
Enter the numbers from your glasses or contact lens prescription and this tool will explain, in plain English, exactly what they mean.
Sphere, the main correction
The sphere value is the core lens power needed to bring your vision into focus. A minus (–) sphere means you are short-sighted (myopic): you see clearly up close but the world blurs at a distance. A plus (+) sphere means you are long-sighted (hyperopic): your eye has to work hard to focus, which can cause blurring at any distance and often causes headaches and eye strain. The further the number from zero, the stronger the correction needed.
Cylinder and axis, the astigmatism correction
If a cylinder value is present, your eye has astigmatism. This means the front surface of your eye (the cornea) is not perfectly spherical, it is shaped more like the back of a spoon than a football. This causes blur at all distances, often with a sense of shadow or ghosting around letters. The cylinder value describes the degree of astigmatism; the axis (a number between 1 and 180) describes the orientation of the irregularity. Neither can be interpreted meaningfully without the other.
Add, the reading addition
The Add value only appears in prescriptions for reading glasses or varifocal lenses. It is always a positive number and is added to the sphere power to create the reading portion of the lens. Its presence means that the natural focusing mechanism of your eye, the lens inside the eye, is losing flexibility with age. This is called presbyopia and affects virtually everyone from their mid-forties onwards. It is a normal part of ageing, not a disease.
What the numbers do not tell you
A prescription describes optical correction but says nothing about the health of your eyes. High myopia (sphere more negative than –6.00) does carry a modestly increased risk of certain eye conditions over a lifetime, and is worth discussing with your optometrist or ophthalmologist. But a strong prescription is not in itself a sign that anything is wrong.