What Is Axis on an Eye Prescription?
Most numbers on an eye prescription describe how much of something you need. Axis is the exception. Axis describes where, and that is why a number like 175 next to a number like -0.75 looks so strange.
The short answer
Axis is an angle, written in degrees from 1 to 180. It tells whoever makes your lenses what orientation to place the astigmatism correction at. It carries no strength information whatsoever.
An axis of 180 is horizontal. An axis of 90 is vertical. Everything else falls in between. A high number is not a worse result than a low one, and comparing your 165 to someone else's 20 tells you nothing about whose eyes are better.
Why the angle exists at all
A cornea without astigmatism is close to evenly curved, like a football: whichever way you slice through it, the curve is about the same. Light lands at a single focus.
With astigmatism, the cornea is curved a little more in one direction than another, closer to a rugby ball. Light entering along one meridian focuses at a slightly different point from light entering across it, which is why untreated astigmatism can leave things blurry or smeared at every distance rather than only near or far.
Correcting that needs two pieces of information: how much of a difference there is, and which way round it sits. The first is CYL, the cylinder value. The second is AXIS. One without the other is not enough to make a lens.
Axis never travels alone
If your prescription has an axis value, it has a cylinder value too. If the cylinder column is blank or reads "DS" or "Sph", there will be no axis either, because there is no astigmatism to orient.
So a prescription might read:
OD -2.00 -0.75 x 180
That is: right eye, sphere -2.00, cylinder -0.75, axis 180. The "x" is simply read as "axis".
Why it matters more for contact lenses
In eyeglasses, the axis is fixed. The lens is set in a frame, the frame sits on your face, and the correction stays where it was placed.
A contact lens floats on the eye and can rotate. A lens correcting astigmatism has to stay oriented, so lenses made for it, called toric lenses, are built with features that keep them settled in one position, such as slight weighting or thin and thick zones that let the eyelid nudge the lens back on each blink.
This is also why toric lenses usually take longer to fit. Your practitioner is checking not only that the power is right but that the lens sits where it should and returns there after you blink. It is the reason the brand on a contact lens prescription is part of the prescription, since two lenses of identical power can settle quite differently.
You can browse our contact lens range, and the supplies section for solutions and cases.
Small changes are ordinary
An axis shifting by a few degrees between exams is common and does not usually signify anything. Measurement involves some judgment, and eyes are not machined parts. A large change is something your eye care professional will notice and follow up on, which is one of the things an exam is for.
If you want the rest of the abbreviations unpacked, our guide to reading your eye prescription covers SPH, CYL, ADD, PD and the difference between a glasses and a contact lens prescription.