What Does the Axis Number Mean on Your Glasses Prescription?
A glasses prescription can look like a collection of unfamiliar abbreviations and numbers.
One measurement that often causes confusion is AXIS.
You might see a number such as 20, 90 or 175 written beside the CYL section of your prescription. Unlike SPH and CYL, however, AXIS isn't a lens power.
So what does it actually mean?
What is AXIS on a glasses prescription?
AXIS describes the orientation of the cylindrical correction used for astigmatism.
It is expressed as a number between 1 and 180 degrees.
The number tells the person making your lenses how the astigmatism correction needs to be positioned.
Why do some prescriptions have an AXIS and others don't?
AXIS is associated with the CYL measurement.
If your prescription contains a cylindrical correction for astigmatism, an AXIS measurement will normally accompany it.
If there is no CYL correction, an AXIS measurement generally isn't required.
What do the numbers mean?
AXIS runs from 1 to 180.
For example, your prescription might show:
CYL: -1.00
AXIS: 90
The -1.00 describes the amount of cylindrical lens power, while 90 describes its orientation.
An AXIS of 90 doesn't mean you have a stronger prescription than somebody with an AXIS of 20.
They simply describe different orientations.
Is AXIS measured in degrees?
Yes.
Think of AXIS as describing a direction rather than an amount of power.
That's why you won't normally see a plus or minus sign in front of it.
Does a higher AXIS mean worse astigmatism?
No.
An AXIS of 170 isn't stronger or more severe than an AXIS of 30.
The strength of the astigmatism correction is represented by the CYL value.
AXIS simply tells us where that correction needs to be positioned.
Why does entering the correct AXIS matter?
The CYL and AXIS measurements work together.
If your prescription requires an astigmatism correction, the cylindrical power needs to be positioned at the prescribed orientation.
That's why you should copy the AXIS exactly as it appears on your prescription when ordering glasses.
Don't confuse AXIS with CYL
These two measurements are closely connected but perform different jobs.
CYL tells us how much cylindrical correction is required.
AXIS tells us the orientation of that correction.
If your prescription contains both, you'll need both pieces of information when ordering your glasses.
Check the right and left eyes separately
Your right and left eyes can have completely different CYL and AXIS measurements.
When entering your prescription online, carefully check that each measurement has been entered against the correct eye.
It's particularly worth double-checking this before submitting your order.
One number with a very specific purpose
AXIS may look mysterious when you first read a spectacle prescription, but its job is relatively straightforward.
If you have astigmatism, it helps specify exactly how your cylindrical correction should be orientated within the lens.
Understanding what it means makes one more part of your glasses prescription much easier to read.