First, you need to know you Pupillary Distance (PD). It’s a vital measurement and yet is often left off of prescriptions, but you can measure it by yourself or with the help of a friend. If you’ve got your measurements on your prescription already, let’s make sure it’s the right number.
Some prescriptions will have numbers for Distance PD, Near PD, or both. The distance value indicates the primary measurement and will come first. The near value indicates a separate measurement for reading glasses, so unless that’s the type of eyewear you’re ordering you can ignore your Near PD number entirely.
One more thing: you may also have a Prism value on your prescription, measured in prism diopters. Just like Pupillary Distance, prism diopters may be abbreviated as PD. Generally speaking, your prescription will say Prism or use a triangle for shorthand to avoid confusion. If you have a PD value between 0 and 1, however, that's likely a prism diopter value. Higher values are measurements in millimeters and denote your Pupillary Distance.
Second, you need the Sphere (SPH) measurement for both your left and right eyes. SPH indicates the amount of lens power, measured in diopters. These values are used to correct issues near- and farsightedness. Negative numbers mean you’re nearsighted and positive numbers mean you’re farsighted.
You also need the Cylinder (CYL) value for both your left and right eyes as well. CYL determines the lens power necessary to correct astigmatism. You may have no CYL value which means you have no astigmatism, or it’s so negligible you do not require any corrective measures. Just like your SPH value, negative and positive values correlate with near- and farsightedness (respectively).
Your ADD value, if you have one, simply denotes additional magnifying power to correct presbyopia. If you have one, make note of it. If you don't, it doesn't matter.
Third, and finally, you may need to know your Axis and ADD measurements. It's entirely possible you won't have either of these values defined, but if you do they are important.
Let's start with Axis. If you have a cylinder value on your prescription, you will also have an Axis. It denotes the vertical meridian of the eye, and that tells opticians the precise direction needed to position the cylindrical power of your lenses. Confused? This measurement basically makes sure the power necessary to correct your astigmatism is exactly where it needs to be so your eyeglasses do their job.
Think about it this way: a blindfold will successfully block a person's vision when tied around the eyes, but if you didn't know where the eyes were you might tie it around someone's mouth instead. The blindfold still has the ability to do its job, but because you didn't know where to put it that never actually happened