Six Codes Hidden on One Sheet of Paper: How to Read BC, DIA, PWR, CYL, AXIS and ADD
The eye exam is over.
The doctor hands you a sheet of paper.
"Just order these."
You look at it.
BC 8.5. DIA 14.2. PWR -3.25. CYL -0.75. AXIS 180. ADD +1.50.
OD. OS.
It is a code.
You nod and leave.
For the next three years you never read that paper again.
You reorder whatever the box says, again and again, until the day the brand is discontinued.
That is the day you get stuck for the first time.
"What was my prescription again?"
This article is for that day.
Crack the six codes once and you can judge for yourself whether any box on any shelf fits your eye.
Two Numbers That Glasses Never Needed
A glasses prescription is simple.
Power, astigmatism, axis.
A contact lens prescription adds two more.
The reason is distance.
Glasses sit twelve millimetres in front of the eye.
A contact lens sits on it.
Anything that sits on you has to match your shape.
Think of shoes.
Size alone (power) is not enough; width (BC) and length (DIA) have to fit too.
So the six codes come in two groups.
Shoe shape: BC, DIA.
Vision: PWR, CYL, AXIS, ADD.
BC — How Curved the Lens Is
Base Curve.
The curvature of the inner surface, in millimetres.
Usually between 8.3 and 9.0.
Here is the first trap.
The smaller the number, the more curved the lens.
8.4 is steeper than 8.9.
A big ball has a gentle surface; a small ball has a steep one. The number is the radius of that ball.
Steep corneas take a small BC, flat corneas a large one.
If the BC is wrong, the lens slides around even when the power is perfect.
Vision wobbles with every blink and the eye feels gritty by afternoon.
"The power is right but something is off" is usually this number.
DIA — The Width of the Lens
Diameter.
The full width of the lens.
13.8 to 14.5.
BC and DIA travel together.
Same BC with a different DIA means a different area of the cornea is covered.
Here is some good news.
These two are not chosen, only checked.
Each product comes with fixed values, so you simply confirm that the BC and DIA on the prescription match the ones printed on the box.
PWR — The "Power" Everyone Knows, Which Is Not Your Glasses Power
Power. Also written SPH.
Minus for short sight, plus for long sight.
-1.00 is mild, -6.00 is strong.
It moves in steps of 0.25.
Second trap.
Do not copy your glasses power onto a contact lens.
Someone who wears -4.50 glasses usually needs about -4.25 in lenses.
The lens sits closer to the eye, so less power is needed for the same effect.
At low powers the difference is negligible; above -4.00 it opens up.
Buying lenses with a glasses prescription is like buying shoes in someone else's size.
We covered this in [Why Are Your Glasses and Contact-Lens Prescriptions Different?](/blogs/news/why-glasses-and-contact-lens-prescriptions-differ).
CYL — A Box Only People With Astigmatism Look At
Cylinder.
The amount of astigmatism.
If you have none, the box is empty or says "DS" or "0.00".
If you do, it reads -0.75, -1.25, -1.75, -2.25 and so on.
The moment a number appears here, the lenses you can choose change.
Not a regular lens but a toric lens.
A toric lens has different power in different directions and is slightly heavier at the bottom so it does not rotate on the eye.
AXIS — The Direction of Astigmatism, and the Third Trap
Axis.
Which direction the astigmatism runs, from 0 to 180 degrees.
CYL and AXIS always come as a pair.
One without the other is a typo.
The third trap lives here.
The prescription says 85, but the box only offers 80 and 90.
Many toric lenses come in ten-degree steps.
"Just pick the closer one" is not the answer.
A five-degree gap goes unnoticed by someone with mild astigmatism and tilts the whole world for someone with strong astigmatism.
Ask the practice that examined you.
If toric lenses are new to you, start with [Choosing Toric Contact Lenses for the First Time](/blogs/news/choosing-toric-contact-lenses-for-the-first-time).
ADD — The Box That Appears After Forty
Addition.
How much to add to your distance power to read up close.
Always a plus.
From +0.75 up to about +2.50.
Fourth trap.
Many boxes print LOW, MID, HIGH instead of a number.
The same +1.50 is LOW in one brand and MID in another.
Do not copy the number; check the brand's conversion table.
An ADD value means a multifocal lens, and that story continues in [At 43, the Menu Suddenly Moved Away](/blogs/news/first-multifocal-contact-lenses-after-40).
OD and OS — A Trace of Latin
On the left of the prescription: OD, OS.
OD is Oculus Dexter, the right eye.
OS is Oculus Sinister, the left eye.
Latin. Medical terms from centuries ago, still in use.
Some clinics write R and L.
What matters is this.
Very few people have the same power in both eyes.
Many have astigmatism in only one.
So a box of 30 is one month for one eye.
Two eyes need two boxes, and if the powers differ, so do the boxes.
This is where the most common ordering mistake happens.
Buying two boxes of the same power, or swapping right and left.
Where the Numbers Live on the Box
A small table on the side of the box.
BC and DIA are printed as fixed values near the product name.
PWR is printed per box.
Toric boxes carry PWR, CYL and AXIS on one line; multifocals carry PWR and ADD.
Open the box and every blister repeats the same numbers.
Glancing at the blister each morning prevents "the day I wore left and right swapped".
Brands Call the Same Number Different Things
PWR, SPH, POWER, D.
DIA, DIAM.
BC, B.C.
CYL, "astigmatism".
AX, AXIS.
Different labels, same values.
Match the number and the sign (+ or -) and you are done.
Prescriptions Expire Too
Eyes change.
Power, cornea, tears.
That is why many countries put an expiry date on contact lens prescriptions.
In the United States it is one or two years depending on the state.
"I can still see fine" only tells you about power.
How much oxygen the cornea is getting, or whether it has small scratches, is not something you can feel.
The 30-Second Checklist Before You Order
Put the prescription and the box side by side.
- Same BC?
- Same DIA?
- Same PWR sign and number?
- If CYL exists: is it a toric lens, and is the value the same?
- If AXIS exists: is the value the same? (If the angle is not available, ask your practice.)
- If ADD exists: is it a multifocal, and does the level match?
- Did you pick separate boxes for right and left?
- Is the prescription still valid?
All eight match: it is the same lens wherever you buy it.
Any one differs: stop.
Cracking the Code Gives You Choices
If you cannot read the prescription, you have to take whatever someone hands you.
If you can, you check the box yourself, compare brands, and send back the wrong one.
Six codes.
Two for shoe shape, four for vision.
Plus right and left.
A sheet you ignored for three years takes ten minutes to read.
References
- US Food and Drug Administration, Contact Lenses
- American Optometric Association, Contact Lenses
- US Federal Trade Commission, The Contact Lens Rule
- American Academy of Ophthalmology, Contact Lens Prescription
This article provides general information. Prescription values are determined by an eye-care professional. Do not choose or change lens parameters that are not on your prescription. If you experience discomfort, stop wearing the lenses and consult a professional.