The 7 Numbers Your Contact Lens Rx Needs to Order Online

Written by the Charm Optical Team • October 7, 2026

Seven fields: base curve, diameter, power, cylinder, axis, add and brand. That is everything an online contact lens order needs. A glasses prescription contains at most four of them and never the base curve, diameter or brand — which is why a glasses Rx cannot order contact lenses, no matter how recent it is.

This post decodes all seven, shows you exactly which ones a glasses prescription is missing, and covers the three places transcription actually goes wrong: cylinder rounding, axis rounding, and powers above 4.00 D. When you have the seven, you can place the order in about two minutes.

What are the seven numbers on a contact lens prescription?

There is a published list of what a contact lens order needs, and it is shorter than most people expect. The United States Federal Trade Commission's Contact Lens Rule defines a contact lens prescription as one carrying, alongside the patient and prescriber details:

• “The power, material or manufacturer or both of the prescribed contact lens”
• “The base curve or appropriate designation of the prescribed contact lens”
• “The diameter, when appropriate, of the prescribed contact lens”

The same rule spells out what a seller has to send back to your optometrist to check an order: “the contact lens power, manufacturer, base curve or appropriate designation, and diameter when appropriate” plus “the quantity of lenses ordered” (16 CFR §315.5). That is the whole order, in one sentence, written by a regulator.

That rule is American law, not Canadian — we cite it because it is the clearest published statement anywhere of the fields an order actually needs. Canada treats contact lenses as regulated medical devices: Health Canada moved even non-corrective decorative lenses into Class II in 2016, as the Canadian Association of Optometrists reported at the time. A valid prescription is required to ship lenses here as well.

Add astigmatism and reading correction to the FTC's core list and you get seven fields. Here they are, with the abbreviations you will actually see printed:

1. 1

   Base curve BC

   How curved the back of the lens is, in millimetres. Usually one number per lens design, around 8.4 to 8.7.

2. 2

   Diameter DIA

   How wide the lens is, in millimetres. Commonly 14.0 to 14.5 for a soft lens.

3. 3

   Power PWR / SPH / D

   The strength, in dioptres. Minus for nearsighted, plus for farsighted.

4. 4

   Cylinder CYL

   The astigmatism correction. Only on toric lenses, and only in a handful of fixed steps.

5. 5

   Axis AX

   Where that astigmatism correction is aimed, from 1° to 180°. Toric lenses only.

6. 6

   Add ADD

   The reading boost on a multifocal lens. Sometimes a number, sometimes just Low, Mid or High.

7. 7

   Brand —

   The exact lens name. Not a preference. It carries the material, the shape and the replacement schedule with it.

Every one of the seven is per eye. Your right eye and left eye often disagree, so fill in both columns even when they look identical.

The order-form transfer

Below is the whole problem in one picture. On the left is a source document. On the right is what an online order form asks for. Flip the source between a contact lens prescription and a glasses prescription and watch which slots stop being fillable.

What do base curve and diameter actually mean?

Base curve is the curve of the back of the lens, written in millimetres of radius. Smaller means steeper. Diameter is how wide the lens is. Together they decide whether the lens centres on your cornea and stays there, or rides high, or drags on your lid with every blink. Neither one is your prescription strength — but a lens that does not centre or stay put cannot deliver the strength it carries, which is why fit and vision are not separable, and why the wrong pair of numbers shows up first as a lens you cannot wear for twelve hours.

Here is the part that surprises people: most modern soft lenses come in exactly one base curve. There is nothing to choose. But the single base curve is different for each lens design, even inside one manufacturer's own family:

Scroll the table sideways →

Three different base curves — 8.4, 8.6 and 8.7 — and three different diameters, across five lenses from one company. That is why base curve is not a setting you can carry from one lens to another. It belongs to the specific lens you were fitted in.

Which fields does a glasses prescription not contain?

Three of the seven are simply absent, and the other four need work. This is the single most useful table in the post.

The traffic runs both ways, by the way. A glasses prescription often carries a PD — your pupillary distance, the gap between your pupils in millimetres. It is essential for making a pair of glasses and completely irrelevant to a contact lens, which centres itself on your eye. Each document carries a field the other has no use for.

Scroll the table sideways →

A glasses prescription is not a weaker version of a contact lens prescription. It is a different document, describing a lens that sits 12 mm away from your eye instead of on it.

Why is a contact lens power weaker than the same glasses power?

Because your glasses are not touching your eye. A lens held further away behaves as though it were a different strength, and the effect grows fast as the prescription gets stronger. Move a minus lens closer and it acts stronger; move a plus lens closer and it acts weaker. So a contact lens, sitting right on the cornea, needs less minus and more plus than the glasses that correct the same eye.

The threshold is not a rule of thumb. The National Academy of Opticianry's continuing-education course on the subject states it plainly: “For contact lenses, all powers greater than 4.00 Diopters in either principal meridian must be compensated” (Vertex Distance, Effective Power, and Compensated Power, Diane F. Drake, 2016).

The same course gives a simple way to see the size of the effect. Square the power, divide by 1,000, multiply by how many millimetres the lens moves — then add or subtract depending on which way it moved and which sign it is:

Scroll the table sideways →

At -10.00 the gap is more than a full dioptre. That is five clicks of the dial, and it is the difference between a lens you can read a phone through and one you cannot. Two honest caveats about that table. The 12 mm is an assumption — your optometrist works from the distance actually measured for your frame. And this is the quick method; the exact optical calculation lands a little differently at the extremes. Treat the table as the shape of the effect, never as your prescription.

How do cylinder and axis get rounded for a toric lens?

If you have astigmatism, your contact lens order needs a cylinder and an axis. Neither is a free number. Look again at the parameter table above: Biofinity Toric and MyDay Toric are each made in exactly four cylinder powers — -0.75, -1.25, -1.75 and -2.25 — and axes on a 10° grid. That is the whole menu.

So take the glasses prescription in the transfer above: -3.50 -1.00 × 175 in the right eye.

• A cylinder of -1.00 is not made. The nearest available step is -1.25.
• An axis of 175° is not made either. And 175 sits exactly halfway between 170 and 180, so there is no rounding rule to apply. Somebody has to choose.

That is the whole reason a fitting exists. The choice between 170 and 180 is made by putting a lens on your eye and checking that you can actually see out of it, then writing down the combination that worked. It is a two-minute decision for a trained fitter and an unanswerable question for an order form. A contact lens exam and fitting at Charm Optical is $135, separate from the $99 comprehensive eye exam.

These three toric lenses are all in our contact lens range, and all three ask for the same five numbers:

Daily toric

MyDay Toric 90 Pack

Base curve 8.6, diameter 14.5, axes on the 10° grid.

$110

Monthly toric

Biofinity Toric

The flatter one at 8.7, so its base curve is not interchangeable.

$70

Daily toric

Acuvue Oasys 1-Day for Astigmatism

A 30-pack, so one box is about a month for one eye.

$65

Why can the axis on your lens differ from the axis on your Rx?

A toric lens has to know which way is up. It is weighted or shaped so it settles into one orientation on your eye, and it carries a physical mark so the orientation can be seen. CooperVision lists MyDay Toric's as a “single line at 6 o'clock”.

The mark matters because lenses do not always settle where the design intends. Your lid pressure and the shape of your cornea can park the lens a few degrees off, and a toric correction that is rotated is a toric correction that is aimed at the wrong meridian. So the fitter looks at where the line actually sits and adjusts the axis they order to compensate.

This is exactly what the FTC's rule describes a fitting as: “an examination to determine lens specifications” followed by “an initial evaluation of the fit of the contact lens on the eye” (16 CFR §315.2). The axis on a finished contact lens prescription is the number that worked on your eye. The axis on your glasses prescription is the number your eye needed before any lens was involved. They are allowed to differ, and often do.

Does the brand on your prescription matter?

Yes, and more than most people expect. The brand is not the label on the box, it is a bundle of specifications: base curve, diameter, material, water content, oxygen transmission and replacement schedule. Change the brand and you have changed four of the other six fields at once. That is why the FTC rule names the manufacturer as part of the prescription itself, and why it forbids a seller from quietly swapping it — substituting “the name of a manufacturer or brand other than that specified by the patient's prescription” counts as altering the prescription (16 CFR §315.5(f)).

The practical version: if your prescription says MyDay Toric, order MyDay Toric. If you want to try something else — a monthly instead of a daily, or a different material because your eyes get dry by 4pm — that is a good conversation to have, and it is a conversation, not a substitution. Call us at (780) 490-0090 and we will tell you what your prescription allows and what would need a new fitting.

What else does the order form ask for?

Beyond the seven, three practical things:

1. Both eyes, separately. Almost every field can differ between your right eye (OD) and your left (OS). Fill in both columns even when they match.
2. How many boxes, per eye. For dailies, a 30-pack is about one month for one eye and a 90-pack about three months. Two 90-packs per eye is a six-month supply. For monthlies, a 6-pack is roughly six months for one eye.
3. The replacement schedule you were fitted for. Daily, biweekly or monthly is part of the prescription, not a budget choice. A monthly lens worn daily and thrown out is safe but expensive; a daily lens worn for a month is neither.

We ship contact lenses across Canada, with free shipping on orders over $100, and the code CHARMCONTACTS takes $5 off a contact lens order. Once your order is placed you can check on it any time with our order tracking form. If you want the longer version of how ordering works here, that lives on our contact lenses in Edmonton page.

One exception worth naming plainly: if your prescription has lapsed, do not guess and do not re-use an old box. Call us at (780) 490-0090 and we will tell you what you can still order, and whether you need a new fitting or just an updated prescription. If it turns out you are due, our eye exam and contact lens services page has the details and you can book an appointment online. We see patients by appointment.

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