How to Read a Contact Lens Prescription

What does a contact lens prescription contain?

A contact lens prescription contains eight items that federal law names, plus the per-eye optical and geometric fields the lens design needs.

Under 16 CFR § 315.2, the Contact Lens Rule defines a contact lens prescription as one carrying enough information for the complete and accurate filling of a prescription for contact lenses, including: the name of the patient; the date of examination; the issue date and expiration date; the name, postal address, telephone number and facsimile number of the prescriber; the power, material or manufacturer or both; the base curve or appropriate designation; the diameter, when appropriate; and, for a private label lens, the manufacturer’s name, the private label brand’s trade name and, where applicable, the trade name of the equivalent brand.

Those eight are the floor. What a practice actually prints is a grid with one row per eye, and inside each row the fields below.

Not every row appears on every prescription: a spherical soft lens prints power and geometry and stops. Nothing in the list is arithmetic on a glasses prescription — every optical value was read through a trial lens on the eye it belongs to.

What do BC, DIA and power mean?

BC, DIA and power are the three fields every contact lens prescription carries, and § 315.2 names all three.

Power is the spherical power of the lens in diopters. It is printed as PWR, SPH, P or D depending on the form or the manufacturer, and those labels are one field, not four. A minus value corrects myopia and a plus value corrects hyperopia; the sign is part of the value. Values step in 0.25 D increments — −1.25, then −1.50 — and where no spherical correction is needed the field reads plano or PL rather than 0.00. Some labels print BVP, back vertex power, which is the same field named for the surface at which the power is measured. Whether power and sphere are the same thing on a contact lens is answered on is contact lens power the same as sphere.

BC is the base curve: the radius of curvature of the back surface of the lens, in millimeters, where a larger number is a flatter lens. § 315.2 requires “the base curve or appropriate designation”, which is why a design supplied in a single back-surface geometry can print a name or a code where a reader expected a number and still be a complete prescription. The full treatment is on Base Curve of Contact Lenses.

DIA is the diameter: the overall edge-to-edge size of the lens, in millimeters. § 315.2 requires it “when appropriate”, which is why a blank DIA field is not a missing field. The full treatment is on Diameter of Contact Lenses, and the way base curve and diameter act together is sagittal depth.

Where do the fields appear, and when does the document expire?

The same fields appear on three surfaces: the prescription itself, the contact lens box, and the foil blister pack.

The prescription is the complete document. The box carries the correction and geometry but not the prescriber’s details or the expiration date. The blister foil is the smallest subset — it identifies what is sealed inside that one foil. If the values disagree across the three, that is a finding to raise with the practice that wrote the prescription, not something to reconcile by picking one.

Issue date. § 315.2 defines the issue date as the date on which the patient receives a copy of the prescription at the completion of a contact lens fitting — not the date of the eye examination, which the same section requires as a separate field.

Expiration. 16 CFR § 315.6 sets the floor. A contact lens prescription expires on the date set by the law of the state in which it was written, if that date is one year or more after the issue date; otherwise not less than one year after the issue date. A prescriber may specify an earlier date, but only where that date rests on the prescriber’s medical judgment about the ocular health of the patient — and § 315.6(b) then requires the specific reasons to be documented in the patient’s record in enough detail for review by a qualified professional. A shorter expiry is a clinical decision on the record, never a default.

What do CYL, AX and ADD mean?

CYL, AX and ADD appear only on the lens types that need them: cylinder and axis on a toric lens, an add on a multifocal.

CYL is the cylinder: the astigmatic power of the lens, in diopters, printed alongside the sphere rather than inside it. A prescription without a CYL field is a spherical lens, not a lens with zero astigmatism. Published sources disagree about the sign, and this page records the disagreement rather than averaging it: Warby Parker states that contact lens cylinder “can be a positive or negative number”, while Alcon states it is “always a minus (−) on contact lenses”. Both describe real labels — minus form is the prevailing contact lens convention, and the same refraction written in plus form is still that refraction. Rewriting one as the other is arithmetic, and it belongs on the Cylinder Transposition Calculator. What the number itself means is on what CYL means.

AX is the axis: the meridian the cylinder sits on, in degrees, written 001 to 180. It is an orientation, not a strength, and it accompanies every cylinder value. What the number means is on what axis means.

ADD is the near addition on a multifocal. It is printed either as a power in diopters or as a designation — LOW, MED or HIGH — which is the manufacturer’s band for a range of near powers within that design. A prescription reading ADD MED is complete. Some multifocal designs also print D/N, marking the dominant and nondominant eye, because the two are corrected differently; where the field is absent, the design does not use it. The designation is covered on add designation, and starting near add is calculated on the Multifocal Contact Lens Calculator.

Why does the federal minimum name power but not cylinder?

The federal minimum names power as one field because cylinder, axis and add ride inside the power of the prescribed lens rather than beside it.

§ 315.2 lists “the power, material or manufacturer or both of the prescribed contact lens” as a single item. It does not enumerate cylinder, axis or add. That is not an omission: the section defines the information needed to fill a prescription completely and accurately for the prescribed lens, and for a toric lens the power of that lens is a sphere, a cylinder and an axis together. For a multifocal it is a distance power and an add. A toric prescription that printed a sphere and stopped would not satisfy § 315.2, because it would not describe the lens that was prescribed.

The practical consequence is the one a reader cares about. The three universal fields — power, base curve, diameter — are the ones to check first, because the rule names them explicitly. The toric and multifocal fields are present when the design requires them and absent when it does not, and their absence on a spherical prescription is correct.

What a contact lens prescription is not

A contact lens prescription is not a converted glasses prescription, and reading one does no conversion.

It is not arithmetic on a spectacle power. Moving a spectacle sphere to the corneal plane matters once power passes roughly ±4.00 D, and that compensation is done on the Contact Lens Conversion Calculator and explained on vertex-compensated contact lens power.

It does not settle whether a glasses prescription can be used for contacts. That question is answered on Starting Contact Lens Parameters Are Not a Prescription.

Two sentences, two links. Everything else on this page describes fields that already exist on a document a prescriber wrote.

Starting contact lens parameters still require on-eye fit

Starting contact lens parameters still require on-eye fit, because a calculated number is not one of the fields described above.

§ 315.2 defines a contact lens fitting as the process that begins after an initial eye examination and ends when a successful fit has been achieved, and it defines the issue date as the day the patient receives the prescription at the completion of that fitting. Every value on the sheet is therefore downstream of a trial lens: power checked with an over-refraction, fit read at the slit lamp, and only then a document. Converted numbers sit upstream of all of it.

Sources

Clinical and regulatory claims on this page are attributed to the publications below.

• 16 CFR Part 315, Contact Lens Rule— § 315.2 definitions: the eight items a contact lens prescription contains; “the power, material or manufacturer or both”; “the base curve or appropriate designation”; “the diameter, when appropriate”; the private label clause; the issue date as the day the patient receives a copy at the completion of a contact lens fitting; contact lens fitting as the process ending when a successful fit has been achieved. § 315.6: expiration on the state-law date where that is one year or more after issue, otherwise not less than one year; an earlier date only on the prescriber’s medical judgment as to ocular health, with the specific reasons documented in the patient’s record. Retrieved from the eCFR, 13 August 2026 (Title 16 current as of 8/11/2026).
• Acuvue, How to read contact lens prescriptions(medically reviewed by Dr. Giovanna Olivares, OD, FAAO; published 21 August 2024, updated 17 March 2026) — O.D. and O.S. as oculus dexter and oculus sinister; power labelled PWR, SPH or D; minus for myopia and plus for hyperopia; 0.25 D steps; plano or PL where no correction is needed; ADD as the additional power for presbyopia; BC as curvature in millimeters; DIA as width in millimeters.
• Warby Parker, How To Read a Contact Lens Prescription(Dr. Nikki Shah, updated 2 April 2026) — the grid layout with one set of numbers per eye; PWR and SPH as interchangeable labels; base curve sometimes not printed at all where a design has only one; ADD as a diopter value or as low, medium or high; D/N for the dominant and nondominant eye; the box omitting the expiration date and the prescriber’s details.
• Alcon PRECISION, Contact Lens Prescription 101— PWR, BC and DIA as the three numbers every label carries; cylinder and axis as the two the toric label adds.
• Lenstore, What is Base Curve and Diameter— base curve and diameter printed on the box, the blister pack and the prescription.
• Lens.com, Back Vertex Power— back vertex power as the effective power of a lens measured at its back surface.