A contact lens is fully described by a small set of numbers and labels. Once you can read them fluently, any manufacturer catalogue (and every ODAtlas table) becomes quick to scan and compare. This guide works through each parameter in turn: what it is, what it controls clinically, the typical range you'll see, and the traps to watch for. It is organised into four families: fit, optics, material & physiology, and handling & scheduling.
The short version
- Fit: base curve + diameter combine into sagittal depth, which is what really determines tightness.
- Optics: sphere, plus cylinder/axis for astigmatism, plus an add for presbyopia.
- Physiology: Dk/t (transmissibility), not Dk alone, describes how much oxygen reaches the cornea.
- Handling: wear schedule (overnight or not) and replacement schedule (how often discarded) are two different things.
1. Fit parameters
Base curve (BC / Base)
The base curve is the radius of curvature of the lens's back surface, in millimetres. A smaller number is a steeper curve; a larger number is flatter. Most soft lenses fall between roughly 8.3 mm and 9.0 mm, with about 8.4–8.7 mm being the common range. Because soft materials drape to the cornea, many mass-market soft lenses are offered in just one base curve; others provide two (for example 8.4 and 8.8) for steeper or flatter eyes. Rigid lenses, by contrast, are almost always fitted with a specific base curve chosen from keratometry/topography.
The aim is a lens that centres well, covers the cornea, and shows a small amount of movement on blink: enough for tear exchange, not so much that vision or comfort suffers.
Diameter (DIA / Diam)
The overall diameter of the lens in millimetres, typically 13.8–14.5 mm for soft lenses. The lens should vault the cornea and rest on the conjunctiva with adequate coverage in all directions of gaze.
Why base curve and diameter must be read together: sagittal depth
Here is the single most useful concept in soft lens fitting. Neither base curve nor diameter alone determines how tight or loose a lens feels; what matters is the sagittal depth (the vaulting height of the lens over the eye), which depends on both together. A steeper base curve or a larger diameter increases sagittal depth and makes the lens fit tighter; a flatter base curve or smaller diameter loosens it. This is why two lenses with the same base curve can fit very differently if their diameters differ, and why comparing base curve alone between products is misleading.
2. Optical parameters
Sphere power (PWR / Sph / D)
The spherical power in dioptres (D) that corrects myopia (minus) or hyperopia (plus). Soft lens powers are usually made in 0.25 D steps through the common range, with larger steps (0.50 D) at high powers. Above roughly ±4.00 D the contact lens power differs from the spectacle prescription because the lens sits on the eye rather than ~12 mm in front of it. See our vertex distance guide and the conversion tool.
Cylinder (CYL) and axis
For toric (astigmatic) lenses, the cylinder is the amount of astigmatic correction, commonly available in steps such as −0.75, −1.25, −1.75 and −2.25 D, with wider ranges in some products, and the axis is its orientation in degrees (1–180). Stock toric lenses are often limited in the axes and cylinders they offer, especially at oblique axes. Toric lenses also incorporate a stabilisation design so the axis stays put on the eye. See the toric & astigmatism guide. Note that prescriptions can be written in plus- or minus-cylinder form; the transposition tool converts between them.
Add power (near add)
For multifocal lenses, the add is the reading (near) boost. In contact lenses it is usually banded as Low / Medium / High (sometimes with numeric ranges) rather than a single dioptric value, because the optical design distributes power across zones rather than at one point. Selection starts from the spectacle add and is refined on the eye. See the multifocal fitting guide.
3. Material & physiology parameters
Dk (oxygen permeability) and Dk/t (transmissibility)
Dk is how readily oxygen passes through the lens material; Dk/t is the oxygen transmissibility of a lens of a given thickness (t). Dk is a property of the material; Dk/t is a property of the actual finished lens and is what the cornea experiences. Because thickness is in the denominator, transmissibility falls as a lens gets thicker, so a high-power (thicker) lens transmits less oxygen than a low-power lens of the same product. Silicone hydrogels transmit far more oxygen than conventional hydrogels. Read the full explanation in Understanding Dk, Dk/t & oxygen transmissibility.
Water content
The percentage of water in the hydrated material. In conventional hydrogels, oxygen is carried by water, so higher water content historically meant more oxygen (up to a ceiling). In silicone hydrogels, oxygen travels through the silicone, so water content is decoupled from oxygen performance. These lenses are often lower-water yet far higher in Dk. Water content still influences comfort, dehydration behaviour, and handling, but on its own it is not a proxy for oxygen delivery.
Modulus (stiffness)
Modulus describes how stiff or soft the material is. Early silicone hydrogels were relatively stiff (higher modulus), which improved handling and lens integrity but was linked to mechanical complications such as superior epithelial arcuate lesions (SEALs) and contact lens papillary conjunctivitis in some wearers. Newer materials achieve high oxygen with lower modulus for better comfort. Modulus isn't always in the headline spec but is worth knowing when troubleshooting mechanical issues.
Surface / wetting technology
Silicone is intrinsically hydrophobic, so silicone hydrogels use surface treatments, internal wetting agents, or plasma processes to keep the surface wettable and comfortable. This is a differentiator between products that two lenses with similar Dk can still feel very different.
FDA material groups
Soft lens materials are classified by the US FDA into groups based on water content and surface charge (ionicity), which affect deposit tendencies and solution interactions. Silicone hydrogels were added as Group V (and are sometimes sub-divided). A quick reference:
| Group | Water content | Surface charge | Notes |
|---|---|---|---|
| I | Low (<50%) | Non-ionic | Low deposit tendency |
| II | High (≥50%) | Non-ionic | Prone to protein/lipid deposition |
| III | Low (<50%) | Ionic | Moderate deposition |
| IV | High (≥50%) | Ionic | Highest protein deposition tendency |
| V | Varies | Silicone hydrogel | High oxygen; own deposit & care profile |
UV blocking
Some lenses incorporate a UV-blocking chromophore. This is a useful adjunct but does not replace UV-protective eyewear, because a lens does not cover the conjunctiva, lids or peri-ocular skin.
4. Handling & scheduling parameters
Wear schedule (WEAR)
Whether the lens is approved for daily wear (removed each night) or extended / continuous wear (approved for overnight wear for a stated number of nights). Overnight wear markedly increases the risk of microbial keratitis and demands much higher oxygen transmissibility; it is a clinical decision, not merely a label.
Replacement schedule (REPL)
How often the lens is discarded and replaced: daily disposable, two-weekly, monthly, or quarterly/ conventional being the usual options. This is separate from wear schedule. Our replacement schedules guide compares the trade-offs; you can browse by modality on the daily, two-week and monthly pages.
Quantity (Qty)
The number of lenses per box. This matters for cost-per-day calculations and for working out how many boxes an annual supply needs.
Putting it together: reading one line
Suppose a table row reads: silicone hydrogel, Dk 100, BC 8.5, DIA 14.2, powers +6.00 to −12.00, daily wear, monthly replacement, 6/box. You now know this is a high-oxygen material suitable for a broad range of prescriptions, on an average fit (that will still depend on sagittal depth versus the eye), to be removed nightly and replaced monthly, with an annual supply of roughly four boxes per eye. If your patient is a −9.00 D myope, remember the lens is thicker at that power, so its effective Dk/t is lower than the headline Dk suggests, and the −9.00 spectacle Rx should be vertex-converted.
How to compare two lenses fairly
- Fit: compare sagittal depth intent (base curve and diameter together), not base curve alone.
- Oxygen: compare Dk/t at the relevant power, not just material Dk.
- Optics: confirm the exact power/cylinder/axis/add is actually available, not just the nominal range.
- Comfort factors: consider modulus, water content and wetting technology, which the headline numbers don't fully capture.
- Logistics: replacement schedule and box quantity drive real-world cost and compliance.