Three numbers on every contact lens package determine whether the lens will fit your eye. Most people ignore them. Here is what each one means and why getting them wrong matters.
BC — Base Curve: The Shape of the Lens
The base curve is the curvature of the back surface of the lens, measured in millimeters. Yilala lenses use an 8.6mm BC. A lower number (8.3mm, for example) means a steeper curve. A higher number (8.9mm) means a flatter curve.
Your eye has a natural curvature. If the lens and your eye match within about 0.2mm, the lens sits flush against your cornea and stays in place when you blink. If the mismatch is larger, the lens either slides around with every blink (too flat) or creates a suction effect that feels tight and uncomfortable (too steep).
The 8.6mm BC fits roughly 80% of adult eyes. If a lens at 8.6mm does not feel right after trying different orientation and hydration levels, your eye may need a slightly different curve. An eye doctor measures this during a contact lens fitting.
DIA — Diameter: How Much of Your Eye the Lens Covers
Diameter is the width of the lens from edge to edge, measured in millimeters. Yilala lenses are 14.5mm DIA. The average human iris is about 12mm. At 14.5mm, the colored portion of the lens extends beyond your natural iris border by about 1.25mm on each side. This is what creates the eye enlargement effect.
The difference between standard and circle lens terminology is a matter of diameter. A lens with a diameter over 14.0mm is technically a circle lens. Below that, it is a standard colored lens. Yilala’s 14.5mm sits at the midpoint — enough to create visible enlargement, not so large that the lens looks artificial. Read more about circle lenses vs colored contacts.
A lens with a diameter smaller than your iris allows your natural eye color to show around the edges, creating a ring effect. A lens significantly larger than 14.5mm can extend too far onto the white of the eye, making the lens edge visible in certain lighting.
WT — Water Content: How Hydrated the Lens Stays
Water content is the percentage of the lens weight that is water. Yilala lenses are 38% WT. This means 38% of the lens material by weight is water held within the hydrogel polymer structure.
Higher water content sounds better. It is not. Lenses above 60% water content feel extremely comfortable when first inserted but dehydrate rapidly — sometimes within four to six hours — because the high water load evaporates faster. As the lens loses water, it pulls moisture from your tear film to compensate, which makes your eyes feel dry. The 38% range dehydrates more slowly and maintains stable comfort across an eight-to-ten-hour wear period.
The Parameter Combiner: Why These Numbers Work Together
A lens with the right BC but wrong DIA will fit well but look artificial. A lens with the right DIA but wrong BC will look good but feel uncomfortable. A lens with the right BC and DIA but wrong WT will feel great for two hours and terrible by hour five. All three parameters need to work together, which is why manufacturers like Yilala publish them clearly rather than hiding them in fine print.
| Parameter | Yilala Value | What It Controls |
|---|---|---|
| BC (Base Curve) | 8.6mm | How the lens sits on your cornea |
| DIA (Diameter) | 14.5mm | How much of your eye the lens covers |
| WT (Water Content) | 38% | How long the lens stays comfortable |
If a seller does not publish these three numbers on their product page, they are either hiding low-quality specifications or did not bother to test their lenses. Either way, look elsewhere. Browse lenses with full specifications published.