Two lenses with the exact same power can feel completely different — one comfortable and clear, the other tight, dry, and a little blurry. The difference is usually fit, and fit comes down to two numbers on your prescription: base curve and diameter.
Base curve and diameter, explained
- Base curve (BC): how curved the lens is, in millimetres. It should match the curve of your cornea so the lens sits stable and comfortable — too flat or too steep feels off. Typical range 8.3–8.8.
- Diameter (DIA): the total width of the lens, in millimetres. It should cover the cornea with a little margin. Typical range 13.8–14.5.
- Both are set by your eye-care professional during a fitting and printed on your contact lens prescription.
Why fit = base curve + diameter (the "sag")
Here is the part most guides miss. What actually decides how a lens sits is its sag — the depth the lens "scoops in" from its edge to its centre. And sag is not set by one number; base curve and diameter decide it together:
- A steeper base curve (lower number) → deeper sag → sits tighter.
- A flatter base curve (higher number) → shallower sag → sits looser.
- A larger diameter → deeper sag; a smaller diameter → shallower sag.
Because they work together, they can offset each other — a flatter curve can be "made up" by a larger diameter. The real differences are tiny, often just 0.1–0.2mm, but clinically that is enough to change how a lens feels all day.
What right vs. wrong fit feels like
Sag too deep (too tight): the centre vaults off the eye and tears pool, while the edge pinches and rubs your lid. The result is a foreign-body feeling, a tear film that dries out on screens, and an off-centre fit that can blur vision in dim light.
Sag just right (seated and centred): the edge seats evenly all the way around, so there is no foreign-body feeling even on long screen days, and the lens stays centred on your pupil for clear vision in any light.
A note for colored lenses
With colored lenses, diameter does double duty: it affects the fit and the look — a larger diameter creates more of an "enlarged eye" effect. It is tempting to chase the biggest diameter for the look, but comfort still comes first. Stay within the BC/DIA your eyes were fitted for, and choose the color and enlargement effect within that safe range.
Fit, quick answers
How do I find my BC and DIA? They are on your contact lens prescription, set during a professional fitting. A glasses prescription alone will not have them.
What if a lens is slightly different? Small differences can still fit, but always match your prescribed BC/DIA — if a lens feels too tight, too loose, or moves a lot, check with your prescriber.
General information, not medical advice. Always wear lenses fitted and prescribed for your eyes.