If you already wear contact lenses and just want to reorder, you don’t necessarily need a fresh appointment to know your size, the two key measurements are already written down for you. But it’s important to be clear up front: contact lens sizing is a medical measurement, and if you have never worn lenses before, those numbers should come from a professional fitting, not a guess. This guide shows you how to find your size when you already have it, and where the line is for needing an eye-care professional.
The two numbers that define lens size
Contact lens “size” comes down to two values:
- Base Curve (BC): how curved the lens is, so it matches the curve of your eye. Common values fall in a narrow range, and the right one keeps the lens comfortable and stable.
- Diameter (DIA): the overall width of the lens, measured across in millimetres.
Together, BC and DIA determine whether a lens sits well on your eye. A lens with the wrong base curve can feel too tight or move around too much, which is uncomfortable and can affect eye health, so these aren’t numbers to estimate.
Where to find your size
You almost certainly already have these numbers in one of two places:
- Your contact lens prescription: BC and DIA are listed alongside your power values. This is the authoritative source.
- Your current lens box or blister foil: manufacturers print BC, DIA and power right on the packaging. If you’re happily wearing a pair now, the box tells you exactly what to reorder.
If you’re comfortable in your current lenses, reordering the same BC, DIA and power from a trusted seller like Contact Lens PK is straightforward. Just match every value on the box.
What you can’t safely do yourself
You cannot reliably measure your own base curve at home, it’s determined by an eye-care professional using proper equipment during a fitting. So while you can read your size off an existing prescription or box, you can’t derive it from scratch without a fitting. For first-time wearers, the base curve and diameter must be measured professionally, because the wrong fit isn’t just uncomfortable, a poorly fitting lens can disrupt the tear film and oxygen flow your cornea needs.
When to see a professional anyway
Even if you have your numbers, see an eye-care professional if you’re a first-time wearer, if your prescription has expired, if your eyes feel dry, red or irritated in your current lenses, or if your vision isn’t sharp. Eyes change over time, and a fitting that was right two years ago may not be right today. Reordering known-good lenses is fine; ignoring discomfort or skipping an overdue check is not.
Frequently asked questions
Can I just guess my base curve?
No. Base curve is matched to your eye’s shape by measurement. Guessing risks an uncomfortable or unhealthy fit. Read it off your prescription or box, or get it measured.
Is the size the same as my power?
No, size (BC and DIA) is separate from power (SPH, and CYL/AXIS for astigmatism). You need all of them correct to order the right lens.
I lost my box and prescription. What now?
Book a quick fitting or eye test. There’s no safe way to recover your exact size without one, and it’s a good chance to confirm your eyes haven’t changed.
The shortcut is simple: your base curve and diameter are on your prescription or your lens box, so reordering is easy once you know where to look. But measuring those numbers from scratch, especially for a first pair, is a job for a professional, because comfort and eye health depend on the right fit.