What Do PWR, BC, and DIA Mean? Reading a Contact Lens Prescription

Look at a contact lens box or prescription and you'll see numbers like "PWR -3.00," "BC 8.6," "DIA 14.2." When reordering or buying online, not knowing what they mean is unsettling. Here's a gentle explanation of what each number represents. Note: these values are based on an eye exam, so don't change them on your own judgment.

PWR (power): the number that sets your vision

PWR (also written as P or SPH) is the power — how much your vision is corrected.

Nearsighted people have a minus (−) value; farsighted people have a plus (+). The bigger the number, the stronger the correction. For example, "−5.00" is for stronger nearsightedness than "−3.00." Values typically come in steps of 0.25.

Glasses power and contact power aren't necessarily the same, because the distance between your eye and the lens differs. Contact power needs to be measured separately at an eye clinic.

BC (base curve): fitting your eye's curve

BC (base curve) describes how curved the lens is, written in millimeters like "8.6" or "8.7."

A smaller number means a steeper (tighter) curve; a larger number means a flatter one. Choosing a BC that matches your cornea's curve helps the lens fit snugly.

If the BC doesn't match, the lens may shift around or cling too tightly, causing discomfort or trouble. The available BC options vary by product.

DIA (diameter): the lens size

DIA (diameter) is the overall size of the lens itself, also in millimeters, like "14.0" or "14.2."

For regular clear lenses you rarely think about it, but for colored lenses it pairs with the "colored diameter" as an important number affecting your look.

"Colored diameter" for cosmetic lenses

Colored lenses have a separate number called the colored diameter — the size of the tinted portion of the lens.

A larger colored diameter makes the iris look bigger, a more dramatic effect. A smaller one looks more natural. You choose based on the look you want, but the overall DIA and power should still be based on an eye doctor's prescription.

The numbers come from your prescription

All the numbers here are determined by an eye exam. Avoid choosing on your own judgment with reasoning like "my friend uses this product" or "it's about the same as last time." Lenses with the wrong BC or power cause not just poor vision but eye trouble.

Get regular eye exams and know the values that suit you. Your vision and eye condition change over time, so the longer it's been since your last purchase, the more worth getting a fresh exam.

Once you know your prescription, manage it

Knowing your power, BC, and DIA makes reordering and stock management smoother. Not running out of your specific lens spec is the foundation of comfortable contact life.

Lenslog tracks the replacement date and remaining count of the lenses you register, and reminds you when it's time to replace or restock. It prevents the "ran out without realizing" and "wore expired lenses" situations. Get your prescription numbers right at the eye clinic, and leave daily management to the app. That way you keep using the lenses that suit you, at the right time, every time.