What is DIA and BC in Contact Lenses?
BC (Base Curve) describes the curvature of the back of a contact lens, while DIA (Diameter) describes its total width from edge to edge. When ordering contacts, select the exact BC and DIA shown for your prescribed lens brand and model, together with the correct Power and—if you wear toric lenses—CYL and AXIS.
Example contact lens packaging showing common ordering parameters. The numbers are examples only—use the values for your own lenses.
BC, DIA, Power, CYL and AXIS: Quick Reference
| Parameter | Meaning | What it describes | Example | When you enter it |
|---|---|---|---|---|
| BC | Base Curve | The curvature and design of the back surface of the lens | 8.6 mm | When the product offers a BC selection |
| DIA | Diameter | The total width of the lens from edge to edge | 14.2 mm | When the product offers a DIA selection |
| Power (PWR/SPH) | Lens power | The amount of vision correction, shown with a minus or plus sign | -2.50 | For each eye |
| CYL | Cylinder | The amount of astigmatism correction | -1.25 | For toric contact lenses |
| AXIS | Axis | The orientation of the cylinder correction, measured from 0° to 180° | 180° | For toric contact lenses, together with CYL |
The values above are examples, not ordering recommendations. Always copy the parameters for your right and left eye from your current contact lens prescription or the packaging of the exact lens you already wear.
Ready to reorder the lenses you already know? Shop contact lenses online and enter the parameters separately for each eye.
What Are BC and DIA in Contact Lenses?
BC and DIA are two of the measurements used to describe the size and design of a contact lens. They may appear on your contact lens prescription, the blister pack, the side or back of the box, and the product page used for ordering.
BC describes the shape of the lens surface that faces the eye. DIA describes the overall size of the lens. Both can influence how a lens covers the cornea, centers, moves when you blink, and feels during wear.
However, BC and DIA are only part of the complete lens design. Material, thickness, flexibility, water content, edge profile, peripheral curves, and manufacturing method can also affect fit. Two lenses with identical BC and DIA values are therefore not automatically interchangeable.
What Is the Base Curve of a Contact Lens?
The Base Curve, abbreviated as BC, describes the curvature and design of the back surface of the contact lens—the side positioned closest to the cornea. BC is expressed in millimeters.
Because BC represents a radius of curvature, a lower number generally describes a steeper curve and a higher number a flatter curve. Many standard soft lenses have a BC of approximately 8.3–9.0 mm, although specialty and rigid lenses can use different values.
BC should not be treated as a simple measurement of the eye or as the only factor controlling fit. It works together with DIA and the complete lens construction.
Does the Same BC Mean the Same Fit?
No. Two contact lens models can both show BC 8.6 and still fit differently. They may vary in:
- diameter and sagittal depth;
- lens material and water content;
- center and edge thickness;
- flexibility;
- edge design and peripheral curves;
- manufacturing method.
These differences can change how the lens moves, centers, and feels. This is why you should order the same prescribed brand and model instead of selecting another product based only on matching numbers.
How Do I Find My Contact Lens BC?
Look at your contact lens prescription or the box of the exact lenses you currently wear. The value is usually printed next to BC and may look like 8.4, 8.5, 8.6, or another number.
Some contact lens models are manufactured with only one base curve. In that situation, a store may display the BC as fixed information instead of showing a selectable field.
What Can Happen If the BC Is Unsuitable?
A poorly fitting contact lens may cause discomfort, excessive or restricted movement, unstable vision, dryness, redness, or difficulty keeping the lens centered. The effect cannot be predicted from the BC number alone because DIA, material, and the rest of the lens design also influence fit.
Remove the lenses and contact an eye-care professional if you experience persistent pain, redness, light sensitivity, discharge, or reduced vision.
What Is DIA on Contact Lenses?
DIA means Diameter. It is the total width of a contact lens from one edge to the other and is measured in millimeters.
Many soft contact lenses have a DIA of approximately 13–15 mm. Rigid gas-permeable lenses are usually smaller, while scleral and other specialty lenses can be larger.
DIA affects how much of the cornea and surrounding tissue the lens covers. Together with BC and the overall design, it influences positioning, movement, stability, and comfort.
How Do I Find the DIA of My Contacts?
Check the box, blister pack, or contact lens prescription for DIA or D. It may appear as a value such as 14.0, 14.2, or 14.5. Copy the number exactly when the online product page asks for it.
As with BC, some products come in only one diameter. The value may then be shown in the product specification rather than as a selectable option.
What Can Happen If the DIA Is Unsuitable?
A lens with an unsuitable diameter may provide too little or too much coverage, move excessively, move too little, decenter, feel uncomfortable, or produce unstable vision.
A larger DIA is not automatically tighter, and a smaller DIA is not automatically looser. Base curve, diameter, material, thickness, and peripheral geometry work together.
What Do Power, CYL and AXIS Mean?
Power, PWR or SPH
Power is the strength of the contact lens prescription, measured in diopters. A minus value is commonly used for nearsightedness, while a plus value is used for farsightedness. Contact lens boxes and ordering forms may label this field as Power, PWR, or SPH.
Pay close attention to the plus or minus sign. Enter the value separately for the right and left eye.
CYL and AXIS
CYL (Cylinder) and AXIS are used for toric lenses that correct astigmatism. CYL describes the amount of astigmatism correction, while AXIS describes its orientation in degrees.
CYL and AXIS must be selected as a pair for each eye. Do not copy these values from an eyeglass prescription because contact lens and glasses prescriptions are not necessarily the same.
If your prescription includes CYL and AXIS, shop contact lenses for astigmatism and choose the exact model you already wear.
How to Order Contact Lenses Using BC and DIA
Before ordering, place your current contact lens box or prescription beside you. Complete the following steps for each eye:
- Choose the exact brand and model. Similar BC and DIA values do not make two different products interchangeable.
- Select the Power. Check the number and its plus or minus sign carefully.
- Check BC and DIA. Select the values shown on your box or prescription. If the product has only one option, the value may already be fixed.
- Enter CYL and AXIS when required. These fields apply to toric lenses for astigmatism.
- Enter ADD when required. Multifocal lenses may also require an addition power or a brand-specific multifocal design.
- Repeat the process for the other eye. Never assume that both eyes use identical parameters.
- Review the order before payment. Compare the brand, model, Power, BC, DIA, CYL, AXIS, ADD, pack size, and quantity with your current lenses.
If you need help interpreting the abbreviations, read our complete guide on how to read a contact lens prescription. You can also use our online contact lens buying guide before placing an order.
Order with confidence: once you have confirmed the exact lens model and values for each eye, find your contact lenses at ContactLenses4Us.
Can You Switch to a Different BC, DIA or Brand?
Do not change the BC, DIA, brand, or contact lens model based only on a comparison of the printed numbers. A lens with the same Power, BC, and DIA can still behave differently because its material, thickness, edge profile, flexibility, and overall geometry may not match your current lens.
Likewise, a model with a slightly different BC or DIA may sometimes fit appropriately because the rest of its design is different. An eye-care professional can assess the lens on the eye and check centration, movement, coverage, comfort, vision, and eye health.
Caring for Contact Lenses After Ordering
Correct parameters do not replace proper handling. Follow the instructions for your lens type and replacement schedule.
- Wash and dry your hands before touching the lenses.
- Use the recommended contact lens care system.
- Never use water or saliva to clean or store lenses.
- Discard daily disposable lenses after one use.
- Replace reusable lenses according to schedule.
- Use fresh solution and replace the lens case regularly.
- Remove the lenses if you develop pain, persistent redness, discharge, light sensitivity, or reduced vision.
For step-by-step advice, see our guide to caring for contact lenses.
Frequently Asked Questions
Where Can I Find BC and DIA on a Contact Lens Box?
Look on the side or back label of the box and on the individual blister pack. BC and DIA are usually printed near Power/PWR/SPH and the lens name. The exact layout varies by manufacturer.
Is BC 8.4 the Same as BC 8.6?
No. They are different base curve values. Do not switch between them independently. The complete lens design matters, so use the value and model selected for you.
Can I Order Contacts If the Store Does Not Let Me Select BC or DIA?
Some products are made in only one BC or DIA, so the parameter is fixed rather than selectable. Check the product specification and confirm that it matches the exact brand and model you already wear.
Do Both Eyes Have the Same BC and DIA?
Sometimes, but not always. Check the information for the right and left eye separately. Power and other parameters can also differ.
Do All Contact Lenses Need CYL and AXIS?
No. CYL and AXIS are generally required for toric contact lenses that correct astigmatism. Standard spherical lenses do not normally use these fields.
Can I Use My Glasses Prescription to Order Contacts?
No. A glasses prescription does not contain all the fitting information required for contact lenses and its power may differ. Use your contact lens parameters.
Conclusion
BC means Base Curve and describes the back curvature of a contact lens. DIA means Diameter and describes its total width. Power provides the vision correction, while CYL and AXIS are used for toric lenses that correct astigmatism.
When reordering, match the exact lens brand and model and copy every value separately for the right and left eye. Do not use BC and DIA alone to substitute one contact lens for another.
Have your parameters ready? Shop contact lenses at ContactLenses4Us and select the values for the lenses you already know.
