# How is contact lens base curve measured?

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Generally, your eye doctor will use a keratometer to measure the curve of your cornea, which is the front surface of the eyes – where contacts rest. These numbers help to determine the lens diameter and base curve that appear on your contact lenses prescription.

## How are base curve contacts measured?

Simple Rule for estimating lens base curve on a PLUS POWER lens

1. For plus power use the spherical equivalent (SE) and add 4.00 diopters to that. For example, if you have an Rx of + 2.00 sphere, the base curve for the lens will be approximately 6.00.
2. Rx +2.00Sph -> [+2.00 +4.00D] = 6.00BC.

## How is base curve calculated?

Figuring out the proper base curve based upon Rx is fairly simple: Plus Power – Use the Spherical Equivalent (Sphere power plus half the cylinder power) and add 4.00 diopter to that. Example – Rx of +2.50, the base curve will be approximately 6.50.

## Is 8.4 or 8.6 base curve?

Studies show that a single base curve of 8.4mm managed a “good or better” fit in approximately 90% of individuals,1 and base curves of 8.4mm and 8.6mm together encompassed 98% of individuals.

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LENS BASE CURVES AVAILABLE (mm) DIAMETERS AVAILABLE (mm)
5 8.6 14.0
6 8.5, 9.0 14.2
7 8.4, 8.8 14.0
8 8.7 14.2

## What is the average contact lens base curve?

Typical base curve values range between 8.0 and 10.0 mm, though it can be flatter (from 7.0mm) if you have a rigid gas-permeable lens. A person with a higher base curve number has a flatter cornea (the clear, front surface of the eye) compared to someone with a lower base curve number, which indicates a steeper cornea.

## What happens if you wear the wrong base curve?

If your lenses have the wrong diameter or base curve, you’ll likely feel that something is always in your eye. If the lenses are too flat, your eyelids will tend to dislodge them when you blink. The wrong size lenses can even cause an abrasion of your cornea.

## What does BC and DIA mean for contacts?

BC – Base Curve (usually a number between 8 and 10) DIA – Diameter (usually a number between 13 and 15) Brand – The brand/type of contact lens that your doctor has fitted you for.

## Does base curve have to be exact?

The base curve number would be a number between 8.0 and 10.0 millimeters and would be more precise because these lenses need to fit just right. Now that most contact lenses dispensed are soft lenses, this measurement doesn’t need to be quite as precise.

## What is the best base curve for a lens?

The most basic rule is that you always want the base curve to be as close to +6.00 as you can get and still have the Rx work. In theory +6.00 should always give you the best possible combination of curves for weight, optics, etc.

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## How do you convert from base curve to Keratometry?

Base Curve

1. Determine the patient’s mean keratometric reading by taking the average of the flattest keratometry and the steepest keratometry values: (flat K + steep K)/2.
2. Determine the “effective K,” which is a number that embodies both the central keratometric readings and the HVID.

## Is there a big difference between 8.5 and 8.6 base curve?

No there is not a big diff between the two base curves. However, it’s the relationship between diameter and base curve that is more important. Also, the material of the lens can also affect the fit. You can have 3 diff contact lenses with the same BC, Diameter and power and they will all fit differently.

## Is there a big difference between 8.3 and 8.4 base curve?

Usually yes, that is a big difference. 8.3 are for corneas that are steeper than average , while 8.6 is for flatter curvatures. Someone who requires an 8.3 or 8.4 usually can’t wear an 8.6 because they just keep sliding all over the eye.

## Is there a big difference between 8.6 and 8.7 base curve?

“Is there a big difference between 8.6 and 8.7 base curve in contact lenses?” No, the difference is small. The 8.7 curve is . 1mm flatter, but since these are soft lens curvatures, and soft lenses assume some of the shape of the cornea, the fitting value won’t be changed dramatically.