Power in contact lenses is the strength of the lens, measured in units called diopters. It tells you how much the lens bends light to correct your vision. You’ll see it on your prescription and lens packaging labeled as PWR or SPH, and it’s the single most important number in determining which contacts correct your specific eyesight.
How Diopters Work
A diopter measures how strongly a lens redirects light. The concept is straightforward: a 1-diopter lens brings light into focus at 1 meter away. A 2-diopter lens focuses light at half a meter. A 3-diopter lens focuses at a third of a meter. The shorter the focusing distance, the more powerful the lens. Your contact lens power tells you exactly how much correction your eye needs to bring images into sharp focus on the retina.
Most soft contact lenses are manufactured in a range from +8.00 diopters to -12.00 diopters. Standard lenses come in 0.25-diopter steps (so you might be -2.25 or -2.50, but not -2.30). At higher powers, typically above +6.00 or -6.00, the steps widen to 0.50 diopters.
Plus vs. Minus: What the Sign Means
The positive or negative sign in front of your power number tells you which direction your vision needs correcting. Eye care professionals call these “plus” and “minus” powers.
A minus number (like -3.00) means you’re nearsighted. You can see things up close but distant objects look blurry. The lens diverges light slightly before it enters your eye, pushing the focal point back onto your retina. The higher the negative number, the stronger the correction and the more nearsighted you are.
A plus number (like +2.00) means you’re farsighted. Distant objects are clearer, but close-up work is difficult. The lens converges light to shift the focal point forward. Again, a higher positive number means a stronger prescription. Someone with +1.00 needs much less correction than someone with +4.50.
Additional Power Values for Astigmatism
If you have astigmatism, your cornea is shaped more like a football than a basketball, curving more steeply in one direction. Standard spherical lenses can’t fully correct this, so you’ll wear toric lenses that have two extra power values on the prescription: cylinder (CYL) and axis.
Cylinder measures how much additional correction the astigmatism requires. Most toric contact lenses start at -0.75 diopters of cylinder power and increase in 0.50-diopter steps. Axis is a number between 1 and 180 that specifies the angle of the astigmatism on your eye, telling the lens which direction to apply that extra correction. Even small errors in axis alignment can noticeably affect vision, especially at higher cylinder powers. Research shows that undercorrecting cylinder by as little as 0.75 diopters causes measurable drops in visual clarity.
ADD Power for Presbyopia
If you’re over 40 and losing the ability to focus on close objects, your prescription may include an ADD value. This appears on multifocal or bifocal contact lenses and represents the extra magnifying power built into part of the lens for reading and other near tasks. ADD power is always a positive number, and it’s layered on top of your distance prescription. It’s typically categorized as low, medium, or high, depending on how much near-vision boost you need.
On multifocal lens packaging, you may also see a “D” or “N” marking. These stand for dominant and non-dominant eye, since some multifocal designs use slightly different power distributions in each eye to balance near and distance vision.
Why Contact Lens Power Differs From Glasses
If you wear both glasses and contacts, you’ve probably noticed the numbers aren’t identical. This isn’t an error. Glasses sit about 12 to 13 millimeters in front of your eye, while contacts rest directly on the cornea. That gap, called vertex distance, changes how much power is needed to focus light on the same spot.
For mild prescriptions, the difference is negligible. But once your power reaches roughly +/-4.00 diopters or higher, the gap becomes clinically significant. A person who wears -7.00 glasses, for example, will typically need a slightly weaker contact lens to achieve the same correction. Your eye care provider does this conversion automatically when writing your contact lens prescription, which is one reason you can’t simply use your glasses prescription to order contacts.
How Lens Fit Affects Your Actual Vision
Power isn’t the only number that matters for sharp vision. Two other values on your prescription, base curve (BC) and diameter (DIA), describe the physical shape and size of the lens. Base curve is the curvature of the back surface, and diameter is the overall width.
These numbers determine how the lens sits on your cornea. Research on soft lenses has found that even when a lens appears to fit well on the eye, the wrong base curve can reduce visual acuity, particularly with thinner lenses. A lens that’s too steep or too flat may shift on your eye or create subtle optical distortions that undermine the intended power correction. This is why a contact lens fitting involves more than just determining your prescription strength.
Reading Your Lens Box
Every box of contact lenses prints the key values on the side panel. Here’s what you’ll see:
- PWR or SPH: Your spherical power, the main correction number
- BC: Base curve, measured in millimeters
- DIA: Diameter of the lens in millimeters
- CYL: Cylinder power (toric lenses only)
- AX or AXIS: Axis angle (toric lenses only)
- ADD: Near-vision addition (multifocal lenses only)
Your left and right eyes almost always have different power values, so check that you’re putting the correct lens in each eye. The prescription will list OD (right eye) and OS (left eye) separately. If your powers are close together, mixing them up may not be immediately obvious, but it will cause subtle strain and blurriness throughout the day.