Glasses to contacts calculator: why the power changes

How to use it

People with strong glasses are often puzzled that their contact lens box shows a different number. Neither is a mistake. Glasses sit a little in front of the eye and contact lenses sit on it, and that gap changes how strong a lens needs to be.

Enter the sphere, cylinder and axis for one eye, with their signs. The vertex distance is the gap between the back of the glasses lens and the front of the eye. The tool assumes 12 mm, the usual figure. Strong prescriptions sometimes state the distance they were measured at, often marked BVD (back vertex distance); if yours does, enter that.

You get two results. The exact power at the eye is given to three decimal places. The rounded power uses normal lens steps: the sphere in 0.25 steps up to 6.00 and 0.50 steps above 6.00, the cylinder in 0.25 steps. The axis doesn't change, and a note says whether the rounded result differs from your glasses.

How the numbers work

A lens of power F (in diopters) worn d meters in front of the eye acts like a lens of power F ÷ (1 - d × F) on the eye. With glasses at 12 mm, d is 0.012.

An astigmatic prescription has two main meridians with different powers, so the tool converts each one. The first is the sphere alone; the second is sphere plus cylinder. The new sphere is the converted first meridian, and the new cylinder is the difference between the two converted values.

The tool opens on -6.00 -1.50 x 180 at 12 mm:

• First meridian: -6.00 ÷ (1 - 0.012 × -6.00) = -6.00 ÷ 1.072 = -5.597
• Second meridian: -7.50 ÷ (1 - 0.012 × -7.50) = -7.50 ÷ 1.090 = -6.881
• Cylinder: -6.881 + 5.597 = -1.284
• Rounded: -5.50 -1.25 x 180

So glasses of -6.00 -1.50 x 180 correspond to about -5.50 -1.25 x 180 at the eye (exactly -5.597 and -1.284).

Minus powers get weaker at the eye and plus powers get stronger. Moving a minus lens closer increases its effect, so less is needed. Moving a plus lens closer reduces its effect, so more is needed.

Why it matters above about ±4.00

Fitters usually allow for vertex distance once a prescription passes about 4.00 in either direction. Below that the change is small: at -3.00 it is about a tenth of a diopter and disappears in rounding. At -4.00 it is just under 0.20, enough to round to -3.75, and at -10.00 it is more than a full diopter. Between 3.00 and 4.00 the rounded result can tip by one quarter step, the kind of detail a fitting settles.

What this tool can't tell you

A contact lens prescription holds much more than a power. It lists the base curve and diameter, which decide how the lens sits on your eye, the lens type and material, and how often to replace it. Those come from a fitting, where an optometrist or ophthalmologist checks how the lens centers and moves, how healthy your cornea and tear film look, and how well you see with the lens in. Toric lenses for astigmatism come in a limited set of cylinder powers and axes, often 10 degrees apart, and small amounts of astigmatism are sometimes left uncorrected, so the final lens may differ again from anything shown here.

In the US, contact lenses can only be sold with a valid contact lens prescription, which is separate from a glasses prescription. That includes colored and cosmetic lenses, which are medical devices too. Rules elsewhere vary, but the risks don't: a badly fitting lens can scratch the cornea or cut its oxygen supply, and infections can threaten sight. Never order lenses from this number alone. The column on contact lens trouble and care covers safe daily habits.

See an eye doctor for a fitting before your first lenses, and go back if lenses become uncomfortable or your vision with them isn't sharp. Some problems are urgent:

• If an eye hurts, turns red, becomes sensitive to light or your vision blurs while you wear lenses, take the lenses out and see an eye doctor the same day. Keep the lenses and case to show the doctor.
• New flashes, a sudden shower of floaters or a curtain over part of your vision also need same-day care. Strongly nearsighted eyes have a higher risk of retinal detachment.
• For sudden loss of vision in one eye, or a chemical splash (rinse with plenty of clean water for at least 15 minutes first), go to an emergency department or call your local emergency number (such as 911, 999, 112, or 119 in Japan). Eye emergencies and first aid explains what to do on the way.

Questions people ask

Why is my contact lens power weaker than my glasses?

If you are nearsighted, a contact lens needs less minus power than glasses to do the same job, because it sits on the eye rather than 12 mm in front. At -6.00 the difference is about 0.40.

My glasses are about -2.00. Does any of this apply?

Hardly. At -2.00 the exact change is under 0.05 of a diopter, which rounds back to -2.00. Low prescriptions usually differ between glasses and contacts for other reasons, such as how the astigmatism is handled.

Does it work for farsighted prescriptions?

Yes, in the other direction. Plus powers get stronger at the eye: +5.00 becomes +5.319, rounded to +5.25.

Why is the astigmatism on my contact lens prescription different?

Soft toric lenses come in fewer cylinder powers and axis steps than glasses lenses. Rigid lenses can correct much of the astigmatism that comes from the cornea without any cylinder at all, and small amounts are sometimes left out. Your fitter chooses after seeing how lenses behave on your eye. The glasses prescription reader explains cylinder and axis.

Is 12 mm right for my glasses?

It is the standard assumption, and most frames sit close to it. A few millimeters either way barely changes the result at moderate powers but matters more at very high ones, which is one reason strong prescriptions are measured with extra care.

Sources

• The effective power (vertex distance) formula from basic geometric optics
• US Federal Trade Commission Contact Lens Rule, under the Fairness to Contact Lens Consumers Act
• American Academy of Ophthalmology patient pages on contact lens safety (aao.org/eye-health)