The eyeglass prescription in your wallet cannot be used to order contact lenses, and it is short by more than one number. That gap is the honest starting point for contact lenses vs glasses, because the two devices are described by different measurements, released to you under different federal rules, and checked by different people before anything ships. 1-800 Contacts publishes the parameter list it needs before it will fill an order, and that list is the fastest way to see what an eyeglass form leaves out.
This is an independent editorial review. 1-800 Contacts did not review or approve this article.
Contact lenses and eyeglasses are prescription medical devices in the United States. This article explains what the paperwork and the specifications mean; it is not medical advice, and an eye doctor decides what is right for your eyes.
Contact lens and eyeglass specifications compared
Nothing below ranks the two devices. It sets out which measurements each one requires, and who is allowed to hand them to you.
| What the paperwork carries | Eyeglass prescription | Contact lens prescription |
|---|---|---|
| Corrective power | Sphere, and cylinder with axis for astigmatism | The same fields, but recalculated for a lens resting on the eye |
| Curvature of the device | Not specified; the optician works to the frame | Base curve, written on the prescription |
| Size of the device | Decided by the frame you choose | Diameter, written on the prescription when appropriate |
| The exact product | Any lens the lab grinds to the numbers | Material and manufacturer, plus the brand name for a store brand |
| Pupillary distance | Needed to center the lenses in the frame | Not used; the lens centers itself on the cornea |
| Minimum validity | Set by state law | A federal floor of one year, with narrow exceptions |
| What it takes to get one | An eye exam | An eye exam plus a separate contact lens fitting |
| Before an online order ships | The seller fills it | The seller must verify it with the prescriber |
Why contact lenses vs glasses is two documents, not one
An eyeglass form describes light. A contact lens form describes an object that has to rest on a living surface for hours, so it carries shape as well as power. That is the practical core of contact lenses vs glasses, and why an optical shop cannot read one and produce the other.
The FDA lists what a contact lens prescription has to contain:
- The patient’s name and the date of the examination
- The issue date and the expiration date
- The prescriber’s name, address, phone number and fax number
- The power of the prescribed contact lenses
- The material and, or, the manufacturer of the prescribed lens
- The base curve, or an appropriate designation for it
- The diameter, when appropriate
- For a store brand, the manufacturer’s name and the trade name of that brand
Base curve and diameter have no eyeglass equivalent at all. Base curve describes how steeply the back of the lens is domed so it matches the front of your eye; diameter describes how far across the cornea it reaches. 1-800 Contacts lists the same set on its ordering pages, giving power (also called sphere or SPH), base curve (BC), diameter (DIA), cylinder and axis for toric lenses, and add power for multifocals, and says plainly that contact lens prescriptions “are different from glasses prescriptions (they include measurements like base curve and diameter).”
The eyeglass side has a measurement of its own that contacts never need: pupillary distance, the gap between your pupils, which tells the lab where to put the optical center of each lens. The FTC notes that in some states a prescriber is allowed to charge you for taking it.
Vertex distance: the same eye, two different powers
Here is where the numbers genuinely stop matching. A refraction is measured with test lenses held a short way in front of the eye, conventionally 12 to 14 millimeters, which is roughly half an inch. A contact lens sits at zero. Move a lens nearer or further and its effective power changes, so the power that corrected you in the exam chair is not the power that will correct you on the cornea.
The correction is a fixed piece of arithmetic: the compensated power equals the original power divided by one minus the change in distance, in meters, multiplied by the original power. Below about 4.00 diopters the shift is smaller than the 0.25 diopter step a phoropter turns in, so nobody bothers. Above it, the difference becomes clinically real. Worked through, a minus 8.00 diopter contact lens corresponds to roughly a minus 8.75 to minus 9.00 diopter spectacle lens, and a phoropter reading of minus 8.00 sphere with minus 5.25 cylinder at 85 degrees, taken at 12 millimeters, converts to about minus 7.30 sphere with minus 4.13 cylinder at the same axis once it is moved onto the eye.
So a strongly corrected wearer will see different figures on their two prescriptions, and neither is a typo. The further a lens sits from the eye, the more it also changes the apparent size of what you are looking at, which is why strong spectacle powers can make a room feel slightly the wrong scale while the equivalent contacts do not.
The trouble with asking contact lenses vs glasses which is better
There is no specification that answers it. Asking contact lenses vs glasses which is better is asking a question the measurements are not built to settle, because the two devices are not ranked against each other anywhere in the standards that govern them. ISO 18369-2:2017 sets tolerance limits for “the principal optical and physical parameters of rigid corneal, rigid scleral and soft contact lenses at the time of manufacture,” and says nothing at all about spectacles.
What the specifications do decide is which choice your own numbers make awkward:
- A strong correction magnifies or shrinks the image more through spectacles than through contacts, because of the same vertex distance above
- Spectacle lenses correct only where the frame sits, so a glance past the edge of the lens is uncorrected; a contact lens moves with the eye
- A contact lens covers the cornea, so oxygen supply and cleaning become part of the product; a spectacle lens has neither concern
- A presbyopic correction can be an add power on a multifocal contact or a second zone in a spectacle lens, and the paperwork differs for each
Those are trade-offs, not a scoreboard.
Buying contact lenses online or eyeglasses: what the federal rules give you
The rules diverge sharply here. Ordering contact lenses online or eyeglasses starts with the same right to hold your own prescription, but only one of the two comes with a clock attached.
Under the FTC’s Contact Lens Rule, a prescriber must “immediately give a copy of the contact lens prescription to the patient at the end of the contact lens fitting — even if the patient doesn’t ask for it,” and may deliver it digitally if you consent to the method. The Rule “sets a floor, or minimum, expiration date of one year unless there is a legitimate medical reason for setting a shorter expiration date,” and a prescriber who shortens it has to document the reason in enough detail for professional review and keep the record for three years.
Then comes the part that has no eyeglass equivalent. A seller who cannot confirm your prescription from the document itself has to contact your prescriber, and the Rule counts the prescription as verified if the prescriber “fails to communicate with the seller within eight business hours of getting a complete verification request.” Direct communication means a completed communication by phone, fax or email. Silence is a yes.
1-800 Contacts describes its own side in the same terms, saying “the FCLCA has determined that a seller of contact lenses who must verify a prescription must wait 8 business hours once the required information has been successfully communicated to the eye care provider office,” and that “if the seller does not receive verification response within that given time frame the seller is allowed to ship the contact lens order.” It sends the request through a text-to-speech phone system with a fax response option, which matters because the eight hours only begin once the request has actually arrived.
Wear schedules and returns: where contact lenses vs specs really differ
A spectacle lens is bought once. The comparison of contact lenses vs specs turns, in the end, on the fact that one of them is a consumable with a schedule printed into its approval.
The FDA separates lenses by how long they may stay in. A disposable lens is “used once and discarded,” meaning a fresh pair every day. Other soft lenses run on frequent or planned replacement. Extended wear lenses are approved for overnight use “ranging from one to six nights or up to 30 days,” and that range is an approval limit rather than a suggestion. Silicone-hydrogel materials exist to push more oxygen through to the cornea during those hours.
1-800 Contacts publishes policies that fit a consumable: every order ships free with no minimum and no hidden fees, returns and exchanges are free with no deadline on unused lenses, and the company covers return shipping. It says it runs three distribution centers across the United States and has sold lenses since 1995. Those terms change nothing about the optics, but they do change what an unopened box left over from an old prescription is worth.
Before you place an order, check the listing against the prescription rather than against your memory:
- Confirm the expiration date has not passed, because a seller cannot verify an expired prescription into a valid one
- Match the brand exactly, including the store-brand trade name if there is one
- Match the base curve and the diameter for each eye separately, as they are often different
- Match the power for each eye, and the cylinder and axis if the lens is toric
- Check the replacement schedule on the box against the one you were fitted for
- Check the prescriber’s phone and fax details, since a wrong number is what stalls the eight-hour clock
Frequently Asked Questions
The questions below come up because the two prescriptions look similar at a glance and are governed very differently underneath.
Can I use my glasses prescription to buy contact lenses?
No. An eyeglass prescription contains no base curve and no diameter, and its power figures have not been adjusted for a lens sitting on the eye. A contact lens prescription is issued only after a contact lens fitting, which is a separate step from the eye exam.
Why does an online seller call my eye doctor?
Because the Contact Lens Rule requires it when the seller cannot confirm the prescription from the document itself. The prescriber has eight business hours from receiving a complete request to respond; if nothing comes back, the prescription counts as verified and the order may ship.
Does a steeper base curve mean a better fit?
No. Base curve is a match, not a grade. A number that suits one cornea will sit too tight or too loose on another, which is what the fitting is for. It is also why two people with the same power can need different lenses.
How long is a contact lens prescription valid?
At least one year under the federal floor, unless your prescriber documents a legitimate medical reason for a shorter term. State law can set a longer minimum, and the expiration date printed on the prescription is the one a seller has to work from.
Bottom line
An eyeglass prescription and a contact lens prescription describe the same eye with different measurements, and the second adds shape, a product name and a verification step the first never needed. Read the base curve, the diameter and the expiration date before you compare anything else, and let the fitting rather than the price decide what goes on your cornea.
