Glasses to Contacts Conversion: Everything You Need Before Making the Switch

Quick Answer: For prescriptions up to ±4.00D, your glasses SPH power is typically the same as your contact lens SPH power. For prescriptions above ±4.00D, a vertex distance adjustment applies: minus prescriptions reduce slightly, and plus prescriptions increase slightly. This adjustment only affects SPH power. Base curve and diameter cannot be derived from a glasses prescription.

If you wear glasses and are thinking about switching to contact lenses, you have probably noticed that the two prescriptions do not always match. The glasses to contacts conversion is one of the most searched topics among UK lens wearers, yet most guides either oversimplify it to the point of being useless or load it with technical jargon that leaves readers more confused than before. This article explains the conversion clearly, provides a complete reference chart, and covers what the chart cannot tell you.

Why Glasses and Contact Lens Prescriptions Are Different

The glasses to contacts conversion exists because of a fundamental physical difference in where the two types of lens sit in relation to your eye. When people search for a glasses to contacts conversion, they are really asking about a principle called vertex distance. Spectacle lenses rest on your nose bridge, typically 12 to 14 millimetres in front of your cornea. Contact lenses rest directly on the corneal surface, making that distance effectively zero. This gap changes the effective optical power of any corrective lens placed over the eye.

When the Difference Matters

The vertex distance effect is only clinically significant for prescriptions above ±4.00 dioptres. Below this threshold, the power difference between a glasses prescription and the equivalent contact lens power is less than 0.25D, which is smaller than the manufacturing step size of most contact lenses. In practice, this means:

• Prescriptions from -4.00 to +4.00: glasses SPH and contact lens SPH are the same value

• Prescriptions stronger than ±4.00: the contact lens power differs from the glasses power and conversion is needed

• Minus (myopia) prescriptions above -4.00: the contact lens power is slightly less minus than the glasses

• Plus (hyperopia) prescriptions above +4.00: the contact lens power is slightly more plus than the glasses

This is because moving a minus lens closer to the eye increases its effective power (so you need less correction at the corneal plane), while moving a plus lens closer to the eye decreases its effective power (so you need more correction at the corneal plane).

The Vertex Distance Formula in Plain English

The glasses to contacts conversion uses a formula called vertex distance compensation. Without algebra, the logic works like this: take your glasses SPH power, apply a small adjustment based on the 12mm gap, and round the result to the nearest 0.25D step. For a -6.00D glasses prescription, the calculation produces approximately -5.50D for the contact lens. For a +5.00D glasses prescription, the result is approximately +5.25D. The chart in the next section shows these values pre-calculated for the most common prescription ranges.

Glasses to Contacts Conversion Chart

The table below shows the approximate contact lens SPH equivalent for glasses prescriptions above ±4.00D, based on a standard 12mm vertex distance. Values are rounded to the nearest 0.25D manufacturing step. This glasses to contacts conversion chart covers the most common prescription ranges encountered in UK contact lens fittings.

Note: this chart is for educational reference only. It is not a contact lens prescription and cannot be used to order contact lenses in the UK without a professionally issued prescription. Prescriptions up to ±4.00D do not require conversion: the contact lens SPH value matches the glasses SPH value.

The left side of the table covers short-sighted (myopic) wearers with minus prescriptions. The right side covers long-sighted (hyperopic) wearers with plus prescriptions. Notice that the minus adjustments become progressively larger as the prescription strengthens: the difference between glasses and contact lens power at -5.00 is only 0.25D, while at -10.00 it grows to 1.25D. The same magnifying effect occurs on the plus side.

Worked Example: Converting a -5.50D Glasses Prescription

Your glasses prescription shows -5.50D SPH. Using the chart, the approximate contact lens SPH is -5.25D, which is 0.25D less minus. Your optician confirms this on-eye and rounds to the nearest available manufacturing step. You order -5.25D contact lenses.

Worked Example: Converting a +5.00D Glasses Prescription

Your glasses prescription shows +5.00D SPH. Using the chart, the approximate contact lens SPH is +5.25D, which is 0.25D more plus. The lens sitting on the cornea is weaker than the same lens sitting 12mm away from the eye, so more power is needed to achieve the same corrective effect.

What a Glasses to Contacts Conversion Cannot Tell You

The glasses to contacts conversion chart handles only one of several parameters needed for a complete contact lens prescription. Understanding what the conversion cannot provide helps explain why a separately issued contact lens prescription is essential rather than optional.

Base Curve and Diameter

A contact lens prescription always includes two values that have no equivalent in a glasses prescription:

• Base Curve (BC): measured in millimetres, this describes the curvature of the back surface of the contact lens, which must match the curvature of your cornea closely enough for comfort, stability, and oxygen flow. Base curve is measured during a contact lens fitting using corneal topography or keratometry. It cannot be estimated or derived from a glasses prescription under any circumstances.

• Diameter (DIA): the physical width of the lens as it sits on the eye, also measured in millimetres. Most soft disposable lenses come in diameters between 13.8mm and 14.5mm. Coloured contact lenses frequently use slightly larger diameters (typically 14.0mm to 14.5mm) to achieve full iris coverage.

Both values are determined during the contact lens fitting appointment by a registered optician and specified on the resulting prescription. They are the reason why a glasses-to-contacts conversion, however accurately calculated, cannot substitute for a professional fitting.

Cylinder and Astigmatism

If your glasses prescription includes a CYL (cylinder) value, this indicates you have astigmatism. The glasses to contacts conversion does not automatically resolve how this should be handled for contact lens wear. The relevant rules are:

• CYL of 0.75D or less: mild astigmatism that can often be accommodated in a standard spherical contact lens using a spherical equivalent calculation (glasses SPH plus half the CYL value, rounded to the nearest 0.25D). An optician makes this decision based on the individual.

• CYL above 0.75D: typically requires a toric contact lens, which has a different optical design that corrects astigmatism specifically. Toric lenses have additional fitting parameters including axis alignment and lens stabilisation, which require professional fitting.

For an explanation of how lens materials relate to prescription type, our guide to what contact lenses are made of covers the full range of hydrogel and silicone hydrogel options available for different prescriptions.

Glasses to Contacts Conversion in the UK: What the Law Requires

In the United Kingdom, the glasses to contacts conversion chart is a useful reference tool but it does not satisfy the legal requirements for purchasing contact lenses. All contact lenses in the UK are classified as medical devices by the MHRA (Medicines and Healthcare products Regulatory Agency). The following rules govern contact lens supply in the UK:

• A separate contact lens prescription is required to purchase contact lenses from any UK-registered retailer, including online. This is a legally distinct document from a glasses prescription.

• Contact lens prescriptions are issued by a registered optometrist after a contact lens fitting, which includes assessment of corneal health, tear quality, and lens fit in addition to refraction.

• Contact lens prescriptions in England and Scotland are generally valid for two years from the date of examination. After this period, a new examination is required even if vision appears unchanged.

• Contact lens fittings in the UK typically cost between £25 and £60 at chain opticians (Specsavers, Vision Express) and £40 to £80 at independent practices, usually in addition to a standard eye test.

Your glasses prescription can serve as useful preparation before a contact lens fitting. Knowing your approximate glasses SPH power helps you and your optician anticipate which contact lens powers are likely to be trialled. However, the fitting and the professionally issued prescription remain necessary before any purchase.

Once you have a valid UK contact lens prescription, Bella Lense's prescription coloured contact lenses are available from -8.00 to +4.00 SPH across multiple collections. Enter your SPH power at checkout alongside your chosen shade.

For guidance on caring for contact lenses correctly once you have your prescription, our complete contact lens care guide covers the full cleaning, storage, and replacement routine.

Contacts to Glasses Conversion: The Reverse Direction

The same vertex distance principle applies when converting in the opposite direction. If you have a contact lens prescription and want to estimate what your glasses prescription power might be, the adjustment simply reverses:

• Minus contact lens powers become slightly more minus for glasses: -5.25D contacts approximate to -5.50D glasses

• Plus contact lens powers become slightly less plus for glasses: +5.25D contacts approximate to +5.00D glasses

• Contact lens prescriptions below ±4.00D: glasses SPH is the same as contact lens SPH

The contacts to glasses conversion is useful if you need to order backup glasses and your most recent prescription is a contact lens one. However, as with the glasses-to-contacts direction, a contacts-to-glasses estimate is not a verified glasses prescription and a separate glasses eye examination is the correct route to an accurate spectacle prescription. For a full comparison of how the two prescription formats differ, our article on contact lenses vs glasses covers the practical and optical differences in detail.

For wearers who need long-sighted correction specifically and are considering contact lenses, our guide to contact lenses for long-sightedness explains how plus prescriptions work in contact lens format and what options are available.

Important: The conversion chart in this article is for educational and reference purposes only. It is not a contact lens prescription and must not be used to order contact lenses. Always obtain a professionally issued contact lens prescription from a registered UK optometrist before purchasing any contact lens.

FAQ

Can I use my glasses prescription to order contact lenses in the UK?

No. UK law requires a separately issued contact lens prescription from a registered optometrist. A glasses prescription cannot substitute for this, even if the SPH values appear similar. Contact lens prescriptions include additional parameters, such as base curve and diameter, that glasses prescriptions do not contain.

Is the glasses to contacts conversion the same for every prescription?

No. For prescriptions up to ±4.00D, no conversion is needed: the glasses and contact lens SPH values are the same. For prescriptions above ±4.00D, the degree of adjustment increases with prescription strength. At -10.00D glasses, for example, the contact lens power is approximately -8.75D.

Do I need to convert my CYL value when switching from glasses to contacts?

Not always. Mild astigmatism (CYL of 0.75D or less) can often be incorporated into a standard spherical contact lens using a spherical equivalent calculation. Higher cylinder values require toric contact lenses. Your optician determines the appropriate approach during fitting.

What is vertex distance and why does it affect prescription conversion?

Vertex distance is the physical gap between the back surface of a spectacle lens and the front of your cornea, typically 12 to 14 millimetres. Contact lenses sit directly on the cornea so this distance is zero. The effective power of a corrective lens changes as it moves closer to or further from the eye, which is why a glasses power and a contact lens power are not always identical.

Can I order coloured contact lenses using a glasses prescription?

No. Coloured contact lenses are medical devices in the UK and require a valid contact lens prescription from a registered optometrist, exactly like clear lenses. The glasses to contacts conversion chart can help you understand what power range you are likely to need, but the prescription itself must be professionally issued.

My glasses prescription is -3.50. Is my contact lens prescription the same?

Yes, almost certainly. Prescriptions below ±4.00D do not require vertex distance conversion. Your contact lens SPH will be -3.50D, the same as your glasses prescription. Your optician will also specify base curve and diameter during fitting, which are not on your glasses prescription.

Final Thoughts

The glasses to contacts conversion is simpler than most guides suggest. For the majority of UK wearers with prescriptions below ±4.00D, the SPH power is identical between glasses and contacts. For stronger prescriptions, the glasses to contacts conversion chart above provides the approximate starting point. What the conversion cannot provide are the fitting parameters, base curve, and diameter, that only an optician can determine, and which UK law requires before any contact lens is dispensed. Use the chart to prepare for your fitting, then explore Bella Lense's coloured prescription range once your prescription is in hand.