What Lens Index Do I Need for My Prescription?

Choosing the wrong lens index is one of the most common and most avoidable mistakes when ordering prescription glasses online. The lens index you need is determined directly by your prescription strength — and getting it right is the difference between lenses that are thin, light, and comfortable, and lenses that are thick, heavy, and cosmetically unflattering. This guide explains exactly what lens index do I need for every prescription range, from mild corrections to extreme powers, with specific thresholds and measurable outcomes at every step.

If you have your prescription in front of you, you will know your answer by the end of this article.

What Lens Index Actually Means — and Why It Matters for Your Prescription

Lens index — also called refractive index — is a number that describes how efficiently a lens material bends light. A higher refractive index means the material bends light more powerfully per millimetre of thickness. This allows the same corrective power to be achieved in a thinner, lighter lens.

The refractive index of a material is defined as the ratio of the speed of light in a vacuum to the speed of light through that material. Standard optical plastic (CR-39) has a refractive index of 1.50. High-index materials range from 1.56 to 1.74. The higher the index, the thinner the lens can be made for any given prescription power.

A 1.74 index lens is approximately 50% thinner than a 1.50 index lens at the same prescription power. A 1.67 index lens is approximately 35% thinner. A 1.61 index lens is approximately 20% thinner. These are not marketing claims — they are the direct optical consequence of the refractive index formula. According to the College of Optometrists, lens index selection is a core dispensing decision that directly affects the optical and physical performance of the finished spectacle lens.

For a full breakdown of how prescription values translate to lens requirements, see our guide on how to read an eyeglass prescription.

What Lens Index Do I Need: The Complete Prescription Threshold Guide

The answer to “what lens index do I need” is determined primarily by the SPH (sphere) value on your prescription, with CYL (cylinder) contributing additional effective power for astigmatism corrections. Here are the recommended index thresholds used by optical professionals:

1.56 index — SPH up to ±2.00, CYL up to ±1.50: The entry-level high-index material. Approximately 10% thinner than standard CR-39 (1.50). Suitable for mild prescriptions where lens thickness is not a significant concern. The most affordable index option and the standard choice for low-power single vision lenses.

1.61 index — SPH ±2.00 to ±4.00, CYL up to ±2.00: The mid-range index. Approximately 20% thinner than CR-39. The recommended choice for moderate prescriptions. Offers a meaningful reduction in edge thickness for minus lenses and centre thickness for plus lenses without the cost premium of higher index materials.

1.67 index — SPH ±4.00 to ±6.00, CYL up to ±3.00: The high-index standard. Approximately 35% thinner than CR-39. Strongly recommended for prescriptions in this range. At -5.00 SPH, a 1.67 index lens is typically 1.5–2.0mm thinner at the edge than a 1.56 index lens in the same frame — a difference that is clearly visible and significantly affects comfort and appearance.

1.74 index — SPH above ±6.00, CYL above ±3.00: The ultra-high index. Approximately 50% thinner than CR-39. The recommended choice for strong prescriptions where lens thickness would otherwise be cosmetically and physically problematic. At -8.00 SPH, a 1.74 index lens reduces edge thickness by approximately 3–4mm compared to 1.56 index in a standard frame.

For a detailed comparison of the 1.61 and 1.67 index options specifically, see our dedicated guide on best lens index for high prescriptions: 1.56 vs 1.61 vs 1.67.

How CYL Power Affects What Lens Index You Need

The SPH value alone does not always determine what lens index you need. The CYL (cylinder) value — which corrects astigmatism — adds effective power to the lens and increases thickness in specific meridians. A prescription with a moderate SPH but a high CYL may require a higher index than the SPH alone would suggest.

The standard method for calculating effective lens power when CYL is present is to add half the CYL value to the SPH value. For example, a prescription of -3.00 SPH / -2.00 CYL has an effective power of approximately -4.00 diopters — placing it in the 1.67 index range rather than the 1.61 range that the SPH alone would indicate.

This is why opticians and online retailers ask for your full prescription — SPH, CYL, and AXIS — rather than just the sphere value. For a full explanation of each prescription field, see our guides on SPH, CYL and AXIS explained and ADD, PD and Segment Height explained.

What Lens Index Do I Need for Progressive Lenses

Progressive lenses introduce an additional consideration when determining what lens index you need. The ADD power — the additional near correction built into the lower zone of a progressive lens — increases the total power range of the lens and affects thickness at both the top and bottom of the lens blank.

For progressive lenses, the recommended index thresholds shift slightly upward compared to single vision lenses. A prescription that would comfortably use 1.61 index in a single vision lens may benefit from 1.67 index in a progressive, particularly if the ADD power is +2.00 or above.

The general progressive lens index recommendations are: 1.56 for low distance prescriptions with ADD up to +1.50; 1.61 for moderate distance prescriptions with ADD up to +2.00; 1.67 for distance prescriptions above ±3.00 or ADD above +2.00; 1.74 for distance prescriptions above ±6.00 regardless of ADD.

For a detailed look at how progressive lenses work and what affects their fit and comfort, see our guide on progressive lenses over 40: what they actually feel like.

What Lens Index Do I Need for High Prescriptions

For prescriptions above ±6.00 SPH, the question of what lens index you need becomes critical rather than cosmetic. At these powers, a standard or mid-index lens produces edge thickness (for minus prescriptions) or centre thickness (for plus prescriptions) that is physically heavy, optically distorting at the periphery, and cosmetically problematic in most frame styles.

At -8.00 SPH in a standard 54mm frame, a 1.56 index lens has an edge thickness of approximately 9–11mm. The same prescription in 1.74 index reduces that edge thickness to approximately 5–6mm — a reduction of nearly 50% that transforms the wearability of the glasses entirely.

For prescriptions above ±8.00, frame selection also becomes important. Smaller frames reduce the lens diameter required, which directly reduces edge thickness regardless of index. Rimless and semi-rimless frames are generally not recommended above ±6.00 because the lens edge is exposed and the thickness is more visible. For a full guide to frame and lens selection at high powers, see our expert guide to glasses for high prescriptions and our article on rimless glasses and high prescriptions.

What Lens Index Do I Need: Frame Size and Shape Matter Too

The frame you choose directly affects how thick your lenses will be — regardless of index. This is because lens thickness is calculated at the widest point of the lens blank required to fill the frame. A larger frame requires a larger lens blank, which means more material at the edges for minus prescriptions and more material at the centre for plus prescriptions.

As a practical rule: reducing frame width by 5mm reduces edge thickness by approximately 0.5–1.0mm at moderate prescriptions, and by 1.0–2.0mm at high prescriptions. This means that choosing a smaller, rounder frame can achieve a similar thinning effect to upgrading one index level — and the two effects compound when combined.

For guidance on lens thickness and how to minimise it through frame selection, see our guide on navigating lens thickness: tips for comfort and style. For single vision lens options across all index levels, see our complete single vision lens guide.

Affordable Lenses at Every Index Level at FuzWeb

At FuzWeb, prescription lenses are available across the full index range — 1.56, 1.61, 1.67, and 1.74 — for single vision, bifocal, and progressive prescriptions. Every lens at every index includes UV400, HMC (Hard Multi-Coat), anti-reflective, hydrophobic, and oleophobic coatings on both surfaces as standard. These are not paid upgrades — they are included in the base price at every index level.

Four frame collections offer excellent compatibility across all prescription strengths and index options:

• Bclear frames — a wide range of full-rim and rimless styles, with multiple frame widths to suit different prescription strengths and index requirements.
• Hotochki frames — lightweight designs with precise geometry, well-suited to higher index lenses where frame weight balance matters.
• Gmei Optical frames — a broad selection of shapes and sizes, including smaller frames that help minimise edge thickness at higher prescriptions.
• Zirosat frames — full-rim and rimless options with strong structural integrity suited to 1.67 and 1.74 index lenses.

If you are unsure which index is right for your specific prescription, email info@fuzweb.com with your full prescription details and the FuzWeb team will confirm the recommended index before you order. For the full ordering process, visit our lens ordering guide.

Frequently Asked Questions: What Lens Index Do I Need

What lens index do I need for a -3.00 prescription?

A prescription of -3.00 SPH with no significant CYL is comfortably served by 1.61 index. This index reduces lens edge thickness by approximately 20% compared to standard CR-39, producing a noticeably thinner lens without the cost premium of 1.67 or 1.74. If your CYL is -2.00 or above, consider 1.67 index instead.

What lens index do I need for a -5.00 prescription?

A prescription of -5.00 SPH falls in the 1.67 index range. At this power, 1.67 index reduces edge thickness by approximately 35% compared to CR-39 and produces a lens that is both lighter and cosmetically acceptable in most frame styles. 1.61 index is technically possible but will produce noticeably thicker edges, particularly in wider frames.

Is 1.74 index worth the extra cost?

For prescriptions above ±6.00 SPH, yes — unambiguously. The thickness reduction at these powers is substantial enough to affect comfort, weight, and optical performance at the lens periphery. For prescriptions below ±6.00, the additional cost of 1.74 index over 1.67 index delivers diminishing returns and is generally not necessary.

Does a higher lens index affect optical clarity?

Higher index materials have a slightly higher Abbe value trade-off — meaning they can introduce marginally more chromatic aberration (colour fringing) at the lens periphery compared to lower index materials. In practice, this is only noticeable at very high prescriptions and in large frames. For most wearers, the thinness and weight benefits of higher index far outweigh any marginal optical trade-off.

What lens index do I need for progressive lenses?

For progressive lenses, add approximately one index level to the recommendation for single vision lenses at the same distance prescription. A distance prescription of -3.00 that would use 1.61 in single vision typically benefits from 1.67 in a progressive, particularly with ADD powers of +2.00 or above. Always confirm with your retailer before ordering.

Can I use 1.56 index for a strong prescription?

Technically yes, but it is not recommended above ±3.00 SPH. At higher powers, 1.56 index produces lenses that are noticeably thick and heavy, which affects comfort, appearance, and in some cases optical performance at the lens periphery. The cost saving over 1.61 or 1.67 index is not worth the compromise in lens quality at moderate to strong prescriptions.

Does lens index affect the coatings on my glasses?

No. Lens coatings — UV400, anti-reflective, HMC, hydrophobic, oleophobic — are applied to the lens surface regardless of index. At FuzWeb, all five coatings are included as standard on every lens at every index level. For a full explanation of what each coating does, see our guide on understanding lens coatings for eyewear.

Your Prescription Tells You Exactly What Lens Index You Need

The answer to “what lens index do I need” is not a matter of preference or budget — it is a direct function of your prescription power. SPH up to ±2.00 → 1.56. SPH ±2.00 to ±4.00 → 1.61. SPH ±4.00 to ±6.00 → 1.67. SPH above ±6.00 → 1.74. CYL values above ±2.00 shift the recommendation one level higher. Progressive lenses shift the recommendation one level higher again.

Getting the index right before you order means thinner lenses, lighter glasses, better comfort, and better optics at the lens periphery. Getting it wrong means paying to remake lenses that are too thick to wear comfortably.

If your prescription is at a borderline between two index levels, always go higher — the marginal cost difference is small, and the optical and cosmetic benefit is real. For further reading, explore our guides on why are my glasses lenses so thick, affordable progressive lenses, and how to order prescription lenses online.