Decoding Prescription Terminology: ADD, PD, and Segment Height Explained

If you've ever looked at your eyeglass prescription and found yourself puzzled by values beyond SPH, CYL, and AXIS, you're not alone. For anyone who needs multifocal, bifocal, or progressive lenses, three additional values appear on the prescription: ADD, PD, and segment height. Understanding ADD, PD, and segment height explained in plain language is essential for ordering the right lenses — and for understanding why these numbers matter as much as the primary prescription values. Get any one of them wrong and the lenses won't work as intended, no matter how accurate the rest of the prescription is.

This guide covers each value with precision, explains how they interact, and shows how FuzWeb uses them to produce lenses that deliver seamless, comfortable vision at every distance.

What ADD, PD, and Segment Height Explained Means for Multifocal Lenses

Standard single-vision prescriptions correct vision at one distance — either far or near. But as people age, the eye's natural lens loses flexibility, making it harder to shift focus between distances. This condition, known as presbyopia, typically begins in the mid-40s and affects virtually everyone eventually.

Multifocal lenses — including bifocals and progressives — solve this by incorporating multiple focal zones within a single lens. To produce these lenses accurately, the optician needs three additional data points beyond SPH, CYL, and AXIS: the ADD value (how much near power to add), the PD (where to centre the optical zones relative to the pupils), and the segment height (where to position the near zone within the frame). All three must be correct for the lenses to function properly.

FuzWeb's prescription review process verifies all six values — SPH, CYL, AXIS, ADD, PD, and segment height — before any lens enters production. Learn more at ordering prescription lenses from FuzWeb in 6 easy steps.

ADD Explained: The Near Vision Power in ADD, PD, and Segment Height

ADD stands for Addition and represents the additional positive power added to the distance prescription to correct near vision. It is always a positive value, typically ranging from +0.75 to +3.50 diopters, and it is the same for both eyes in the vast majority of prescriptions.

The ADD value compensates for the loss of accommodation — the eye's ability to change focus — that occurs with presbyopia. A lower ADD value such as +1.00 indicates early-stage presbyopia, where only mild near correction is needed. A higher value such as +2.50 or +3.00 indicates more advanced presbyopia, where the eye has lost most of its natural focusing ability for close distances.

In a progressive lens, the ADD power is gradually introduced from the distance zone at the top of the lens through an intermediate corridor to the near zone at the bottom. In a bifocal lens, the transition is abrupt — a visible line separates the distance and near zones. The ADD value determines the optical power of that near zone in both cases.

It is important to note that ADD does not replace the distance prescription — it is added to it. A person with a distance SPH of -2.00 and an ADD of +2.00 will have a near zone power of Plano (0.00), meaning no net power for near vision correction. The ADD simply shifts the focal point to accommodate reading distance.

For a full explanation of how the distance prescription values work, see FuzWeb's guide to SPH, CYL, and AXIS explained.

PD Explained: Pupillary Distance in ADD, PD, and Segment Height

PD stands for Pupillary Distance — the measurement, in millimetres, of the distance between the centres of the two pupils. This measurement tells the lens manufacturer exactly where to position the optical centre of each lens so that it aligns precisely with the wearer's line of sight.

PD can be expressed in two ways. A single PD (such as 64mm) represents the total distance across both eyes and is measured from pupil centre to pupil centre. A dual PD (such as 32/32 or 33/31) gives a separate measurement for each eye — right eye first, left eye second — measured from the centre of the nose bridge to each pupil. Dual PD is more precise and is preferred for progressive lenses, where optical centration is critical.

An incorrect PD measurement is one of the most common causes of discomfort with new glasses. Even a 1–2mm error can cause the optical centres to misalign with the pupils, producing prismatic effects — a subtle but persistent sense of visual displacement that causes eye strain, headaches, and difficulty focusing. For progressive lenses, where the near corridor is narrow and precisely positioned, PD accuracy is especially important.

FuzWeb makes PD measurement straightforward with the FittingBox PD Finder — a digital tool that uses your smartphone's front-facing camera and a plastic loyalty card (with the black strip) to measure your pupillary distance accurately in seconds, without a ruler or a second person. It works best on a smartphone with a selfie camera and takes the guesswork out of one of the most critical steps in ordering prescription glasses online. For full instructions on how to use it, see FuzWeb's guide on how to measure your pupillary distance at home.

Prefer a manual method? The same guide also covers a step-by-step ruler and mirror technique that works well for single-vision lenses and requires no additional tools or apps.

For a broader overview of how to read your full prescription document, FuzWeb's article on demystifying your eyeglass prescription is the recommended starting point.

Segment Height Explained: Lens Positioning in ADD, PD, and Segment Height

Segment height is the vertical measurement, in millimetres, from the bottom edge of the lens to the point where the near vision zone begins. It tells the lab exactly how high or low to position the near segment within the finished lens — and it is specific to the frame chosen, not to the prescription itself.

This is why segment height cannot be determined until a frame has been selected. The measurement depends on the frame's lens height, the wearer's pupil position within the frame, and their natural reading posture. A frame with a tall lens will have a different segment height than a compact frame, even for the same wearer with the same prescription.

For progressive lenses, the minimum segment height is typically 17–18mm, though most labs recommend at least 20mm to allow the full progressive corridor to develop properly. Frames with very short lens heights may not accommodate a full progressive design — a consideration worth checking before purchasing a frame if progressive lenses are required.

For bifocal lenses, the segment height determines where the visible line sits. Too high and the near segment intrudes into the distance zone; too low and the wearer must tilt their head uncomfortably to access near vision. Correct segment height placement is what makes bifocal lenses feel natural rather than awkward.

FuzWeb's optical team calculates segment height based on the frame dimensions and the wearer's facial measurements provided at the time of ordering. If you're unsure how to measure segment height for your chosen frame, the glasses frame measurement guide covers the process in detail.

How Lens Quality Supports ADD, PD, and Segment Height Precision

Accurate ADD, PD, and segment height values are only as useful as the lens quality that executes them. A poorly manufactured progressive lens with optical distortions in the corridor will undermine even a perfectly measured prescription.

Every prescription lens ordered through FuzWeb includes UV400 protection, HMC (Hard Multi-Coat), and AR (Anti-Reflection) coating as standard — not as paid upgrades. UV400 blocks 100% of UVA and UVB radiation. HMC provides scratch resistance that extends lens life significantly. AR eliminates surface reflections that cause glare and visual fatigue, which is particularly important for progressive wearers who are already adapting to a new optical design.

Optional upgrades include Photochromic lenses that darken outdoors, Anti Blue Light lenses for screen-heavy use, Tinted lenses, and Polarized lenses for driving and outdoor activities.

For higher ADD values combined with significant distance prescriptions, lens thickness can become a concern. FuzWeb's Bobbie MR lens series offers high-index options — MR-8 (1.60 index), MR-7 (1.67 index), and MR-10 (1.74 index) — that produce thinner, lighter progressive lenses without compromising optical clarity. Explore the range at Bobbie lenses at FuzWeb.

For a full breakdown of available lens coatings and what each one does, read FuzWeb's guide to lens coatings for eyewear.

How to Read ADD, PD, and Segment Height on Your Prescription

A typical multifocal prescription might look like this:

OD (Right Eye): SPH -1.50 / CYL -0.50 / AXIS 90 / ADD +2.00
OS (Left Eye): SPH -1.25 / CYL -0.75 / AXIS 85 / ADD +2.00
PD: 32.5 / 31.5
Segment Height: 20mm (frame dependent)

Reading this: both eyes have mild myopia with mild astigmatism, and both require +2.00 ADD for near vision — typical of moderate presbyopia. The dual PD of 32.5/31.5 indicates a slight asymmetry between the two eyes, which is entirely normal. The segment height of 20mm means the near zone begins 20mm from the bottom of the lens.

When ordering online, enter the ADD value exactly as written. Do not round it. Do not assume it is the same as a previous prescription without checking — ADD values typically increase over time as presbyopia progresses. Use the dual PD values if available, as they produce more accurate centration than a single combined PD for progressive lenses.

If your prescription does not include segment height, each brand at FuzWeb has its own team of optical experts who will calculate it using a factory-suggested formula based on the frame's lens height — so the correct value is always determined for the specific frame chosen. You can also find guidance in FuzWeb's article on choosing the right eyeglass lenses.

Ready to order your progressive or bifocal lenses? Browse the full frame collection at FuzWeb frames and upload your prescription at checkout.

Frequently Asked Questions: ADD, PD, and Segment Height Explained

What does ADD mean on an eyeglass prescription?

ADD stands for Addition and represents the extra positive power added to the distance prescription to correct near vision. It is used for progressive, bifocal, and reading lenses, and is always a positive value, typically between +0.75 and +3.50 diopters.

What does PD mean on an eyeglass prescription?

PD stands for Pupillary Distance — the measurement in millimetres between the centres of the two pupils. It tells the lab where to position the optical centre of each lens so it aligns with the wearer's line of sight. An incorrect PD can cause eye strain, headaches, and distorted vision.

What is segment height and why does it matter?

Segment height is the vertical distance from the bottom of the lens to the start of the near vision zone. It determines where the reading segment sits within the frame and must be measured for the specific frame chosen. An incorrect segment height makes progressive or bifocal lenses uncomfortable and difficult to use.

Is ADD the same for both eyes?

In the vast majority of cases, yes — the ADD value is identical for both eyes. This is because presbyopia affects both eyes equally. However, some prescriptions may list slightly different ADD values for each eye, which is less common but not unusual.

Can I measure my own PD at home?

Yes — two methods are available. FuzWeb's FittingBox PD Finder uses your smartphone's front-facing camera to measure your PD digitally in seconds. Alternatively, the manual ruler and mirror method works well for single-vision lenses. Both are covered in FuzWeb's guide on how to measure your pupillary distance at home.

What happens if segment height is wrong?

If segment height is set too high, the near zone intrudes into the distance viewing area, causing blurred distance vision. If set too low, the wearer must tilt their head down uncomfortably to access near vision. Either error makes the lenses difficult to adapt to and may require remake.

Do I need ADD, PD, and segment height for single-vision lenses?

No. Single-vision lenses only require SPH, CYL, AXIS, and PD. ADD and segment height are only needed for progressive, bifocal, or reading lenses. If your prescription does not include an ADD value, you do not need multifocal lenses.