Ordering glasses online involves two measurements that most people have never paid attention to in a physical practice, because the optician handled them without explanation. Pupillary distance and frame width. Get both right and the glasses perform as the prescription intends. Get either wrong and the result ranges from mild discomfort to genuinely poor optical performance, and the problem is rarely obvious enough to identify without knowing what to look for.
This guide covers both, in plain language, so that the next online order goes smoothly.
What Pupillary Distance Actually Is
Pupillary distance, almost always abbreviated to PD, is the measurement in millimetres between the centres of the two pupils. It matters because the optical centre of each lens in a pair of prescription glasses needs to align precisely with the corresponding pupil for the prescription to work correctly.
A lens is ground with its optical centre at a specific point. When that point sits directly in front of the pupil, the wearer looks through the most accurate part of the lens. When it does not, the eye is looking through a part of the lens that introduces unwanted prism, a slight deflection of the light path that the visual system compensates for through sustained muscular effort. That effort is not dramatic, but accumulated across a full day of wear it contributes to eye strain and fatigue that most wearers attribute to other causes.
For mild prescriptions, a PD error of a millimetre or two is unlikely to cause noticeable problems. For stronger prescriptions, particularly those with significant cylinder correction for astigmatism, accurate PD becomes more critical. For varifocal lenses, where the intermediate and near zones are positioned relative to the pupil centre, PD accuracy is essential to the lens performing as designed.
Single PD Versus Dual PD
A PD measurement comes in two formats. A single PD is one number representing the total distance between both pupils, typically somewhere between 58 and 72 millimetres for most adults. A dual PD gives two separate measurements, one for each eye from the nose bridge to the pupil centre, written as something like 32/30.
Dual PD is the more precise measurement and is worth obtaining where possible, because most people’s pupils are not perfectly symmetrical relative to the nose bridge. The difference between the two sides is often small, but for stronger prescriptions and varifocal lenses, having the accurate individual measurements produces a better result than using half the single PD for each eye.
How to Obtain a PD Measurement
The simplest way is to ask the optician who conducted the eye test. PD is measured routinely during a dispensing appointment and can be requested at the time of the examination. It is not always included on the standard prescription printout, which sometimes requires a specific request.
For buyers who already have an up-to-date glasses prescription but no PD measurement, some online opticians provide a smartphone-based measurement tool that uses the camera to estimate PD from a photograph taken at a standard distance. These tools work adequately for most standard single vision prescriptions. For higher prescriptions, varifocals, or anyone who wants maximum accuracy, a professionally measured PD is the better option.
Measuring manually with a ruler and a mirror is possible and gives a reasonable approximation. Hold a millimetre ruler against the bridge of the nose, look straight ahead in a mirror, and measure from the centre of one pupil to the centre of the other. The limitation is that it requires one eye to be closed for part of the measurement, which introduces the kind of small errors that accumulate more in complex prescriptions.
Frame Width: Why It Matters as Much as PD
Frame width is the measurement that determines whether a pair of glasses fits the face, and it is the measurement most people skip when ordering glasses online because it requires knowing their own face width, which most people have never measured.
The frame width is the total horizontal measurement of the frame from one outer temple edge to the other. It should roughly correspond to the width of the face at the temples. A frame too narrow for the face will press against the temples, sit too close to the face, and cause discomfort across extended wear. A frame too wide will slide down the nose, sit too far from the face, and reduce the accuracy of the prescription alignment.
Frame measurements are almost always printed on the inside of the temple arm, in a sequence of three numbers. A frame labelled 52-18-145 has a lens width of 52 millimetres, a bridge width of 18 millimetres, and a temple length of 145 millimetres. The total frame width is calculated as lens width multiplied by two, plus the bridge width, which in this example gives 52 + 52 + 18 = 122 millimetres.
Measuring face width at the widest point, usually at the temples, with a flexible tape measure or a piece of string gives the number to compare against. A frame width within about 5 millimetres of the face width in either direction typically fits well. Further outside that range and the fit will be noticeable.
Bridge Width and Why It Gets Overlooked
The bridge width, the middle number in the temple arm sequence, determines how the frame sits on the nose. A bridge too narrow for the nose will cause the frame to ride high and may create pressure marks. A bridge too wide will let the frame slide down constantly.
This is the measurement that most online buyers discover matters more than expected after a purchase that looked right from the front but sits incorrectly on the nose. Knowing the bridge width of a pair of glasses that fits well and using it as a reference when ordering is one of the simpler and more reliable ways to avoid the problem.
Putting It Together for an Online Order
For anyone ordering prescription glasses online for the first time, the process of getting the fit right comes down to a short sequence. Obtain the PD measurement before ordering, ideally from the optician or through a reliable measurement tool. Measure face width at the temples. Find the measurement label on the temple arm of a pair that currently fits well and use those numbers as a reference for the new order.
Most reputable online opticians provide full frame measurements in their product listings. Cross-referencing those against known measurements takes a few minutes and significantly reduces the risk of a poor fit that only becomes apparent once the glasses arrive.
Glasses are worn every day, all day. The measurements that determine whether they sit correctly on the face are worth the brief effort of establishing accurately before the order is placed rather than through a process of trial and return that takes considerably longer.
