What are the numbers on the inside of eyeglass frames?

The numbers typically represent: eye size (A measurement) in mm, followed by the bridge size (DBL) in mm, followed by the temple length in mm. For example, 54 □ 17 - 145 means a 54 mm eye size, 17 mm bridge, and 145 mm temple length. These measurements follow the boxing system standard.

How do you calculate frame PD?

Frame PD = A measurement (eye size) + DBL (bridge size). For example, a frame with a 52 mm eye size and 18 mm bridge has a Frame PD of 70 mm. This value is compared to the patient's PD to determine how much each lens must be decentered.

What is the effective diameter?

The effective diameter (ED) is twice the longest distance from the geometric center of the lens opening to any point on the edge. It represents the maximum dimension a lens must span to fill the frame completely. ED is the critical measurement for calculating minimum blank size and may be larger than the A measurement for non-round frame shapes.

Boxing System Measurements: Standard Frame Dimensions for Lens Ordering

Q: What is the difference between the boxing system and datum system?

The boxing system draws a rectangle around the lens opening and measures the rectangle's width (A) and height (B). The datum system uses the horizontal midline of the lens opening as a reference. The boxing system's A measurement is typically larger than the datum measurement for non-rectangular frames. The boxing system is the current industry standard used on the ABO exam.

What Is the Boxing System?

The boxing system is the standardized method for measuring eyeglass frame dimensions used throughout the optical industry. It works by drawing an imaginary rectangle ("box") around each lens opening at the tightest fit, then measuring the dimensions of that box. The boxing system ensures consistent communication between opticians, labs, and manufacturers.

Core Measurements

A Measurement (Eye Size)

The A measurement is the horizontal width of the boxing rectangle, representing the widest horizontal dimension of the lens opening in millimeters. Also called the eye size, it is the first number in the standard frame marking (e.g., "52" in 52-18-140).

B Measurement (Vertical Depth)

The B measurement is the vertical height of the boxing rectangle, representing the deepest vertical dimension of the lens opening. This measurement is critical for progressive lens fitting, as the B dimension must accommodate the full progressive corridor.

DBL (Distance Between Lenses)

The DBL (also called bridge size) is the shortest horizontal distance between the two lens openings, measured at the bridge. It is the second number in the frame marking (e.g., "18" in 52-18-140).

Effective Diameter (ED)

The ED is twice the longest distance from the geometric center of the lens opening to any point on the edge. For round openings, ED equals the A measurement. For non-round openings, ED may be larger than A because the farthest edge point may not be on the horizontal axis.

Key Calculations

Calculation	Formula	Example
Frame PD	A + DBL	52 + 18 = 70 mm
Geometric Center (GC)	A/2 horizontal, B/2 vertical	26 mm from each side, B/2 from top and bottom
Total Decentration	Frame PD - Patient PD	70 - 64 = 6 mm total
Per-eye Decentration	(Frame PD - Patient PD) / 2	6 / 2 = 3 mm per eye
Minimum Blank Size	ED + (2 x decentration per eye) + 2	56 + 6 + 2 = 64 mm

Frame PD = A + DBL is one of the most frequently tested calculations on the ABO exam. It connects the frame dimensions to the patient's PD and determines the required decentration. Memorize this formula and practice calculating it quickly.

Standard Frame Marking

Frames are typically marked on the inside of the temple with three numbers separated by a box symbol (□) or dash: Eye Size □ DBL - Temple Length.

For example: 54 □ 17 - 145

54 = A measurement (eye size) in mm
17 = DBL (bridge size) in mm
145 = Temple length in mm

The B measurement and ED are not typically marked on the frame but can be measured with a frame gauge.

Geometric Center vs. Optical Center

The geometric center (GC) is the center point of the boxing rectangle. It is a fixed property of the frame shape. The optical center (OC) is where the lens power is centered, determined by the patient's PD and fitting requirements. These are different points unless the patient's monocular PD exactly matches A/2.

The distance between the GC and OC is the decentration.

When helping a patient select frames, mentally calculate the Frame PD (A + DBL) and compare it to the patient's PD. A Frame PD close to the Patient PD means minimal decentration, thinner lenses, and smaller blank requirements. This is especially important for high-power prescriptions where excessive decentration produces thick, heavy lenses.

Boxing System vs. Datum System

The datum system is an older measurement method that uses the horizontal midline of the lens opening as a reference. The boxing system's A measurement is typically larger than the datum measurement for non-rectangular frames. The boxing system is the standard in the United States and is used on the ABO exam.

Using the datum system values when the lab expects boxing system measurements. Almost all modern labs, edgers, and frame manufacturers use the boxing system. Unless specifically requested, always provide boxing system dimensions.

Clinical Relevance

The boxing system is the language of frame measurement. Every lens order, frame selection decision, and fitting calculation depends on these standardized dimensions. Fluency with the boxing system is a core competency for every dispensing optician and a significant portion of the ABO exam.

Key Takeaways

The boxing system draws an imaginary rectangle around the lens opening to define measurements
A = horizontal width (eye size), B = vertical height, DBL = bridge width
ED = longest dimension from geometric center to edge, doubled
Frame PD = A + DBL (critical for decentration calculations)
Decentration per eye = (Frame PD - Patient PD) / 2
The boxing system is the industry standard; the datum system is older and rarely used