How to Measure Eyewear Correctly

Eyewear Measurements Explained

Precise eyewear measurements ensure comfort, optimal vision, and lens accuracy. Learn about frame box measurements, PD (pupillary distance), segment heights, and ocular center heights for proper lens alignment.

1. Frame Box Measurements

These numbers are usually printed on the inside of your glasses and follow the format: Eye Size – Bridge – Temple Length. Example: 52▭18 140

Term Description
Eye Size (A) Horizontal width of one lens (in mm).
Bridge Size (DBL) Distance between lenses over the nose (in mm).
Temple Length Length of the arm from hinge to tip (in mm).
B Size (Lens Height) Vertical height of the lens (used for progressive eligibility).
ED (Effective Diameter) Largest diagonal lens measurement + 2mm (used to assess lens blank size).

2. Pupillary Distance (PD)

PD is the distance between the centers of the pupils, typically in millimeters. It's essential for aligning lenses properly with the patient’s visual axis.

  • Binocular PD: Single measurement between pupils (e.g., 62 mm).
  • Monocular PD: Distance from bridge to each pupil (e.g., 31/31 mm).
  • Near PD: Slightly smaller than distance PD due to convergence when reading.
Tip: Always use a PD ruler or digital tool for precise measurement. Incorrect PD can cause eyestrain or headaches.

3. Segment Heights

Segment height refers to where the multifocal or progressive lens segment begins, measured from the bottom of the lens to the fitting cross or bifocal line.

Lens Type Standard Measurement Where It's Measured
Bifocal Typically 17–20 mm From bottom of lens to lower eyelid
Trifocal Typically 22–25 mm From bottom of lens to pupil
Progressive (PAL) Minimum of 18 mm (varies by design) From bottom of lens to pupil center (fitting cross)

4. Ocular Center Height

This is the vertical distance from the bottom of the lens to the optical center. It must align with the patient’s pupil height to ensure clear vision and minimize unwanted prism.

  • Used for all lens types, especially high-Rx and PALs.
  • Always measured with the patient wearing the frame.
  • Important for adjusting pantoscopic tilt and wrap angles.
Remember: Proper ocular center alignment helps avoid image displacement, distortion, or unnecessary prism effect.

Optical Diagrams

Quick Quiz: Frame & Lens Measurements

1. What does the “A” measurement in eyewear refer to?

Temple length
Lens width ✅
Bridge width
Segment height

2. What is the typical range for adult pupillary distance (PD)?

45–50 mm
55–65 mm ✅
65–75 mm
70–85 mm

3. The bridge size (DBL) measures the:

Width of the frame
Vertical lens height
Distance between lenses over the nose ✅
Lens curvature

4. What is segment height used to measure?

Lens width
Bridge spacing
Temple fit
Placement of multifocal lens zones ✅

5. A bifocal segment height is typically measured from:

Center of the lens
Lower eyelid margin ✅
Bridge center
Brow ridge

6. Which lenses most often require accurate ocular center height measurements?

Single vision reading
Plano sunglasses
Progressive lenses ✅
Contact lenses

7. What tool is commonly used to take accurate PD measurements?

Autorefractor
Retinoscope
PD ruler ✅
Slit lamp