How to Measure Your Segment Height (SH)

Segment height is the vertical distance from the bottom of your lens to your pupil. Getting it right ensures progressive, bifocal, or multifocal lenses are positioned exactly where you need them for clear, comfortable vision.

Progressives: 18–24 mm Bifocals: 10–14 mm Frame-specific

What is Segment Height?

Segment height (SH) is the distance from the bottom of your eyeglass lens to the center (for progressives) or lower eyelid (for bifocals). It ensures the reading and intermediate zones of your lenses are aligned with your natural line of sight.

Before You Start (Setup Checklist)

  • Wear the exact frame you’ll be ordering lenses for.
  • If you are ordering new glasses from us, use frames that are similar in lens height to the new pair you are going to order.
  • Stand in good, even lighting and look straight ahead.
  • Use a millimeter ruler or ask a friend to help.
  • Keep posture upright—don’t tilt your head.
  • Take multiple readings for consistency.

Why Segment Height Matters

Accurate SH ensures your reading and near zones are exactly where you need them. If SH is off, you may notice:

  • Blurry or distorted near vision
  • Neck strain from tilting your head
  • Difficulty switching between distance and near zones
  • General discomfort with multifocals

Measure Your Own SH (Mirror Method)

Tools: millimeter ruler + mirror. Repeat 3–4 times for accuracy.

  1. Put on your chosen frame and stand in front of a mirror.
  2. Hold a millimeter ruler vertically against the lens bottom.
  3. Look straight ahead. Note the mm mark aligned with your pupil center (progressives) or lower eyelid (bifocals).
  4. Record the measurement in millimeters.

Tip: Don’t tilt your head—stand naturally.

Have a Friend Measure (Most Accurate)

  1. Wear your frame and look at a target 10–20 ft (3–6 m) away.
  2. Your friend places the ruler at the lens bottom.
  3. They note the mm mark at pupil center (progressives) or lower eyelid (bifocals).
  • Keep posture natural and eyes forward.
  • Don’t follow the ruler—focus on the far target.
  • Take multiple readings for consistency.
Download Millimeter Ruler (PDF)

Progressive vs Bifocal Measurements

Progressives:

Measure to the center of the pupil.
Typical range: 18–24 mm

Bifocals:

Measure to the lower eyelid.
Typical range: 10–14 mm

Which Segment Height Do I Use?

  • Single-vision: No SH needed.
  • Progressives: Pupil center to lens bottom.
  • Bifocals: Lower eyelid to lens bottom.
  • Occupational lenses: May be adjusted higher—check with provider.

Common Mistakes (and Quick Fixes)

  • Using the wrong frame: Always measure with the exact frame ordered.
  • Looking at the ruler: Keep eyes forward, not down.
  • Head tilt: Maintain natural posture.
  • Only one reading: Take multiple for accuracy.

Where to Find Segment Height on a Prescription

Look for any of these notations:

  • SH, Seg, or Seg Ht
  • Separate values for OD/OS (right/left eye)

Quick Reference

  • Progressives: 18–24 mm
  • Bifocals: 10–14 mm
  • Measure with the actual frame on
  • Take multiple readings
Tools
  • Millimeter ruler
  • Mirror (self-measure) or a friend (more accurate)
Download Millimeter Ruler (PDF)

FAQs & Glossary

Is SH always on my prescription?
Not always. Many providers only include it if you order multifocals or progressives.
Do I need SH for single-vision glasses?
No—segment height only applies to bifocals, multifocals, and progressives.
Why do my eyes have different SH measurements?
That’s normal—faces aren’t perfectly symmetrical. Use the values measured for each eye.
Glossary: OD / OS
OD = right eye, OS = left eye.

Ready to Order?

Have your prescription and segment height handy for a precise order.