**What it is**
The PI ratio is the diameter of the pupil expressed as a percentage of the total iris diameter. It is the primary measurement of pupil size relative to the iris.
**Formula**
“`
PI Ratio = (Pupil diameter ÷ Iris diameter) × 100
“`
**Normal range**
A PI ratio of **20–30%** is considered physiologically normal for adults in standard indoor lighting. The app displays an interpreted label alongside the number:
| PI Ratio | Label |
|———-|——-|
| < 15% | Miosis (very constricted) |
| 15–19% | Constricted |
| 20–30% | **Normal** |
| 31–40% | Dilated |
| > 40% | Mydriasis (very dilated) |
**Age-normalised comparison**
Because pupil size decreases with age, PupilMetrics compares the measured diameter against an age-specific reference range. The patient’s date of birth must be entered for this comparison to be shown.
| Age Group | Expected Diameter | Normal Range |
|———–|——————|————-|
| Infant (< 1 yr) | 2.2 mm | 2.0 – 2.5 mm |
| Child 1–5 | 4.0 mm | 3.5 – 4.5 mm |
| Child 6–11 | 4.3 mm | 3.8 – 4.8 mm |
| Teen | 4.2 mm | 3.5 – 5.0 mm |
| Adult 20–39 | 3.5 mm | 3.0 – 4.2 mm |
| Adult 40–59 | 3.0 mm | 2.5 – 3.5 mm |
| Senior 60+ | 2.7 mm | 2.3 – 3.2 mm |
The estimated diameter is derived from the PI ratio multiplied by an assumed average iris diameter of 12 mm.
