---
title: 3.2 PI Ratio (Pupil–Iris Ratio)
description: "****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…"
url: "https://cnri.edu/docs/3-2-pi-ratio-pupil-iris-ratio/"
updated: "2026-03-24"
category: Reading the Analysis Results
---

# 3.2 PI Ratio (Pupil–Iris Ratio)

****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.