Reviewed September 30, 2026 by the editorial team. Educational content; follow your facility's protocols.
Normal pupil size
Pupil size is set by two muscles in the iris. The sphincter, driven by the parasympathetic fibers of the oculomotor nerve (cranial nerve III), makes the pupil smaller. The dilator, driven by sympathetic fibers, makes it larger. Light, focusing on a near object, emotion, age and many drugs shift the balance.
Because light changes the size so much, always record the lighting when you document a measurement. Pupils also tend to be larger in children and young adults and smaller in older adults.
| Condition | Typical normal diameter |
|---|---|
| Bright light | 2-4 mm |
| Dark or dim room | 4-8 mm |
| Difference between the eyes | Usually 0.4 mm or less; seldom more than 0.8 mm in physiologic anisocoria |
Pupil size chart from 1 to 9 mm
Most neuro assessment sheets print a gauge of black circles from 1 to 9 mm. Hold the gauge next to the eye, at the same distance, and pick the circle that matches best. The descriptions below are general guides for a patient in ordinary room light; interpret them with the rest of your assessment.
| Size | Usual description | What it may suggest in room light |
|---|---|---|
| 1 mm | Pinpoint | Opioid effect, miotic eye drops, or a pontine (brainstem) lesion |
| 2 mm | Small | Normal in bright light; small in a dim room |
| 3 mm | Normal | Typical adult size in room light |
| 4 mm | Normal | Typical in room light; upper end of normal in bright light |
| 5 mm | Normal to slightly large | Common in dim light, children and young adults |
| 6 mm | Large | Normal in the dark; large in bright light |
| 7 mm | Dilated | Possible sympathetic stimulation, anticholinergic or stimulant effect, or dilating drops |
| 8 mm | Dilated | Near the upper limit of normal in the dark; check reactivity |
| 9 mm | Widely dilated | Mydriatic drops, severe drug effect, or a third nerve problem; report if new |
A size alone is rarely the concern. What matters is a change from baseline, a difference between the two eyes, and how briskly each pupil reacts.
How to check pupils and document PERRLA
PERRLA stands for Pupils Equal, Round, Reactive to Light and Accommodation. Record it only when you have actually checked every part. If a part is abnormal, describe the finding instead.
- Dim the room and ask the patient to look at a distant point, so focusing does not shrink the pupils.
- Note the resting size of each pupil in mm with a pupil gauge, and check that both are round.
- Bring a penlight in from the side and hold it on one eye for a few seconds. Watch that pupil constrict (direct response) and the other pupil constrict at the same time (consensual response). Repeat on the other eye.
- Grade the speed: brisk, sluggish, or nonreactive (fixed).
- Test accommodation: ask the patient to look at a distant object, then at your finger or thumb held about 15-20 cm in front of the face. The pupils should constrict and the eyes converge.
- Document size, shape and reaction for each eye, for example: "R 3 mm brisk, L 3 mm brisk, round, accommodation intact".
Ask about eye drops, eye surgery, artificial lens, glass eye and recent drug use before you interpret an abnormal pupil.
Anisocoria: when pupils are unequal
About 10-20% of healthy people have physiologic anisocoria, a small difference that stays about the same in light and dark and reacts normally. To find out which pupil is the abnormal one, compare the difference in bright light and in the dark.
| Finding | Abnormal pupil | Possible causes |
|---|---|---|
| Difference greater in bright light | The larger pupil (it does not constrict well) | Third nerve palsy, Adie tonic pupil, dilating drops, iris trauma |
| Difference greater in the dark | The smaller pupil (it does not dilate well) | Horner syndrome, miotic drops, inflammation |
| Difference the same in light and dark | Neither | Physiologic anisocoria |
New anisocoria with headache, drooping eyelid, double vision, neck pain or reduced consciousness needs emergency evaluation. A dilated pupil with ptosis can signal an aneurysm compressing the third nerve; a small pupil with ptosis after neck pain can signal carotid dissection.
What pupil changes mean during neuro checks
In patients with head injury, stroke, brain surgery or raised intracranial pressure, pupils are part of every neuro check along with the Glasgow Coma Scale and limb strength. The oculomotor nerve runs along the edge of the tentorium, so a mass pushing the brain downward can compress it.
- One pupil becoming larger and more sluggish than before: early warning of uncal herniation on the same side; notify the provider at once
- A fixed, dilated pupil on one side with falling level of consciousness: a neurologic emergency
- Both pupils fixed and dilated: severe brain injury, profound hypoxia or drug effect; interpret with the full clinical picture
- Pinpoint pupils in an unresponsive patient: consider opioid effect or a pontine lesion
- An oval pupil or a new irregular shape: can precede a third nerve palsy in raised pressure; report it
Compare every finding with the previous check. A change, even of 1 mm, matters more than any single measurement.
Scales that go with this guide
Frequently asked questions
What is a normal pupil size in mm?
About 2-4 mm in bright light and 4-8 mm in the dark. In ordinary room light most adults measure around 3-4 mm.
What does PERRLA mean?
Pupils Equal, Round, Reactive to Light and Accommodation. Document it only after you have checked size, shape, the light reflex in both eyes and the near response.
Is it normal for pupils to be different sizes?
A small difference, usually 0.4 mm and seldom above 0.8 mm, is found in about 10-20% of healthy people. A new or larger difference, or one with other symptoms, needs evaluation.
What does a fixed and dilated pupil mean?
A new fixed, dilated pupil in a patient with brain injury can mean third nerve compression from herniation and is an emergency. Dilating eye drops can cause the same finding, so check the medication history.
What causes pinpoint pupils?
Common causes are opioids, miotic eye drops and lesions of the pons. In an unresponsive patient, pinpoint pupils call for immediate assessment of breathing.
More guides and tools
All scales:
Sources: The Pupils - Clinical Methods, 3rd ed. (NCBI Bookshelf) · Anisocoria - StatPearls, NCBI Bookshelf · Brain Herniation - StatPearls, NCBI Bookshelf. Text written by our editorial team. Reviewed September 30, 2026.