Baby Head Percentile Calculator

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Enter sex, age, and head circumference to estimate a percentile and z-score using WHO growth standards. Use results to track growth over time, not as a diagnosis.

Head Circumference Percentile

WHO Growth Standards (0–5 Years)

Measure the widest part, just above eyebrows and ears.
Peer-Reviewed Science
Updated Dec 2025
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Methodology

The calculator compares head circumference with the WHO Child Growth Standards for children of the same age and sex. It uses the daily reference values from birth through 60 completed months. Ages are converted using 7 days per week, 30.4375 days per month and 365.25 days per year; values between whole days are interpolated.

The z-score describes how far the measurement is from the reference median. It is converted to a percentile using the normal distribution. A percentile describes the measurement's position in the reference group, not the chance of a health condition. If your child was born early, enter the corrected age recommended by their clinician.

Sources: WHO head circumference-for-age standards, including the daily tables for boys and girls; WHO age-conversion instructions; and the CDC explanation of LMS z-scores and percentiles.

How to use
  1. Select sex: choose girl or boy.
  2. Enter age: choose weeks, months, or years, then type the age (0 to 5 years).
  3. Enter head circumference: choose inches or centimetres, then type the measurement. Switching units converts a value you have already entered.
  4. Press Calculate: read the percentile band, estimated percentile, and z-score.
  5. Track over time: single readings can be noisy. Re-check with repeated measurements.
What head circumference percentiles measure

Head circumference is the distance around the widest part of the head. It is also called occipito-frontal circumference.

A percentile compares a child's measurement to a reference population of the same age and sex. For example, the 50th percentile means the measurement is close to the middle of the reference distribution.

Clinicians look at the pattern over time. A steady growth pattern can matter more than a single percentile, especially if age or tape placement was slightly off at one visit.

Frequently asked questions
What does head circumference percentile mean?
It is the position of the measurement compared with a reference distribution for the same age and sex. Lower percentiles mean a smaller head circumference relative to the reference, higher percentiles mean a larger head circumference.
What percentile is considered low or high?
Clinicians often pay attention when head circumference is below about the 3rd percentile or above about the 97th percentile, especially if the child is also crossing percentile lines over time. A single measurement is not a diagnosis.
How do I measure head circumference at home?
Use a non-stretch tape. Wrap it around the widest circumference: above the eyebrows, above the ears, and around the most prominent part of the back of the head. Measure three times and record the largest value to the nearest 0.1 cm.
Should I use WHO or CDC growth charts?
This calculator uses WHO head circumference-for-age standards from birth to 5 years. Some services switch charts by age or use local protocols. If you want other tools, browse the full list at All calculators.
My baby's head circumference is below the 3rd percentile. What happens next?
Most clinicians re-check the measurement technique and confirm age. They also look at weight and length, family head sizes, and the pattern across visits. If there are concerns, they may arrange follow-up measurements or further assessment.
My baby was born early. Should I use corrected age?
For babies born preterm, growth is often plotted using corrected (adjusted) age for a period of time. Many clinical guides use corrected age until around 2 years for growth charting, but practices vary by service.