Head circumference reflects brain growth. It is measured at every check-up in the first two years. A head that is very small or very large, or crosses percentile lines quickly, may need investigation.
How to use the Head Circumference Percentile Calculator
- Sex
- Age
- Head circumference
Results update as you type, and every field accepts the units shown in its menu.
How it works
Measure around the widest part of the head, above the eyebrows and ears, with a non-stretch tape, and enter age and sex.
Tips
- Measure lying length for babies under 2 and standing height from age 2.
- Use the exact age in months for babies – a few weeks matters.
- Follow the trend across several visits; crossing two major percentile lines is more important than one value.
- Discuss concerns with your paediatrician.
Common mistakes to avoid
- Comparing a baby with the wrong sex chart.
- Reading the 50th percentile as “normal” and everything else as abnormal.
- Using corrected age incorrectly for premature babies.
Inputs explained
| Field | What to enter |
|---|---|
| Sex | Choose one: Male, Female |
| Age | Units: months, years, weeks |
| Head circumference | Units: cm, in, m, ft, mm |
Example values
How the main result changes when Age changes and everything else stays at the example values above:
| Age (months) | Head circumference percentile (%) |
|---|---|
| 3 | 99 |
| 4.5 | 84 |
| 6 | 49 |
| 7.5 | 22 |
| 9 | 9 |
| 12 | 2 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
z = ((head circumference ÷ M)^L − 1) ÷ (L × S) with WHO LMS values
Microcephaly: below −2 SD (about the 2nd percentile) · macrocephaly: above +2 SD
Worked example
Inputs — Sex: Male; Age: 6 months; Head circumference: 43.3 cm.
Results — Head circumference z-score = -0.03; Head circumference percentile = 49 %; Interpretation = Normal range; Median head circumference = 43.3 cm.
Key terms
- Microcephaly
- Head size below −2 standard deviations for age and sex.
Frequently asked questions
What is a normal head circumference at birth?
About 33–37 cm.
Is a large head always a problem?
No – it often runs in families (benign familial macrocephaly).
How fast does the head grow?
About 2 cm a month in the first 3 months, then slower.