# Head Circumference Percentile Calculator

URL: https://speedycalculator.com/calculators/health/head-percentile/
Last updated: 2026-10-30

> The head circumference percentile calculator compares a child's head size with the WHO Child Growth Standards for age and sex (0–5 years) and flags microcephaly or macrocephaly.

## Inputs

- Sex
- Age (months, years, weeks)
- Head circumference (cm, in, m, ft, mm)

## Results

- Head circumference z-score
- Head circumference percentile
- Interpretation
- Median head circumference

## 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
```

## Explanation

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

FieldWhat to enterSexChoose one: Male, FemaleAgeUnits: months, years, weeksHead circumferenceUnits: 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 (%)3994.5846497.52299122
*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.*

## 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.

## FAQ

**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.


## Sources

- [WHO – Child growth standards](https://www.who.int/tools/child-growth-standards/standards)