Head Circumference Percentile Calculator

The head circumference percentile calculator compares your baby's head size with the WHO standards for the same sex and age (birth to 24 months). Enter age and head circumference in centimeters or inches to get the percentile, z-score, median and the usual 3rd to 97th percentile range. It is a screening aid, not a diagnosis.

How to use the Head Circumference Percentile Calculator

1. Select your baby's sex.
2. Enter age in months and extra days (adjusted age if born early).
3. Enter the head circumference and choose centimeters or inches.
4. Press Calculate to see the percentile, z-score and the median for age.

Formula and method

The WHO publishes three numbers for each sex at each age: L (skewness), M (median) and S (coefficient of variation). The calculator interpolates them to your baby's exact age in months and days and computes z = ((measurement / M)^L − 1) / (L × S), or ln(measurement / M) / S when L is 0. The percentile is the normal distribution function of z times 100. A z-score of 0 is the 50th percentile, −2 is about the 2nd and +2 is about the 98th.

The WHO tables used here are the monthly LMS values from birth to 24 months. Between whole months the calculator interpolates linearly, so results can differ from the WHO daily tables by a small fraction of a percentile point.

Head circumference by age (WHO)

How fast does a baby's head grow?

The head grows fastest in the first months, reflecting rapid brain growth. Typical monthly growth (change in the WHO median from month to month):

Why doctors measure head circumference

Head circumference is measured at well-baby visits because very fast or very slow head growth can be an early sign of a medical or developmental concern. A single measurement matters less than the line your baby follows. Doctors pay particular attention when the measurement is below the 3rd or above the 97th percentile, or when it crosses percentile lines quickly.

How to measure a baby's head

Use a flexible, non-stretch tape. Place it just above the eyebrows and ears and around the widest part of the back of the head. Keep the tape level and snug but not tight. Take three measurements and use the largest. Hair, a wriggling baby or a soft head shape right after birth can change the reading by a few millimeters.

Big head or small head: what it can mean

Many babies with large or small heads are healthy and have parents with the same head size. Because head size is partly inherited, doctors often measure the parents' heads too. A large head alone is not a diagnosis, and a small head alone is not a diagnosis. Your baby's doctor will combine the number with development, the soft spot (fontanelle), neurological findings and the growth line over time.

Head shape is not head size

Flat spots (positional plagiocephaly) and molding after birth change head shape but do not necessarily change head circumference. Ask your doctor about repositioning or tummy time if you notice a flat spot.

Premature babies and adjusted age

Babies born before 37 weeks are usually plotted by adjusted (corrected) age until about 24 months. Adjusted age is the age since birth minus the number of weeks early. Enter the adjusted age in the age boxes and ask your baby's doctor how they interpret the result, because very preterm babies often follow their own curves. The Adjusted Age Calculator works out the adjusted age for you.

Head circumference and the soft spot

The soft spot at the top of the head normally closes sometime between roughly 9 and 18 months, although the range is wide. Your doctor checks it along with the head size and growth. Do not press on it or worry about gentle washing.

When to call the doctor

Call your baby's doctor or nurse if a measurement is below the 3rd or above the 97th percentile, if a percentile has crossed two major lines (for example from the 50th to the 10th), if your baby is not regaining birth weight by about two weeks, is feeding poorly, has fewer wet diapers than usual, seems unusually sleepy or fussy, or if you are worried for any reason. Do not wait for the next routine visit if something seems wrong.

Frequently asked questions

What is a normal head circumference for a newborn?

The WHO median at birth is 34.5 cm for boys and 33.9 cm for girls; roughly 31 to 37 cm covers the 2nd to 98th percentile range.

Is the 95th percentile for head size bad?

Not by itself. Many healthy babies have large heads. Your doctor looks at the growth pattern and development.

My baby’s head is in the 99th percentile. Should I worry?

Talk to your doctor. A high percentile may be normal if it is steady and the parents have large heads, but doctors check for causes if growth is fast or other signs are present.

How often is head size measured?

At routine visits during the first two years, often at every well-baby check.

Can I use inches?

Yes. Choose inches in the unit box.

Does this work after 24 months?

No. This page uses the WHO standards for 0 to 24 months; ask your doctor about head circumference after that.

Sources

• WHO Child Growth Standards
• CDC: WHO Growth Charts for children from birth to 24 months
• CDC: Growth chart percentile data files with LMS values

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