Measuring head circumference requires a flexible, non-stretchy tape measure wrapped around the widest part of the head, just above the eyebrows and ears at the front and around the most prominent point at the back of the skull. The technique is simple enough that parents can do it at home with reasonable accuracy, but small errors in tape placement or tension can throw the reading off by a centimeter or more. Getting it right matters because clinicians use this single number to track brain growth in infants and screen for conditions that benefit from early intervention.
What You Need
The standard tool is a non-elastic measuring tape, the kind used in sewing. Paper or fiberglass tapes work well. Cloth tapes that have stretched over time, retractable metal tapes, or anything with noticeable give should be avoided. The reason is straightforward: a tape that stretches even slightly will understate the circumference, and the degree of stretch varies with how hard you pull. Research comparing different tapes found statistically significant differences in accuracy between brands, with some showing a gap of about two percent between the most and least accurate options.
If you do not have a proper tape measure, a piece of non-stretchy string wrapped around the head and then measured flat against a ruler is a reasonable substitute. It adds a step and a small opportunity for error, but it is far better than guessing or using an elastic band.
The Step-by-Step Technique
The goal is to capture the maximum circumference of the skull. In clinical practice, this is described as the distance from the glabella (the slight bump between the eyebrows) around to the opisthocranion (the most protruding point at the back of the head).1PubMed Central. Head circumference – a useful single parameter for skull volume development in cranial growth analysis? You do not need to know those anatomical terms to do this correctly. Here is what to do:
- Position the person: For an infant, have someone hold the baby upright or lay the baby on a flat surface, face up. For an older child or adult, have them sit or stand comfortably and face forward.
- Place the tape at the front: Set the tape just above the eyebrows, roughly at the widest part of the forehead. The tape should sit above the tops of the ears on both sides.
- Wrap around the back: Bring the tape around the back of the head, finding the point where the skull sticks out the most. On most people this is slightly above the base of the skull, not at the very bottom.
- Keep it level: The tape should be at approximately the same height all the way around. A tape that dips down on one side or rides up in the back will give a smaller reading.
- Pull snug but not tight: The tape should compress any hair without digging into the skin. Think of it as resting firmly against the head, not cinching it.
- Read and record: Note where the tape overlaps at the front, where you started. Record the number in centimeters to the nearest millimeter.
- Measure at least twice: Take two or three readings and use the largest consistent value. If two readings differ by more than a few millimeters, check your tape placement and try again.
Clinical guidelines describe the technique as measuring the distance around the back of the child’s head with a non-elastic tape held above the eyebrows and ears, then plotting the result on an age- and sex-appropriate growth chart.2PubMed Central. Measuring head circumference: Update on infant microcephaly The chart comparison is what gives a single measurement its meaning, because 35 centimeters means something very different for a one-week-old than for a six-month-old.
Common Mistakes That Throw Off the Reading
The biggest source of error is not the tape or the person being measured. It is inconsistent placement. A study of over 1,100 low-birthweight infants compared head circumference readings taken by pediatric residents against those taken by trained ultrasound technologists. The correlation between the two was strong at 0.93, but when the researchers looked at individual disagreements, about five percent of measurements differed by two centimeters or more.3Journal of Clinical Epidemiology. The reliability of neonatal head circumference measurement Two centimeters is enough to shift a baby from one percentile range to another, which could trigger an unnecessary referral or miss a real concern.
The usual culprits are a tape that slips above the brow ridge, a tape that catches on a hair clip or headband mark, or an infant who is squirming so much that the tape is neither level nor snug. Hair itself can be a factor in older children and adults with thick or voluminous hair. Push the tape down through the hair until it contacts the scalp. If the hair is braided tightly or in a style that adds bulk, note that in your records so the next measurement can be taken under similar conditions.
Can Parents Measure Accurately at Home?
Yes, and the data on this is encouraging. In one study, parents of infants aged one to six weeks were given illustrated instructions and a simple tape, then asked to measure their baby’s head twice. Their readings correlated with trained-observer measurements at an intraclass correlation of 0.94, and the average difference between the parent and observer measurements was only about 0.3 percent.4American Journal of Human Biology. Reliability and validity of parental measurements of infant size Of all the infant body measurements parents were tested on, head circumference turned out to be the most reliable one for home use.
A separate study had adults measure their own heads and parents measure their children’s heads, then compared those readings with a researcher’s measurements. For parents measuring their child, the intraclass correlation was 0.99. Even adults measuring their own heads managed an intraclass correlation of 0.84, which is lower but still within the range considered acceptable for a skilled anthropometrist.5PubMed Central. Reliability of self, parental, and researcher measurements of head circumference Self-measurement is trickier simply because you cannot see the back of your own head, but it can still give a useful ballpark.
The practical takeaway: if your pediatrician asks you to track head circumference between visits, or if you want to monitor growth between well-child checks, you can do this at home with confidence. Use the same tape each time, take multiple readings, and write down the number with the date.
Special Considerations in Newborns
The first measurement of a newborn’s head is not always the most accurate one, because the skull changes shape during delivery. Vaginal birth often produces some degree of skull molding, where the soft bone plates overlap slightly to fit through the birth canal, temporarily reducing the circumference. At the same time, swelling on the scalp from pressure during labor (called caput succedaneum) can make the head appear larger than it actually is. These two effects can partially cancel each other out or compound depending on the delivery.
A study that tracked newborns found their average head circumference increased by about 0.17 centimeters between the first day and the third day of life as molding resolved. Babies whose mothers experienced labor pains and those who were small for gestational age showed even larger increases.6PubMed Central. Serial head circumference measurements should be used to classify congenital microcephaly This is one reason many clinicians prefer serial measurements over a single reading when evaluating whether a newborn’s head size is truly concerning. A head that looks small at birth because of molding may plot normally a few days later once the bones have sprung back into position.
How Prenatal Ultrasound Compares to the Tape Measure
During pregnancy, head circumference is estimated from ultrasound images rather than measured directly. These prenatal estimates tend to run smaller than the postnatal tape measurement. One study found that ultrasound underestimated the actual head circumference in about 88 percent of cases, with the discrepancy growing as pregnancy progressed. By the time the baby was past 37 weeks of gestation, the average gap was close to nine millimeters.7PubMed Central. Sonographic Estimation of the Fetal Head Circumference: Accuracy and Factors Affecting the Error When the head was already above the 95th percentile on ultrasound, the error was even larger.
This systematic underestimation matters clinically. A separate study found that prenatal head circumference Z-scores deviated significantly from postnatal occipitofrontal circumference measurements at birth, meaning that a fetus flagged as potentially microcephalic on ultrasound might turn out to have a normal head size once measured with a tape after delivery.8PubMed. Prediction of microcephaly at birth using three reference ranges for fetal head circumference: can we improve prenatal diagnosis? If you have been told during pregnancy that your baby’s head is measuring small, it is worth knowing that the postnatal tape measurement is the more definitive number.
Plotting the Number on a Growth Chart
A raw head circumference measurement means little on its own. The number becomes useful when plotted against a reference population matched for age and sex. For full-term infants and children, the World Health Organization growth standards are widely used internationally, while in the United States, many clinicians use the CDC growth charts. Preterm infants have their own charts; the Fenton growth charts, recently updated using data from about 4.8 million births across 15 countries, track weight, length, and head circumference from 22 weeks of gestation onward.9Wiley Online Library. Fenton Third‐Generation Growth Charts of Preterm Infants Without Abnormal Fetal Growth: A Systematic Review and Meta‐Analysis
Clinicians typically measure head circumference at every well-child visit during the first two years, and periodically after that. The head grows fastest during the first year of life. In a nationally representative German study of over 17,000 children, adult head circumference was reached by about age 16 in girls, while boys’ heads continued growing through at least age 17.10PubMed. German head circumference references for infants, children and adolescents in comparison with currently used national and international references After the early years, the rate of growth slows dramatically, so the window when tracking matters most is infancy and early childhood.
What clinicians watch for is not any single reading but the trajectory. A head that consistently tracks the 75th percentile is not concerning, even though it is larger than average. A head that was at the 25th percentile and has jumped to the 90th over two visits is more noteworthy, because the acceleration itself may signal something worth investigating.
When the Head Is Too Small or Too Large
Microcephaly is generally defined as a head circumference at least two standard deviations below the mean for age and sex, with severe microcephaly at three or more standard deviations below.11PubMed Central. Characteristics of children of the Microcephaly Epidemic Research Group Pediatric Cohort who developed postnatal microcephaly In practical terms, this means a head that is substantially smaller than the vast majority of children the same age and sex. The causes range from genetic conditions to prenatal infections, and the clinical implications vary enormously depending on the underlying reason.
On the other end, macrocephaly means the head is unusually large. In a study of 90 infants referred for abnormally increasing head circumference, the single most common cause was familial megalencephaly, accounting for about 59 percent of cases. Hydrocephalus was responsible for roughly a third, and miscellaneous causes made up the rest. Among the infants with familial megalencephaly, over 90 percent had a parent or sibling with a large head.12PubMed Central. A Study on Causes and Types of Abnormal Increase in Infants’ Head Circumference in Kashan/Iran In other words, the most frequent reason for an unusually big head in a baby is simply that big heads run in the family, and in those cases the children typically develop normally.
A related condition, idiopathic external hydrocephalus, can also cause heads to cross above the 95th percentile during the first year. A study of 36 infants with this condition found that it resolved without treatment after 18 to 24 months in all cases, with normal development in 32 of the 36. A family history of macrocephaly was present in 88 percent.13Pediatrics. Idiopathic External Hydrocephalus: Natural History and Relationship to Benign Familial Macrocephaly This is worth knowing if your pediatrician flags a rapidly growing head: a large proportion of cases turn out to be benign, inherited, and self-resolving.
That said, the screening value of any single percentile threshold is limited. A retrospective study in a primary care network found that commonly used head circumference thresholds had low sensitivity and low positive predictive value for detecting new pathology associated with head enlargement.14PubMed Central. The test characteristics of head circumference measurements for pathology associated with head enlargement: a retrospective cohort study The serial trajectory, not any one number, is the stronger diagnostic signal.
Does Head Size Reflect Brain Size or Intelligence?
Head circumference correlates strongly with intracranial volume. In one analysis, the overall correlation was 0.81, and in certain skull types the correlation was even higher, approaching 0.98.15Annals of Plastic Surgery. Can Head Circumference Be Used as a Proxy for Intracranial Volume in Patients With Craniosynostosis? So a larger head does, on average, mean a larger brain cavity. But the leap from brain size to cognitive ability does not follow as neatly as people often assume.
A study examining the relationship between head circumference, brain volume, and cognition in children found that while head size and brain volume were positively correlated, neither head circumference nor total brain volume correlated with IQ. No cognitive scores correlated with head circumference or brain volume in either the study group or the control group, even with sample sizes of roughly 90 to 140 per group.16PLoS ONE. Relationships between Head Circumference, Brain Volume and Cognition in Children with Prenatal Alcohol Exposure At the genetic level, early-life head circumference does show positive genetic correlations with adult intracranial volume and measures of education and intelligence, but not with psychiatric or neurological conditions.17PubMed Central. Genetics of early-life head circumference and genetic correlations with neurological, psychiatric and cognitive outcomes The genetic overlap suggests shared biology, but it does not mean that measuring a baby’s head can predict how smart that child will be. The relationship between head size and intelligence is far too weak and indirect for any individual-level inference.
Measuring Head Circumference in Adults
Head circumference is not just a pediatric measurement. It is sometimes used in adult neurology, particularly when evaluating conditions like hydrocephalus in older patients. The technique is the same: a non-elastic tape around the widest point. But adult reference ranges are less standardized than those for children.
A study in the Spanish population proposed that adult men with a head circumference greater than 60 centimeters and women greater than 58 centimeters should be considered macrocephalic, while microcephaly thresholds were 53.6 centimeters for men and 51.3 centimeters for women. Among adult age groups, there were no statistically significant differences in head circumference.18PubMed. Head circumference: the forgotten tool for hydrocephalus management. A reference interval study in the Spanish population These numbers are population-specific and would not necessarily apply to people of different ethnic backgrounds, but they give a sense of what “normal” looks like in adults. The measurement’s primary value in adults is as a baseline: if someone develops symptoms that could indicate increased intracranial pressure, having a prior head circumference on record gives clinicians something to compare against.
Three-Dimensional Scanning as an Alternative
In research settings and some specialty clinics, 3D surface scanners are replacing the tape measure for head shape assessment. These devices capture the full geometry of the skull in seconds, allowing clinicians to calculate not just circumference but also volume and symmetry indices. Studies comparing 3D scanning with traditional tape measurement have found strong correlations. One study using structured light scanning in infants reported correlation coefficients above 0.9 for head circumference, with reasonable consistency between the two methods on Bland-Altman analysis.19PubMed Central. A comparative study between traditional head measurement and structured light three-dimensional scanning when measuring infant head shape
Another comparison study found that intra-rater repeatability for 3D scanners was excellent, with intraclass correlation coefficients above 0.99. Head circumference measured directly and digitally were strongly correlated. However, the researchers cautioned that absolute values from scanners do not always match tape measurements exactly, so the two methods should not be mixed interchangeably when tracking a patient’s growth over time.20PubMed. Three-dimensional surface scanners compared with standard anthropometric measurements for head shape For the foreseeable future, the flexible tape measure remains the standard tool in most clinical and home settings. It is cheap, portable, and when used correctly, more than accurate enough for the job.