You can measure head circumference without a measuring tape by wrapping a non-stretch string around your head, marking where it overlaps, and then measuring that length against a flat ruler or yardstick. This simple swap gives you a reliable measurement in under two minutes. A few other creative methods work too, depending on what you have around the house.
The String and Ruler Method
This is the most accurate alternative to a flexible tape measure, and it works with materials almost everyone has at home. Grab a piece of string, yarn, shoelace, or even a charging cable. The key requirement is that the material should not stretch. Elastic cord, rubber bands, or stretchy hair ties will give you an inflated number.
Wrap the string around the widest part of your head. That means positioning it just above your eyebrows in front and over the most prominent bump at the back of your skull. Keep the string level all the way around, not tilted up on one side or dipping down behind your ears. Pull it snug enough to compress any hair underneath, but not so tight that it digs into your skin. Mark the overlap point with your fingers, a pen mark, or a small piece of tape, then lay the string flat against a ruler or yardstick and read the length.
If you only have a short ruler (15 cm or 6 inches), mark the string at the ruler’s end, reposition it, and add the segments together. Most adult heads measure between 21 and 24 inches (53 to 62 cm), so you’ll likely need to measure in two or three passes with a standard school ruler.
Using a Sheet of Paper
A standard sheet of US letter paper is 11 inches long, and A4 paper is 29.7 cm (about 11.7 inches). You can tape two or three sheets end to end lengthwise, trim them into a narrow strip, and use that strip the same way you would a string. The advantage of paper is that you can write directly on it where the overlap falls, then lay it flat and measure with a ruler. Because paper doesn’t stretch at all, it can actually be more precise than string, though it tears more easily if you pull too hard.
Printable Paper Tape Measures
Several universities and craft sites offer free PDF files of printable measuring tapes with centimeter or inch markings already printed on them. You print the file, cut along the dotted lines, and tape the segments together to create a full-length flexible ruler. The University of South Carolina, for example, hosts a printable 100 cm tape designed for exactly this purpose.
One important catch: home printers sometimes scale documents to fit the page, which shrinks the markings. After printing, hold the tape against a known object (a credit card is 8.56 cm wide, a US dollar bill is 15.6 cm long) to verify the scale printed at 100%. If it didn’t, reprint with “Actual Size” or “No Scaling” selected in your print settings.
Where Exactly to Measure
Placement matters more than the tool you use. The clinical standard, described in the American Journal of Medical Genetics, is to measure from the glabella (the slight bony ridge just above the bridge of your nose, between your eyebrows) around to the most prominent point at the back of your skull. You’re aiming to capture the largest possible circumference.
Remove any hair clips, headbands, ponytails, or braids before measuring. Thick or voluminous hair can add meaningful bulk, so pull your measuring tool tight enough to press through the hair and sit against the scalp. On a baby, the same landmarks apply: just above the brow ridges in front, around the widest point of the back of the head. Take the measurement two or three times and use the largest consistent number.
What About Smartphone Apps?
A few smartphone apps attempt to measure head circumference using the phone’s camera or augmented reality sensors. In theory, newer iPhones and iPads with LIDAR sensors (the Pro models from 2020 onward) can measure distances between two points in 3D space. In practice, these tools aren’t reliable enough for head circumference yet. A 2024 feasibility study testing a camera-based head circumference app found poor agreement with tape measure results, with an interrater reliability score of just 0.34 out of 1.0. Even parents who found the app easy to use got inconsistent readings.
For now, a piece of string and a ruler will outperform any app on your phone.
Converting Your Measurement to Hat or Helmet Size
Once you have your circumference in inches or centimeters, you can match it to standard hat sizes:
- Small (6 3/4 to 6 7/8): 54 to 55 cm, or about 21 1/4 to 21 5/8 inches
- Medium (7 to 7 1/8): 56 to 57 cm, or about 22 to 22 3/8 inches
- Large (7 1/4 to 7 3/8): 58 to 59 cm, or about 22 3/4 to 23 1/8 inches
- XL (7 1/2 to 7 5/8): 60 to 61 cm, or about 23 5/8 to 24 inches
- XXL (7 3/4): 62 cm, or about 24 3/8 inches
Helmet manufacturers often use centimeter ranges instead of numbered hat sizes, so having your measurement in both units is helpful. To convert inches to centimeters, multiply by 2.54.
Why Head Circumference Matters for Babies
If you’re measuring an infant’s head, the stakes are a bit different than hat shopping. Pediatricians track head circumference at every well-child visit because skull size is a reliable proxy for brain volume. A head that falls more than two standard deviations below average for age and sex is classified as microcephaly, which can be an early indicator of developmental concerns including intellectual delay, epilepsy, or cerebral palsy. A head that’s unusually large (megacephaly) can also signal conditions that need evaluation.
Serial measurements over time matter more than any single number. One reading slightly outside the expected range isn’t necessarily alarming, but a pattern of the head growing too slowly or too quickly compared to growth charts is something a pediatrician will want to investigate. If you’re tracking your baby’s head size at home between appointments, the string-and-ruler method is perfectly adequate for spotting trends, though the official measurements at your doctor’s office will be the ones used for clinical decisions.