Child growth
Head Circumference-for-Age Calculator
Enter sex, age, and the largest occipital-frontal circumference. The tool uses WHO LMS tables through 24 months and reports a percentile and z-score. Values below -2 SD or above +2 SD belong in a clinician conversation, not a home diagnosis of microcephaly or macrocephaly.
Reviewed September 2026
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What a head-circumference percentile is for
Head circumference-for-age ranks the largest occipital-frontal circumference (OFC) among children of the same sex and age on WHO standards from birth to 24 months. It is a brain-growth screen, not a hat-size vanity metric and not a weight chart.
CDC-adapted WHO infant packets include head circumference-for-age next to length and weight. US well visits plot it through the second birthday. This page stops at 24 months because that is the WHO infant file CDC hosts for LMS.
Z-scores below -2 or above +2 are a reason to talk to a clinician. They are not a home diagnosis of microcephaly or macrocephaly. Those labels need trend, exam, and sometimes imaging the calculator cannot order.
How to use the head circumference calculator
Choose units and sex. Enter completed years and extra months. A 9-month-old is 0 years and 9 months. Enter the OFC in inches or centimeters from a non-stretch tape.
The highlighted result is the percentile. Z-score and a short reading tell you whether the value sits inside -2 to +2 SD. The last line names the WHO table.
If age is 24 months or more, the tool stops. Ask the clinic which chart they use after that. Do not reuse a 12-month tape number on a 3-year-old.
Inches to centimeters is 2.54, so 17.5 in is 44.45 cm. Rounding to 44 cm on a metric tape you actually used is fine. Typing 17.5 into the centimeter field is not. That would be a newborn head on a 9-month-old.
Sex still matters. Boys' and girls' WHO medians differ by millimeters at some months and more at others. Using the wrong sex is a smaller error than a 2 cm tape miss, and it is still an avoidable error.
LMS formula for OFC
Convert inches at 2.54 cm per inch. Z = ((cm / M)^L - 1) / (L x S) from the WHO head circumference-for-age row for that sex and completed month. Percentile is the normal CDF of Z.
WHO monthly rows run 0 through 24. Linear interpolation fills the rare case of a fractional month if you ever pass one. With year-plus-month inputs, the lookup is an integer month.
How this page reads a WHO head-circumference z-score
| Z-score | Percentile neighborhood | What the card says | What it does not say |
|---|---|---|---|
| Below -2 | Under about the 2nd | Ask a clinician about head growth | Confirmed microcephaly |
| -2 to +2 | About 2nd to 98th | Usual WHO range | That development is "fine" without an exam |
| Above +2 | Above about the 98th | Ask a clinician about head growth | Confirmed hydrocephalus or macrocephaly |
Worked example: 9-month-old at 17.5 inches
Default inputs: boy, 0 years, 9 months, 17.5 in (44.45 cm). WHO boys' head circumference median at 9 months is near 45 cm. Expect a percentile a little under 50th, still well inside -2 to +2.
The same boy at 15.5 in (39.4 cm) falls below -2 SD. Recheck the tape path before you panic, then call. A single low OFC with vomiting or a bulging fontanelle is an urgent visit, not a calculator refresh.
At 19.5 in (49.5 cm) at 9 months, Z is high. Crossed percentiles upward over a few weeks matter more than one high point in a family with large heads. Bring prior numbers.
Tape path: above the brows, around the occiput
Use a flexible, non-stretch tape. Place it just above the eyebrows, above the ears, and around the most prominent point at the back of the head. That largest loop is OFC. A tape that slips onto the forehead or sits on the hair bun under-reads.
Pull snug enough that the tape does not slide, not so tight that it leaves a groove. Take three passes and keep the largest. Hair stays. Hats, bows, and thick clips come off.
A 0.5 cm miss is common at home. On WHO infant tables that can be a third of a standard deviation. If the percentile looks wild next to the clinic card, believe the clinic technique until you match it.
Paper tapes stretch. Cloth dressmaker tapes stretch more. A numbered plastic or laminated insertion tape is what public-health programs buy for a reason. If your tape grew 1 cm over the years, every OFC you take is high.
Measure before a haircut if the style is a high bun, or after if you already took the bun down. Do not compare a bun-on number from March with a bun-off number from June and call it a growth spurt.
Crossing lines versus one odd visit
Head growth velocity in the first months is fast. A jump of several percentiles can be catch-up after a small birth OFC, or it can be a concerning acceleration. Only a series on the same chart, with the same technique, tells those apart.
CDC notes growth charts are not sole diagnostic instruments. A flat OFC while length and weight climb still belongs in the visit, because the brain-growth screen is this number, not BMI.
After you have a percentile
Bring OFC, length, and weight from the same day. Isolated head findings with normal length and weight are still worth a call. Combined small length and small head is a different pattern than a large head with delayed sitting.
This tool does not convert extra juice, added sugar, or hat size into OFC. Those questions have other pages. Head growth is not a nutrition calculator.
Family hat size is a real pattern. Both parents with large heads make a +1.5 SD infant less surprising than the same OFC in two parents at the 20th percentile. It still does not cancel a rapid week-to-week rise or a bulging fontanelle.
If the fontanelle is closed early and OFC is stalling, that is an in-person question. The calculator will still print a percentile. It cannot feel sutures.
Accuracy limits, edge cases, and safety
After 24 months, stop here. School-age head circumference exists in some specialty charts. It is not this WHO infant file.
Plagiocephaly and brachycephaly change skull shape. The largest OFC can still be valid. A single diameter with calipers is not what this table expects.
Craniosynostosis, shunts, and helmets need the specialist's protocol. Do not skip a neurosurgery follow-up because a home tape landed on the 40th percentile.
Premature infants often use corrected age for OFC in the first two years. Calendar age on this page will make a former 30-week infant look small. Ask which age the NICH follow-up clinic plots.
Seek urgent care for a sunken or bulging fontanelle, repeated vomiting, a high-pitched cry, or a sudden OFC jump with lethargy. No percentile on this page clears those signs.
Tape mistakes versus true change
| What happened | Effect on OFC | Effect on percentile | Do this |
|---|---|---|---|
| Tape on the forehead, not above brows | Under-read 0.5 to 1.5 cm | Looks like a drop | Remeasure largest OFC |
| Bow or clip under the tape | Over-read | Looks like a jump | Remove accessories |
| True month of infant growth | About 0.5 cm early on, slower later | Usually stays on a channel | Plot, do not "correct" with the tape |
| Age typed as 9 years instead of 9 months | Tool may refuse (past 24 months) | Garbage in | Use 0 years and 9 months |
Birth OFC, catch-up, and when a jump is not catch-up
A small-for-gestational-age newborn often has a small OFC that climbs toward the family channel in the first months. That climb is expected catch-up. The same climb in a term infant who started on the 50th and hits the 98th in six weeks with vomiting is not catch-up. Bring both stories to the same nurse; the calculator only sees today's tape.
WHO medians at birth are about 34 to 35 cm for term boys and a few millimeters less for girls on the CDC-hosted files. A 32 cm term OFC is low. A 32 cm OFC in a 34-week infant plotted at calendar age looks lower than the neonatology chart that uses corrected age.
Microcephaly pages from CDC exist because OFC is a screening measurement for a set of brain-growth problems, including congenital infections. A home percentile is not that workup. If pregnancy had Zika risk, CMV, or another flagged infection, say so at the visit even if today's z-score is -1.4, inside the usual band.
Do not skip immunizations because a head percentile is high or low. Those are different schedules. The Bright Futures visit still wants OFC plotted with length and weight through age 2.
A tape around the chest is not OFC. Neither is a hat-size conversion chart from a store. If you do not have a tape, wait for the well visit rather than guessing 17 inches from a baseball cap label.
How it works
Z = ((cm/M)^L - 1) / (L x S) from WHO head circumference-for-age. Measure with a non-stretch tape at the largest OFC, above the brows and around the occiput. One off-center pass can move the percentile more than a month of true growth.
Frequently asked questions
After age 2?
This page stops at 24 months, matching the WHO infant files CDC hosts. Later head growth belongs in clinic, not this table.
Hair and hats?
Remove hats and bulky clips. Hair stays. Pull the tape snug without indenting skin.
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References
Kitchen tools are estimates for home cooking. Nutrition tools are screening aids, not a diagnosis. They are not a substitute for a pediatrician or registered dietitian. Food-safety temperatures follow USDA FSIS. Do not reuse leftover meat brine.