WHO growth standards for weight, length and head circumference
Updated October 2026
Age is in months from birth; half months are allowed (a 10-week-old is about 2.3 months). For a baby born before 37 weeks, subtract the weeks born early to get corrected age, which pediatric guidance uses for weight, length and head size until about age 2. Weight is most accurate from the clinic scale, unclothed. Length is measured lying flat, head to heel with the legs straight. Head circumference goes around the widest part of the head, just above the eyebrows and ears; enter 0 to skip it.
Each line of the result shows the percentile, the z-score (standard deviations from the median) and the median for that age and sex, so you can see how far from the middle the measurement sits.
For each month of age the WHO standards give three numbers: L (skew), M (the median) and S (spread). The measurement becomes a z-score with z = ((X ÷ M)L − 1) ÷ (L × S), and the percentile is the share of the normal distribution below that z-score. Between whole months the calculator interpolates the L, M and S values.
For a 6-month-old boy the weight values are L = 0.1257, M = 7.934 kg and S = 0.10958. A weight of 17.5 lb is 7.938 kg, so z = ((7.938 ÷ 7.934)0.1257 − 1) ÷ (0.1257 × 0.10958) = 0.004, which is the 50th percentile.
| Weight | Percentile | z-score |
|---|---|---|
| 14 lb (6.35 kg) | 2nd | -2.00 |
| 15.5 lb (7.03 kg) | 14th | -1.09 |
| 17.5 lb (7.94 kg) | 50th | +0.00 |
| 19.5 lb (8.85 kg) | 84th | +1.00 |
| 21 lb (9.53 kg) | 95th | +1.69 |
A 7-pound spread at this age covers most of the normal range. Using the cutoffs below, the screening limits for a 6-month-old boy are about 14.0 lb (2.3rd percentile) and 21.7 lb (97.7th).
| Age (months) | Boys, weight | Girls, weight | Boys, length | Girls, length |
|---|---|---|---|---|
| 0 | 7.4 lb | 7.1 lb | 19.6 in | 19.3 in |
| 2 | 12.3 lb | 11.3 lb | 23.0 in | 22.5 in |
| 4 | 15.4 lb | 14.2 lb | 25.2 in | 24.4 in |
| 6 | 17.5 lb | 16.1 lb | 26.6 in | 25.9 in |
| 9 | 19.6 lb | 18.1 lb | 28.3 in | 27.6 in |
| 12 | 21.3 lb | 19.7 lb | 29.8 in | 29.1 in |
| 18 | 24.1 lb | 22.6 lb | 32.4 in | 31.8 in |
| 24 | 26.8 lb | 25.3 lb | 34.6 in | 34.0 in |
Since 2010 the CDC has recommended that U.S. clinicians use the 2006 WHO growth standards, not the CDC charts, for children under 24 months, and the CDC growth charts from age 2. The WHO charts are a standard: they describe how healthy infants grow under good conditions, and every baby in the reference group was breastfed for 12 months. For screening, the CDC recommends the 2.3rd and 97.7th percentiles (±2 standard deviations) on the WHO charts rather than the 5th and 95th. Slower weight gain among breastfed babies between about 3 and 18 months is normal on these charts, and the CDC notes that fewer U.S. babies are labeled underweight on them.
Weighing in clothes or a full diaper adds weight that is not the baby’s, which matters most at newborn weights. Measuring length standing up or with bent knees reads short. Using a home bathroom scale (weighing yourself with and without the baby) is too coarse for small changes. Using birth age for a premature baby makes a healthy preemie look small. Comparing one reading to the last overreacts to measurement noise; the trend over several visits is what counts.
A percentile is a measurement, not a diagnosis, and a baby who stays on the 5th or 95th percentile curve may be growing exactly as expected. The American Academy of Family Physicians’ review of faltering growth lists the patterns doctors look for in repeated, accurate measurements: weight below the 5th percentile for sex and corrected age, weight-for-length below the 5th percentile, or a drop across two major percentile lines (95, 90, 75, 50, 25, 10, 5) over time. Rapid upward crossing can also be worth discussing. Ask your pediatrician about a head size that is rising or falling across lines, poor feeding, fewer wet diapers, or a baby who seems unusually sleepy or hard to wake; those need a clinician, not a calculator.