A growth chart is one of the few places where a parent can see, on a single page, how a baby is doing compared with children around the world. This calculator reads the same World Health Organization tables that clinics use. Choose weight, length or head circumference, enter your baby’s age and measurement, and it returns the percentile, the z-score and the band the baby falls in, together with the median and the 3rd, 15th, 85th and 97th percentile values for that age.
The WHO Child Growth Standards were released in 2006 after a study of about 8,500 healthy, breastfed children in six countries, with mothers who did not smoke. They are used from birth in much of the world, and the US CDC recommends them for children up to 2 years. Because they are standards, they describe how healthy children can grow, not just how an average group happened to grow.
How to use the baby growth percentile calculator
- Select boy or girl. The tables differ by sex.
- Enter the age in months. Use the corrected age if your baby was born more than three weeks early.
- Pick the measurement: weight, length lying down, or head circumference.
- Type the value in the unit you have; the calculator converts it.
- Interpret the band. Most healthy babies sit between the 3rd and 97th percentiles.
Growth percentile formula
Percentile = Φ(z) × 100
Value at percentile: X = M × (1 + L × S × z)1/L
L, M and S are the parameters in the WHO tables for each age in months and sex. For weight-for-age they are skewed (L is near zero); for length and head circumference L equals 1, so the distribution is close to normal. Intermediate ages use linear interpolation. The 3rd, 15th, 85th and 97th percentile columns use z = −1.8808, −1.0364, +1.0364 and +1.8808. Method: WHO Multicentre Growth Reference Study Group, Acta Paediatrica Supplement 2006;450.
Worked example
A 9½-month-old girl measures 68.4 cm lying down.
- Between 9 and 10 months her table values change from M = 70.14 to 71.48 cm, and S from 0.03444 to 0.03452. At 9.5 months M = 70.81 cm and S = 0.03448.
- z = (ln(68.4) − ln(70.81)) ÷ 0.03448 ≈ −1.00
- Percentile = Φ(−1.00) × 100 = 15.9th, at the edge of the 3rd–15th and 15th–85th bands.
WHO length-for-age percentiles: girls
| Age (months) | 3rd | 15th | 50th (median) | 85th | 97th |
|---|---|---|---|---|---|
| 0 | 45.6 cm | 47.2 cm | 49.1 cm | 51.1 cm | 52.7 cm |
| 1 | 50.0 cm | 51.7 cm | 53.7 cm | 55.7 cm | 57.4 cm |
| 2 | 53.2 cm | 55.0 cm | 57.1 cm | 59.2 cm | 60.9 cm |
| 3 | 55.8 cm | 57.6 cm | 59.8 cm | 62.0 cm | 63.8 cm |
| 4 | 58.0 cm | 59.8 cm | 62.1 cm | 64.3 cm | 66.2 cm |
| 6 | 61.5 cm | 63.4 cm | 65.7 cm | 68.1 cm | 70.0 cm |
| 9 | 65.6 cm | 67.6 cm | 70.1 cm | 72.6 cm | 74.7 cm |
| 12 | 69.2 cm | 71.3 cm | 74.0 cm | 76.7 cm | 78.9 cm |
| 15 | 72.4 cm | 74.7 cm | 77.5 cm | 80.3 cm | 82.7 cm |
| 18 | 75.2 cm | 77.7 cm | 80.7 cm | 83.7 cm | 86.2 cm |
| 21 | 77.9 cm | 80.5 cm | 83.7 cm | 86.8 cm | 89.4 cm |
| 24 | 80.3 cm | 83.1 cm | 86.4 cm | 89.8 cm | 92.5 cm |
WHO length-for-age percentiles: boys
| Age (months) | 3rd | 15th | 50th (median) | 85th | 97th |
|---|---|---|---|---|---|
| 0 | 46.3 cm | 47.9 cm | 49.9 cm | 51.8 cm | 53.4 cm |
| 1 | 51.1 cm | 52.7 cm | 54.7 cm | 56.7 cm | 58.4 cm |
| 2 | 54.7 cm | 56.4 cm | 58.4 cm | 60.5 cm | 62.2 cm |
| 3 | 57.6 cm | 59.3 cm | 61.4 cm | 63.5 cm | 65.3 cm |
| 4 | 60.0 cm | 61.7 cm | 63.9 cm | 66.0 cm | 67.8 cm |
| 6 | 63.6 cm | 65.4 cm | 67.6 cm | 69.8 cm | 71.6 cm |
| 9 | 67.7 cm | 69.6 cm | 72.0 cm | 74.3 cm | 76.2 cm |
| 12 | 71.3 cm | 73.3 cm | 75.7 cm | 78.2 cm | 80.2 cm |
| 15 | 74.4 cm | 76.5 cm | 79.1 cm | 81.8 cm | 83.9 cm |
| 18 | 77.2 cm | 79.5 cm | 82.3 cm | 85.1 cm | 87.3 cm |
| 21 | 79.7 cm | 82.2 cm | 85.1 cm | 88.1 cm | 90.5 cm |
| 24 | 82.1 cm | 84.6 cm | 87.8 cm | 91.0 cm | 93.6 cm |
WHO head circumference-for-age: median and range
Boys (left) and girls (right), in centimetres.
| Age (months) | Boys 3rd | Boys median | Boys 97th | Girls 3rd | Girls median | Girls 97th |
|---|---|---|---|---|---|---|
| 0 | 32.1 | 34.5 | 36.9 | 31.7 | 33.9 | 36.1 |
| 1 | 35.1 | 37.3 | 39.5 | 34.3 | 36.5 | 38.8 |
| 2 | 36.9 | 39.1 | 41.3 | 36.0 | 38.3 | 40.5 |
| 3 | 38.3 | 40.5 | 42.7 | 37.2 | 39.5 | 41.9 |
| 6 | 41.0 | 43.3 | 45.6 | 39.7 | 42.2 | 44.6 |
| 9 | 42.6 | 45.0 | 47.4 | 41.3 | 43.8 | 46.3 |
| 12 | 43.6 | 46.1 | 48.5 | 42.3 | 44.9 | 47.5 |
| 18 | 44.9 | 47.4 | 49.9 | 43.6 | 46.2 | 48.8 |
| 24 | 45.7 | 48.3 | 50.8 | 44.6 | 47.2 | 49.8 |
Reading the results
- Percentiles are positions, not scores. Being on the 80th percentile is not better than the 30th.
- Consistency matters. A baby who moves across two or more major percentile lines may need a check.
- Parents’ size counts. Small parents tend to have smaller babies, and the Child Height Predictor gives an idea of expected adult height.
- Head size is judged with the whole child. A head percentile much higher or lower than weight and length is something to discuss with a professional rather than to worry over alone.
For a weight-only view with a table by month see the Baby Weight Percentile Calculator; for head size only see the Head Circumference Percentile Calculator; from age 2 use the Child Height Percentile Calculator.
This calculator is for information only. A single percentile does not diagnose a problem; what matters is the pattern over several visits. Talk to your health visitor, midwife or paediatrician about any growth concern.
Sources
- WHO Multicentre Growth Reference Study Group. WHO Child Growth Standards: Methods and development. World Health Organization, Geneva, 2006.
- WHO Multicentre Growth Reference Study Group. Acta Paediatrica Supplement 2006;450 (length/height-for-age, weight-for-age, head circumference-for-age).
- US Centers for Disease Control and Prevention. Use of World Health Organization and CDC Growth Charts for Children Aged 0–59 Months. MMWR 2010;59(RR-9).
- WHO percentile tables (2nd to 98th) for weight, length and head circumference, boys and girls, 0–24 months.
Frequently asked questions
What does it mean if my baby is in the 90th percentile for length?
It means your baby is longer than 90 out of 100 babies of the same age and sex in the WHO reference group. Many tall babies are tall because their parents are tall. Health professionals look at whether the baby has tracked the same percentile region over time.
Why is length measured lying down?
WHO standards for children under 2 years use recumbent length. Standing height is taken from age 2 and is on average about 0.7 cm shorter, which is why a slight step appears in the tables at 24 months. Use a length board or ask a clinic to measure; tape measures at home are rarely accurate.
How is head circumference measured?
The tape goes around the widest part of the head, above the eyebrows and ears and across the most prominent part of the back of the skull. Clinicians take the largest of two or three readings. A single measurement matters less than the line your baby follows from visit to visit.
Do formula-fed and breastfed babies use different charts?
The WHO standards describe the growth of breastfed infants as the norm for all babies, and are recommended for all children under 2 by the CDC. Breastfed babies often gain faster in the first weeks and slower later, so growth may look different to older, formula-based charts.
Is this a medical assessment?
No. It converts measurements into percentiles using published tables but cannot examine your baby or see the full picture. Any worry about growth should be discussed with a doctor, midwife or health visitor.