Parents and carers are often told a baby is “on the 50th percentile” or “tracking along the 25th” without much explanation of what that means. This calculator turns your baby’s weight into a percentile and a z-score using the World Health Organization’s growth standards, and shows the range of weights that fall between the 3rd and 97th percentiles for the same age and sex. It covers birth to 24 months for boys and girls, and accepts weights in kilograms, pounds, ounces or grams.
The WHO standards were built from the Multicentre Growth Reference Study, which followed about 8,500 children from Brazil, Ghana, India, Norway, Oman and the USA whose mothers did not smoke and who were breastfed and given recommended care. The charts therefore show how babies grow under healthy conditions, which is why they are described as standards rather than simple averages.
How to use the baby weight percentile calculator
- Choose your baby’s sex. The standards are different for boys and girls.
- Enter the age in months. Decimals are fine: 3.5 means three and a half months. For a premature baby, use the corrected age.
- Enter the weight from a recent, accurate weighing, ideally without clothes or with a clean nappy.
- Read the percentile and the band. The median and the 3rd, 15th, 85th and 97th percentile weights show what the typical range looks like at that age.
Weight-for-age formula (the LMS method)
z = ln(X ÷ M) ÷ S (L = 0)
Percentile = Φ(z) × 100
Weight at a given z: X = M × (1 + L × S × z)1/L
X is your baby’s weight, and L, M and S are the skewness, median and coefficient of variation published by WHO for each month of age and sex. Between whole months the calculator interpolates L, M and S linearly. Φ is the standard normal cumulative distribution. The tables are those in WHO Multicentre Growth Reference Study Group, Acta Paediatrica Supplement 2006;450:76–85.
Worked example
A girl aged 6 months weighs 7.3 kg. At 6 months the WHO girls’ table gives L = −0.0756, M = 7.297 kg and S = 0.12204.
- (7.3 ÷ 7.297)−0.0756 = 0.99997
- z = (0.99997 − 1) ÷ (−0.0756 × 0.12204) ≈ 0.00
- Percentile ≈ 50th, essentially on the median of 7.30 kg (16 lb 1 oz).
For comparison the median for a boy of the same age is 7.93 kg (17 lb 8 oz).
WHO weight-for-age percentiles: girls
| Age (months) | 3rd | 15th | 50th (median) | 85th | 97th |
|---|---|---|---|---|---|
| 0 | 2.44 kg (5 lb 6 oz) | 2.78 kg (6 lb 2 oz) | 3.23 kg (7 lb 2 oz) | 3.73 kg (8 lb 4 oz) | 4.17 kg (9 lb 3 oz) |
| 1 | 3.22 kg (7 lb 1 oz) | 3.63 kg (8 lb 0 oz) | 4.19 kg (9 lb 4 oz) | 4.82 kg (10 lb 10 oz) | 5.39 kg (11 lb 14 oz) |
| 2 | 4.00 kg (8 lb 13 oz) | 4.48 kg (9 lb 14 oz) | 5.13 kg (11 lb 5 oz) | 5.86 kg (12 lb 15 oz) | 6.53 kg (14 lb 6 oz) |
| 3 | 4.61 kg (10 lb 2 oz) | 5.13 kg (11 lb 5 oz) | 5.85 kg (12 lb 14 oz) | 6.66 kg (14 lb 11 oz) | 7.40 kg (16 lb 5 oz) |
| 4 | 5.09 kg (11 lb 3 oz) | 5.65 kg (12 lb 7 oz) | 6.42 kg (14 lb 3 oz) | 7.31 kg (16 lb 2 oz) | 8.11 kg (17 lb 14 oz) |
| 5 | 5.48 kg (12 lb 1 oz) | 6.08 kg (13 lb 6 oz) | 6.90 kg (15 lb 3 oz) | 7.84 kg (17 lb 4 oz) | 8.70 kg (19 lb 3 oz) |
| 6 | 5.81 kg (12 lb 13 oz) | 6.43 kg (14 lb 3 oz) | 7.30 kg (16 lb 1 oz) | 8.29 kg (18 lb 4 oz) | 9.20 kg (20 lb 4 oz) |
| 7 | 6.09 kg (13 lb 7 oz) | 6.74 kg (14 lb 14 oz) | 7.64 kg (16 lb 14 oz) | 8.68 kg (19 lb 2 oz) | 9.64 kg (21 lb 4 oz) |
| 8 | 6.34 kg (14 lb 0 oz) | 7.01 kg (15 lb 7 oz) | 7.95 kg (17 lb 8 oz) | 9.03 kg (19 lb 14 oz) | 10.03 kg (22 lb 2 oz) |
| 9 | 6.56 kg (14 lb 8 oz) | 7.26 kg (16 lb 0 oz) | 8.23 kg (18 lb 2 oz) | 9.35 kg (20 lb 10 oz) | 10.39 kg (22 lb 14 oz) |
| 10 | 6.77 kg (14 lb 15 oz) | 7.48 kg (16 lb 8 oz) | 8.48 kg (18 lb 11 oz) | 9.64 kg (21 lb 4 oz) | 10.72 kg (23 lb 10 oz) |
| 11 | 6.96 kg (15 lb 5 oz) | 7.69 kg (16 lb 15 oz) | 8.72 kg (19 lb 4 oz) | 9.91 kg (21 lb 14 oz) | 11.03 kg (24 lb 5 oz) |
| 12 | 7.14 kg (15 lb 12 oz) | 7.89 kg (17 lb 6 oz) | 8.95 kg (19 lb 12 oz) | 10.18 kg (22 lb 7 oz) | 11.33 kg (25 lb 0 oz) |
| 15 | 7.67 kg (16 lb 14 oz) | 8.47 kg (18 lb 11 oz) | 9.60 kg (21 lb 3 oz) | 10.93 kg (24 lb 1 oz) | 12.18 kg (26 lb 14 oz) |
| 18 | 8.17 kg (18 lb 0 oz) | 9.02 kg (19 lb 14 oz) | 10.23 kg (22 lb 9 oz) | 11.65 kg (25 lb 11 oz) | 12.99 kg (28 lb 10 oz) |
| 21 | 8.67 kg (19 lb 2 oz) | 9.57 kg (21 lb 2 oz) | 10.85 kg (23 lb 15 oz) | 12.36 kg (27 lb 4 oz) | 13.80 kg (30 lb 7 oz) |
| 24 | 9.16 kg (20 lb 3 oz) | 10.12 kg (22 lb 5 oz) | 11.48 kg (25 lb 5 oz) | 13.08 kg (28 lb 13 oz) | 14.61 kg (32 lb 3 oz) |
WHO weight-for-age percentiles: boys
| Age (months) | 3rd | 15th | 50th (median) | 85th | 97th |
|---|---|---|---|---|---|
| 0 | 2.51 kg (5 lb 8 oz) | 2.86 kg (6 lb 5 oz) | 3.35 kg (7 lb 6 oz) | 3.88 kg (8 lb 9 oz) | 4.35 kg (9 lb 9 oz) |
| 1 | 3.45 kg (7 lb 10 oz) | 3.88 kg (8 lb 9 oz) | 4.47 kg (9 lb 14 oz) | 5.13 kg (11 lb 5 oz) | 5.71 kg (12 lb 9 oz) |
| 2 | 4.39 kg (9 lb 11 oz) | 4.89 kg (10 lb 12 oz) | 5.57 kg (12 lb 4 oz) | 6.32 kg (13 lb 15 oz) | 6.99 kg (15 lb 7 oz) |
| 3 | 5.09 kg (11 lb 4 oz) | 5.64 kg (12 lb 7 oz) | 6.38 kg (14 lb 1 oz) | 7.19 kg (15 lb 14 oz) | 7.92 kg (17 lb 7 oz) |
| 4 | 5.64 kg (12 lb 7 oz) | 6.22 kg (13 lb 11 oz) | 7.00 kg (15 lb 7 oz) | 7.87 kg (17 lb 5 oz) | 8.63 kg (19 lb 1 oz) |
| 5 | 6.08 kg (13 lb 6 oz) | 6.69 kg (14 lb 12 oz) | 7.51 kg (16 lb 9 oz) | 8.42 kg (18 lb 9 oz) | 9.22 kg (20 lb 5 oz) |
| 6 | 6.44 kg (14 lb 3 oz) | 7.08 kg (15 lb 10 oz) | 7.93 kg (17 lb 8 oz) | 8.88 kg (19 lb 9 oz) | 9.72 kg (21 lb 7 oz) |
| 7 | 6.74 kg (14 lb 14 oz) | 7.41 kg (16 lb 5 oz) | 8.30 kg (18 lb 5 oz) | 9.28 kg (20 lb 7 oz) | 10.16 kg (22 lb 6 oz) |
| 8 | 7.01 kg (15 lb 7 oz) | 7.69 kg (16 lb 15 oz) | 8.62 kg (19 lb 0 oz) | 9.64 kg (21 lb 4 oz) | 10.55 kg (23 lb 4 oz) |
| 9 | 7.24 kg (15 lb 15 oz) | 7.95 kg (17 lb 8 oz) | 8.90 kg (19 lb 10 oz) | 9.96 kg (21 lb 15 oz) | 10.90 kg (24 lb 1 oz) |
| 10 | 7.45 kg (16 lb 7 oz) | 8.18 kg (18 lb 1 oz) | 9.16 kg (20 lb 3 oz) | 10.25 kg (22 lb 10 oz) | 11.23 kg (24 lb 12 oz) |
| 11 | 7.65 kg (16 lb 14 oz) | 8.40 kg (18 lb 8 oz) | 9.41 kg (20 lb 12 oz) | 10.53 kg (23 lb 4 oz) | 11.54 kg (25 lb 7 oz) |
| 12 | 7.85 kg (17 lb 5 oz) | 8.61 kg (19 lb 0 oz) | 9.65 kg (21 lb 4 oz) | 10.80 kg (23 lb 13 oz) | 11.83 kg (26 lb 1 oz) |
| 15 | 8.38 kg (18 lb 7 oz) | 9.20 kg (20 lb 4 oz) | 10.31 kg (22 lb 12 oz) | 11.55 kg (25 lb 8 oz) | 12.67 kg (27 lb 15 oz) |
| 18 | 8.87 kg (19 lb 9 oz) | 9.75 kg (21 lb 8 oz) | 10.94 kg (24 lb 2 oz) | 12.27 kg (27 lb 1 oz) | 13.48 kg (29 lb 11 oz) |
| 21 | 9.34 kg (20 lb 10 oz) | 10.28 kg (22 lb 10 oz) | 11.55 kg (25 lb 7 oz) | 12.98 kg (28 lb 10 oz) | 14.27 kg (31 lb 7 oz) |
| 24 | 9.80 kg (21 lb 10 oz) | 10.80 kg (23 lb 13 oz) | 12.15 kg (26 lb 13 oz) | 13.68 kg (30 lb 3 oz) | 15.07 kg (33 lb 4 oz) |
What the numbers tell you
- Growth is fastest in the first months. The WHO median for boys rises from 3.35 kg at birth to 7.00 kg at 4 months and passes double the birth median by month 4. For girls the median doubles by month 5.
- The “normal” range is wide. At 6 months the 3rd to 97th percentile span for girls runs from 5.8 to 9.2 kg, a gap of more than 3 kg.
- The trend matters more than a single point. A baby who has tracked the 15th percentile since birth is following their own healthy curve.
- Weight must be read with length. A short baby is expected to weigh less. Check the Baby Growth Percentile Calculator for length and head size.
When to seek advice
Contact a health professional if your baby falls across two major percentile lines, is not gaining weight over several weeks, feeds poorly, or seems unwell. Also seek advice about any weight concern, whether the percentile is high or low. For head size see the Head Circumference Percentile Calculator; for older children use the Child Weight 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: Length/height-for-age, weight-for-age, weight-for-length, weight-for-height and body mass index-for-age: Methods and development. World Health Organization, Geneva, 2006.
- WHO Multicentre Growth Reference Study Group. Acta Paediatrica Supplement 2006;450:76–85 (weight-for-age).
- US Centers for Disease Control and Prevention. Growth Chart Training: Use of WHO growth standards for children aged 0–2 years.
- WHO weight-for-age percentile tables for boys and girls, 0–24 months (LMS parameters).
Frequently asked questions
What is a baby weight percentile?
A percentile shows how your baby’s weight compares with the WHO reference group of healthy, breastfed infants of the same age and sex. At the 40th percentile, 40 out of 100 babies in that group weigh less and 60 weigh more. It is a position on a scale, not a pass or fail mark.
Which growth chart does this use?
The WHO Child Growth Standards (2006), which the US Centers for Disease Control and Prevention recommends for children under 2 years and which many countries use from birth. The standards describe how children grow when feeding and care follow health recommendations.
Is a low percentile a problem?
Not by itself. Babies on the 5th percentile can be perfectly healthy if they have followed that line steadily. Clinicians pay more attention to a baby who drops across two or more major percentile lines, who stops gaining, or whose weight is far from length. Raise concerns with your child’s health professional.
Should I correct the age for a premature baby?
Yes, usually until about 2 years. Subtract the number of weeks early from the age: a baby born at 32 weeks (8 weeks early) who is now 5 months old should be plotted at about 3 months. Your health team will advise on the exact approach for your child.
How often should my baby be weighed?
Health services usually weigh babies at routine checks, so you rarely need more than that. Weighing more often shows normal day-to-day variation caused by feeds and nappies and can cause needless worry; ask your health visitor about a schedule that suits your baby.