Quick Answer
A baby's weight percentile (centile) shows how their weight compares with other UK babies of the same age and sex — the 50th centile is the median, so a baby on it is heavier than half and lighter than half. This baby weight percentile calculator uses the WHO Child Growth Standards behind the Red Book charts: enter age, sex and weight to get the exact centile and z-score, and see where the baby sits on the nine UK-WHO chart lines. What matters most is not a single number but that the weight tracks steadily along its own centile over time.
How the Baby Weight Percentile Calculator Works
A weight centile (or percentile) is your baby’s rank out of 100 for weight, against other UK babies of the same age and sex. On the 50th centile, half of babies weigh more and half weigh less; on the 9th centile, about 9 in 100 weigh less. This baby weight percentile calculator uses the WHO Child Growth Standards — the same data behind the UK-WHO charts in the Red Book — and the standard LMS statistical method, so the answer matches the printed chart.
Centile: the percentage of babies below that z-score on the normal curve.
Reverse: weight = M × (1 + L × S × z)1/L gives the weight on any chosen centile line.
UK-WHO charts show nine centile lines — 0.4th, 2nd, 9th, 25th, 50th, 75th, 91st, 98th and 99.6th. They are spaced two-thirds of a standard deviation apart, so the gap between each pair is called a “centile space”. Most healthy babies sit between the 2nd and 98th lines and stay close to their own line as they grow.
How to Use the Baby Weight Percentile Calculator
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Pick the sex
Boys and girls have separate WHO charts, so choose the right one first.
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Enter the age
Type the age and pick weeks or months beside it — use weeks for the first few months. The tool covers birth to 24 months.
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Enter the weight
Type the weight and choose kg, g or lb. In lb and oz, add the ounces as sixteenths first (8 lb 6 oz = 8.375 lb).
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Read the centile and ladder
You get the exact centile, the z-score, the nearest chart line and the median for age — plus a nine-line centile ladder for that age with your baby’s own row marked.
Worked Examples: Centiles & Typical Weights
Every figure below is exactly what the calculator prints — enter the same inputs to check.
Centile check
| Baby | Age | Weight | Centile | Z-score |
|---|---|---|---|---|
| Boy | Birth | 3.5 kg (7 lb 11 oz) | 62nd | +0.31 |
| Boy | 4 months | 7.0 kg (15 lb 7 oz) | 50th | 0.00 |
| Boy | 6 months | 8.5 kg (18 lb 12 oz) | 74th | +0.63 |
| Girl | 3 months | 5.0 kg (11 lb 0 oz) | 11th | −1.23 |
| Girl | 12 months | 8.5 kg (18 lb 12 oz) | 34th | −0.42 |
Centile ladder — boy, 6 months
| Centile line | Weight (kg) | lb & oz |
|---|---|---|
| 0.4th | 5.89 kg | 13 lb 0 oz |
| 2nd | 6.35 kg | 14 lb 0 oz |
| 9th | 6.85 kg | 15 lb 1 oz |
| 25th | 7.37 kg | 16 lb 4 oz |
| 50th (median) | 7.93 kg | 17 lb 8 oz |
| 75th | 8.53 kg | 18 lb 13 oz |
| 91st | 9.17 kg | 20 lb 3 oz |
| 98th | 9.85 kg | 21 lb 11 oz |
| 99.6th | 10.57 kg | 23 lb 5 oz |
Average (50th Centile) Baby Weights by Age
The 50th centile is the median — the “middle” weight for age. These are WHO median weights used in the UK-WHO charts. A baby well above or below these can still be perfectly healthy if they track steadily along their own line.
| Age | Boys (50th) | Girls (50th) |
|---|---|---|
| Birth | 3.35 kg (7 lb 6 oz) | 3.23 kg (7 lb 2 oz) |
| 3 months | 6.38 kg (14 lb 1 oz) | 5.85 kg (12 lb 14 oz) |
| 6 months | 7.93 kg (17 lb 8 oz) | 7.30 kg (16 lb 1 oz) |
| 12 months | 9.65 kg (21 lb 4 oz) | 8.95 kg (19 lb 12 oz) |
| 24 months | 12.15 kg (26 lb 13 oz) | 11.48 kg (25 lb 5 oz) |
