Find your baby's weight centile from the WHO growth data behind the UK-WHO Red Book charts, or see the typical weight on any of the nine chart lines by age and sex.
Boy
Girl
months
weeks
kg
g
lb
Uses the WHO Child Growth Standards weight-for-age data (weekly to 13 weeks, then monthly), birth to 24 months — the data the UK-WHO Red Book charts use from 2 weeks. At birth the Red Book uses UK90 data instead, and a baby born before 37 weeks should be entered at corrected age. The nine chart lines (0.4th, 2nd, 9th, 25th, 50th, 75th, 91st, 98th, 99.6th) sit two-thirds of a standard deviation apart.
Weight centile
Weight centile
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Exact percentile
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Z-score (SD)
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Nearest line
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Median for age
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Where your baby sits (centile ladder for this age)
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A guide only, not medical advice. One weight matters less than the trend in your Red Book. A steady line near any centile is usually healthy — talk to your health visitor or GP about any worry, or if the weight crosses centile lines downward.
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Quick Answer
A baby's weight percentile (centile) shows how their weight compares with the WHO standard for healthy, breastfed 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 that the Red Book charts use from 2 weeks: 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 the WHO standard for healthy, breastfed 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 data the UK-WHO charts in the Red Book use from 2 weeks to 4 years — with WHO’s weekly figures for the first 13 weeks and the standard LMS statistical method, so from 2 weeks the answer matches the printed chart.
Z-score: z = ((weight ÷ M)L − 1) ÷ (L × S), where L, M and S are the WHO parameters for that age and sex. 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.
The calculator places your baby’s weight among the nine UK-WHO centile lines (0.4th to 99.6th) and reports the exact centile and z-score.
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.
Two cases where the Red Book works differently. From birth to 2 weeks it has no centile lines, and it plots birth weight on UK90 birth data (British babies measured around 1990) rather than WHO data, so a newborn’s centile here is only a guide — in those first days, weight lost since birth matters more. And for a baby born before 37 weeks, use corrected age (age minus the weeks born early): keep correcting until 12 months if born at 32–36 weeks, and until 24 months for those born earlier.
How to Use the Baby Weight Percentile Calculator
Pick the sex
Boys and girls have separate WHO charts, so choose the right one first.
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.
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).
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
The birth row uses WHO’s birth figures; the Red Book itself plots birth weight on UK90 data, so a newborn’s printed centile can differ.
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, the medians the UK-WHO charts use from 2 weeks; the Red Book plots birth weight on UK90 data instead, so the birth row is WHO’s figure. 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)
This tool is a guide, not medical advice. A healthy baby tends to stay near their own centile — the pattern over time matters far more than one reading. NICE advises a review if weight drops across two or more centile spaces (one space if the birth weight was below the 9th centile), or falls below the 2nd centile. Always weigh on the same accurate scales and speak to your health visitor or GP about any concern.
Baby Weight Percentile Calculator: FAQ
It ranks your baby’s weight out of 100 against the WHO standard for healthy, breastfed babies of the same age and sex, which the UK-WHO charts use from 2 weeks. On the 25th centile, about 25 in 100 weigh less and 75 weigh more. Any centile between the 2nd and 98th is within the normal range — a low or high centile is only a concern if the weight crosses lines over time.
On the WHO standard the UK-WHO charts use, the median (50th centile) boy weighs 7.93 kg at 6 months and 9.65 kg at 12 months, and the median girl 7.30 kg and 8.95 kg. WHO’s birth medians are 3.35 kg for boys and 3.23 kg for girls, but the Red Book plots birth weight on UK90 data instead. Enter any age in the calculator to see the exact median and the full range of centiles.
Yes. The 25th centile is well within the normal band — it simply means the baby is a little lighter than average. A baby who stays steadily on the 25th line is growing normally. Health visitors watch the trend, not the number, so a consistent centile is reassuring even if it is low or high.
NICE guidance flags a fall across two or more centile spaces (one space if birth weight was below the 9th centile, three spaces if above the 91st), or a weight below the 2nd centile, as a reason for review. Newborns also normally lose up to 10% of their birth weight in the first days and regain it by about 3 weeks — more than 10% needs checking. Speak to your GP or health visitor if you are unsure.
Yes — boys are on average a little heavier, so they have their own chart. This calculator uses separate WHO data for boys and girls, so always select the correct sex. From 2 weeks to 4 years, the UK-WHO charts use the WHO standard, built from healthy term babies who were exclusively or predominantly breastfed for their first 4–6 months.
Use corrected age: take the number of weeks your baby was born before 40 weeks off their actual age. The UK-WHO chart guidance keeps correcting until 12 months of corrected age for babies born at 32–36 weeks, and until 24 months for babies born before 32 weeks. Enter the corrected age in weeks or months.
Each ounce is 0.0625 lb, so add the ounces as sixteenths — 8 lb 6 oz is 8 + 6⁄16 = 8.375 lb — then multiply pounds by 0.4536 to get kilograms (8.375 lb ≈ 3.80 kg). Or just pick “lb” in the calculator and enter the decimal pounds; it shows the result in both kg and lb/oz.
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