Baby Weight Percentile Calculator UK
This baby weight percentile calculator uses the UK-WHO growth charts from your baby’s red book: a 6-month-old boy weighing 7.9 kg is on about the 47th centile, close to the middle line. Enter your baby’s date of birth, the date they were weighed and their weight to see their centile.
It works from birth to 4 years and corrects for prematurity.
- ChartsUK-WHO 0–4 years
- Average newbornabout 3.4 – 3.6 kg
- Normal range2nd – 98th centile
Updated November 2026 · Uses UK-WHO growth chart data
Find your baby’s weight centile
Your result
Fill in the details, then press Calculate to see your result with the working shown.
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How UK-WHO baby weight centiles work
The red book growth charts for babies in the UK are the UK-WHO charts. From 2 weeks to 4 years they use the WHO growth standard, which describes how healthy breastfed babies grow. At birth they use UK90 data, because WHO birth data does not cover babies born at different gestations.
Each centile line shows where a given share of babies sit. On the 25th centile, a baby is heavier than 25 in 100 babies of the same age and sex and lighter than 75. The calculator converts weight into a z-score using the published L, M and S values for that age, then into a centile.
Average baby weight by age UK (50th centile)
| Age | Boys | Girls |
|---|---|---|
| Birth | 3.56 kg | 3.41 kg |
| 2 weeks | 3.75 kg | 3.57 kg |
| 6 weeks | 4.93 kg | 4.58 kg |
| 3 months | 6.37 kg | 5.84 kg |
| 6 months | 7.94 kg | 7.30 kg |
| 9 months | 8.90 kg | 8.23 kg |
| 12 months | 9.65 kg | 8.95 kg |
| 18 months | 10.94 kg | 10.23 kg |
| 2 years | 12.15 kg | 11.48 kg |
These are the median (M) weights from the UK-WHO data the calculator uses, rounded.
Premature babies and corrected age
For babies born before 37 weeks, health professionals plot weight at corrected age: their age minus the number of weeks they were born early. The RCPCH advises correcting until 1 year for babies born at 32 to 36 weeks and until 2 years for those born before 32 weeks. The calculator does this automatically.
When to talk to your health visitor
- Your baby’s weight drops through two or more centile spaces.
- Weight is below the 2nd centile or above the 98th.
- Your baby has not regained their birth weight by about 2 weeks.
- You have any worries about feeding.
Frequently asked questions
What is a good weight percentile for a baby?
There is no single good centile. Any centile from the 2nd to the 98th can be healthy if the baby grows steadily along roughly the same line.
What is the average weight of a 6-month-old baby UK?
On the UK-WHO charts the median at 6 months is about 7.9 kg for boys and 7.3 kg for girls.
Is it normal for a baby to drop centiles?
Some movement in the first weeks is common. A fall across two or more centile spaces is worth discussing with your health visitor.
Do I use corrected age for my premature baby?
Yes, if your baby was born before 37 weeks. Enter the weeks of pregnancy at birth and the calculator corrects their age.
Sources and method
We use official UK guidance and recognised formulas, and test each calculator against worked examples. Results are estimates to help you plan, not professional advice. See how we work.