Baby Weight Percentile
Where your baby sits on the growth chart
What a percentile actually means
A percentile tells you how your baby's weight compares with a reference population of healthy babies the same age and sex. If your baby is on the 40th percentile, roughly 40% of babies weigh less and 60% weigh more.
The single most important thing to understand is that a percentile is not a grade. The 90th percentile is not better than the 10th. Both are entirely normal positions. Babies are meant to be different sizes, and someone has to be at each end of a healthy range.
This calculator uses the WHO growth standards, which describe how healthy, well-fed children grow across a range of countries.
The trend matters far more than the number
What paediatricians actually watch is whether a baby is following their own curve consistently over time.
A baby steadily on the 15th percentile from birth is growing exactly as expected. A baby who drops from the 75th to the 25th over a few months is the one who prompts a closer look — not because a lower percentile is bad, but because crossing several lines can indicate a feeding or health issue worth investigating.
Some crossing is normal, particularly in the first few months as babies settle onto their genetically determined curve rather than the one determined by conditions in the womb. A single measurement, taken once, tells you almost nothing.
Things that shift the reading
Measurement conditions affect the number more than people expect. Whether your baby was weighed before or after a feed can shift the result meaningfully, as can a full or recently changed nappy, different scales, and clothing.
For a like-for-like comparison, weigh at a similar time of day, on the same scales, in similar conditions. Small differences between visits are usually measurement variation rather than real change.
Premature babies are typically plotted using corrected age — age from the due date rather than the birth date — for roughly the first two years. Using uncorrected age makes a premature baby look as though they are falling behind when they are growing appropriately.
Breastfed and formula-fed growth patterns differ
Breastfed babies often gain slightly faster in the first two or three months and then slower from around four to six months, compared with formula-fed babies. This is a normal difference in pattern, not a problem with either.
The WHO standards are based on breastfed infants, which is why some breastfed babies appear to “drop” on older charts based largely on formula-fed populations. If you are comparing charts from different sources and seeing different answers, this is often why.
When a percentile is worth discussing
Raise it with your health visitor, GP, or paediatrician if your baby's weight crosses two or more major percentile lines in either direction, if weight gain has stalled or reversed, or if a low percentile comes alongside other signs such as fewer wet nappies, lethargy, or feeding difficulty.
Very high or very low percentiles are usually just normal variation, especially where they run in the family. Small parents often have small babies. Growth is assessed as a whole picture — weight alongside length, head circumference, development, and how your baby is feeding and behaving.
If a chart is causing you anxiety, it is worth saying so at your next appointment. Growth charts are a screening tool intended to help, and a professional who can see the whole trend will give you a far more meaningful answer than a single plotted point.
Common Questions
Is a high percentile better than a low one?
No. Percentiles describe position within a healthy range, not quality. A baby consistently on the 10th percentile is growing just as normally as one on the 90th.
What percentile should my baby be on?
There is no target percentile. What matters is that your baby follows their own curve reasonably consistently over time rather than crossing several lines.
Why did my baby's percentile drop?
Some movement is normal, particularly in the first months as babies settle onto their genetic growth pattern. Crossing two or more major lines is worth discussing with your health visitor or paediatrician.
Do premature babies use the same chart?
They are usually plotted using corrected age - age counted from the due date rather than the birth date - for roughly the first two years, so growth is assessed appropriately.