Baby Weight & Percentile Growth Calculator
Enter your baby's sex, age in months and weight to see roughly where they sit on the WHO weight-for-age growth curve, from newborn to twelve months.
Quick samples
Add age and weight to plot the percentile.
Enter your baby’s sex, age in months (0–12) and weight in pounds, and their estimated percentile appears here.
This compares your baby's weight to the WHO weight-for-age standards for their sex and age. A percentile is one snapshot; your own growth curve across visits matters far more than any single number.
The formula
Percentile = position of your baby's weight between the WHO 3rd, 50th and 97th centile curves for their sex and age
- WHO standards — The World Health Organization growth reference for healthy breastfed and formula-fed babies aged 0–24 months.
- 50th centile — The median — half of healthy babies of that age and sex weigh more, half less.
- 3rd / 97th — The usual outer edges; most healthy babies fall between them.
- Percentile — The share of same-age babies your child weighs more than — a rank, not a mark out of 100.
Approximate WHO median weight-for-age (kg)
| Age | Baby boys | Baby girls |
|---|---|---|
| 1 month | 4.5 kg | 4.2 kg |
| 3 months | 6.4 kg | 5.8 kg |
| 6 months | 7.9 kg | 7.3 kg |
| 12 months | 9.6 kg | 8.9 kg |
Median (50th centile) values for orientation. Most healthy babies fall between roughly the 3rd and 97th centiles around these points.
This calculator places your baby's weight on the World Health Organization growth curve. Enter their sex, age in months up to twelve, and current weight, and it estimates the percentile — roughly where they sit compared with healthy babies of the same age and sex.
Percentiles are one of the most misunderstood numbers in early parenting, so it is worth being clear about what one is: a ranking, not a score. A baby on the 40th percentile is doing exactly as well as one on the 80th; they simply weigh less than more of their peers.
The tool interpolates your baby's position between the published 3rd, 50th and 97th centile lines for their exact age, which turns a single weigh-in into a helpful sense of place on the curve while your health visitor tracks the fuller picture over time.
What a percentile actually means
A percentile compares your baby to a reference population of healthy same-age, same-sex children. On the 30th percentile means about thirty of every hundred such babies weigh less and seventy weigh more.
There is no better or worse end of the scale. A naturally small baby on the 15th and a naturally big baby on the 85th are both perfectly healthy if they are following their own steady curve.
What clinicians watch is consistency. A baby who tracks along the 25th line through their visits is reassuring; the same number is only a concern if the baby has slid off their own curve, up or down, which the tool cannot see from one weigh-in.
Why boys and girls have separate curves
The WHO publishes distinct weight-for-age curves for boys and girls because they grow differently even in the first weeks. At the same age, boys typically weigh a little more than girls, and the gap is consistent from birth.
Using the right curve keeps the percentile honest. Placing a girl against the boys' curve, or vice versa, would shift her apparent position for no real reason.
The sex difference is modest in the first three months and widens gently through the year, which is why this calculator needs your baby's sex and shows you the median it is comparing against. Using the wrong curve would misplace your baby by several percentile points, so this is one input that genuinely changes the answer rather than a cosmetic detail.
How this estimate is worked out
The WHO reference gives smooth 3rd, 50th and 97th centile weights for each month of the first year. The calculator finds the values for your baby's exact age, including part-months by gentle interpolation.
It then measures where your baby's weight falls relative to the median and the outer centiles and converts that into an approximate percentile, treating the spread between the 3rd, 50th and 97th lines as the shape of a normal growth distribution.
That is why the result is described as an estimate. A clinic plots your baby on the official chart to the tenth of a kilogram; this gives a reliable band and number from a home weigh-in, not a replacement for the measured growth record.
Reading your baby's result
If your baby sits in the middle of the range — say the 25th to 75th band — that is unremarkable and the most common place to be. There is nothing to correct or chase.
A very high or very low percentile is also often just your baby's normal, especially if both parents are large or small. One number tells you little on its own; the story is in how your baby's point moves across successive weigh-ins.
The centile becomes clinically interesting when a baby crosses several percentile lines in a short period, either stalling and dropping down or climbing steeply. That pattern, not a single reading, is what prompts a health visitor to look closer at feeding and growth.
Limits, and the 12-month boundary
These curves cover age nought to twelve months. After a first birthday in many countries care moves to CDC-style charts with height and head-circumference added, and weight alone becomes less informative than the ratio to length.
A weight percentile on its own cannot tell you whether a baby is proportionate. A baby on the 90th for weight who is also on the 90th for length is simply bigger, not overweight — something a single weight number cannot show.
This is why your baby is weighed length-for-weight too, and why the real value is the plotted series of points your health visitor keeps. Treat this tool as a helpful between-visit check-in rather than a verdict on your child's growth.
Key terms
- Weight-for-age
- How a baby's weight compares to typical weights for their exact age and sex.
- Centile / percentile
- A baby's rank among same-age peers — the 50th is the middle, not a target.
- WHO growth standards
- The international reference curves describing how healthy babies should grow, birth to two years.
- Growth curve
- The path a single baby traces across weigh-ins; its shape matters more than any one point.
Pro tips
- Weigh at a similar time of day, nappy off, for comparable points on your own curve.
- Follow the trend across visits, not a single percentile reading.
- Enter your baby's sex so the tool uses the correct growth curve.
Common mistakes
- Reading a low percentile as 'behind' when the baby is steadily tracking their own line.
- Judging a single home weigh-in instead of the plotted series of points.
- Using weight alone and forgetting length, which changes what the number means.
Baby weight percentile — your questions
Is a 50th percentile weight good for my baby?
The fiftieth percentile simply means your baby weighs more than about half of same-age, same-sex babies and less than the other half — the exact middle. It is not better than a 20th or an 80th. A healthy baby can sit anywhere along the curve; what matters is that they follow their own steady line over time rather than a particular number. There is no target percentile to hit, and being average is neither good nor bad — it is just average.
Why does the percentile change between visits?
Babies rarely climb a perfectly straight line, and small differences in the scale, the time of day, feeding before a weigh, or just growth spurts can shift a point up or down a little. That normal wobble can nudge the displayed percentile even when your baby is thriving. What clinicians focus on is the overall direction across several weigh-ins, not one movement, so a single bounce is rarely meaningful. A concerning change is a sustained slide across more than one centile space.
How accurate is this calculator versus a clinic?
It is a solid estimate from a home weigh-in, not a clinical measurement. The tool interpolates your baby's weight between published WHO 3rd, 50th and 97th centile values to produce an approximate percentile and band. A health visitor or GP uses calibrated scales, plots to the nearest tenth and adds length for a fuller picture. So use this to understand roughly where you sit and to satisfy curiosity between appointments, and rely on the official growth record for anything that informs care.
Should I worry if my baby is on a low percentile?
Usually not on its own. A baby can be perfectly healthy on the tenth percentile, particularly if the parents are naturally small and the baby is tracking their own steady curve and meeting developmental milestones. What would prompt attention is a baby who was previously higher and has dropped down across two or more centile spaces, or who is not gaining along any line. If your baby is feeding well, alert and following a consistent path, a low number is most likely just their normal.
Does this work after my baby's first birthday?
Not beyond twelve months, because this tool uses the WHO weight-for-age curves that run to about two years but are calibrated here for the first twelve months. After a first birthday, care generally shifts to charts that weigh a child against their length and use BMI-style measures rather than weight alone, since a tall toddler and a short one can share a weight and mean very different things. Enter ages nought to twelve here, and lean on your paediatrician for toddler growth.
Are breastfed and formula-fed babies measured differently?
No, and that is the point of the WHO standards. They were built from healthy breastfed babies in several countries, so they describe how babies should grow regardless of feeding method rather than how a past formula-feeding population did. As a result, a breastfed baby whose growth slows a little after three months is normal by these curves, not underperforming. The same chart applies to every feeding style, which makes comparisons fairer than older national charts that assumed more formula use.