Baby Weight Percentile Calculator
This calculator uses WHO growth standards to show where your baby's weight falls compared to other babies of the same age and sex. Remember — healthy babies come in all sizes.
What does a baby weight percentile actually mean?
A weight percentile shows how your baby's weight compares to a large reference population of healthy babies of the same age and sex. If your baby is at the 40th percentile, it means they weigh more than 40% of babies their age and less than the remaining 60% — nothing more and nothing less. The calculator uses the World Health Organization (WHO) Child Growth Standards, which were developed from data collected across six countries and are considered the international gold standard for infant growth tracking, particularly for breastfed babies. The 50th percentile is not a target — healthy babies naturally distribute across the full range, and a baby consistently tracking at the 15th percentile is just as healthy as one at the 85th, provided their curve is stable.
WHO weight percentiles by age — reference ranges
| Age | Girls 10th–90th | Girls 50th | Boys 10th–90th | Boys 50th |
|---|---|---|---|---|
| Birth | 2.8–3.9 kg | 3.2 kg | 2.9–4.0 kg | 3.3 kg |
| 1 month | 3.4–4.8 kg | 4.1 kg | 3.6–5.1 kg | 4.3 kg |
| 2 months | 4.2–5.9 kg | 5.0 kg | 4.4–6.2 kg | 5.3 kg |
| 4 months | 5.4–7.3 kg | 6.3 kg | 5.6–7.7 kg | 6.6 kg |
| 6 months | 6.1–8.2 kg | 7.1 kg | 6.4–8.6 kg | 7.4 kg |
| 9 months | 7.0–9.4 kg | 8.1 kg | 7.1–9.7 kg | 8.3 kg |
| 12 months | 7.8–10.5 kg | 9.0 kg | 7.8–10.8 kg | 9.2 kg |
| 24 months | 9.8–13.2 kg | 11.3 kg | 10.0–13.6 kg | 11.7 kg |
These are approximate ranges derived from WHO growth standards. Individual variation is normal. Always discuss your baby's growth pattern with your paediatrician or health visitor.
What your baby's percentile result means
- Consistently low percentile (below 10th). A baby who has always tracked around the 5th–9th percentile may simply be naturally small — especially if parents are small-framed. What matters is that the curve is stable. A consistent low percentile with normal feeding, wet nappies, and developmental milestones is reassuring. Bring it up at your next health visitor or paediatrician appointment for context.
- Consistently high percentile (above 90th). A baby tracking above the 90th percentile is not automatically overweight. Large babies are common in families with larger parents, and breastfed babies naturally regulate their own intake. Your health visitor will flag any clinical concern — a single high percentile reading is not cause for alarm.
- Stable percentile — any level. The most reassuring pattern is a baby who follows their own curve consistently over time. A baby at the 25th percentile at 2 months who is still at the 25th percentile at 6 months is growing exactly as expected for their individual trajectory.
- Crossing two or more major percentile lines downward. A significant drop — for example from the 75th percentile to below the 25th — over two or more measurements is the pattern that warrants investigation. This is called faltering growth and your paediatrician will want to assess feeding efficiency, absorption, and overall health.
Factors that influence where your baby sits on the chart
- Genetics.The single strongest predictor of a baby's natural size is parental size. Two smaller parents will typically have a smaller baby — and that baby's natural percentile may sit in the lower ranges throughout infancy without any clinical significance.
- Feeding method and frequency. Breastfed and formula-fed babies grow at slightly different rates, particularly after 3 months. The WHO charts are calibrated for breastfed babies and reflect a natural slowdown in weight gain between 3–6 months that can look alarming on older chart systems.
- Gestational age at birth. Premature babies are typically plotted on corrected age — their age adjusted for how early they were born — until at least 2 years. Using uncorrected age for a premature baby produces artificially low percentiles that do not reflect true growth.
- Birth weight. Very high or very low birth weight can affect early percentile placement before the baby settles onto their natural growth curve, which typically happens around 3–4 months.
- Illness or feeding disruption. A significant illness or feeding difficulty can cause a temporary drop in percentile. Once the issue resolves, most babies return to their established curve within a few weeks.
When to contact your health visitor or paediatrician
Contact your health visitor or paediatrician if your baby drops across two or more major percentile lines between measurements, if they are consistently producing fewer wet nappies than expected for their age, or if they seem uninterested in feeding, excessively sleepy, or are not meeting developmental milestones alongside the weight concern. A single low or high reading on its own is rarely a reason for urgent concern. Growth is a pattern, not a single data point — for a fuller picture, read our guide to understanding baby growth charts.