Baby Weight Calculator
Estimated fetal weight by gestational age, based on a Hadlock-curve approximation of the standard growth chart.
Enter gestational age in weeks. The 50th percentile estimate and a 10th to 90th percentile range are returned. Educational only, not a clinical assessment.
How is this calculated?
Quadratic fit to the Hadlock 50th-percentile EFW reference: log10(EFW grams) = 0.18469 + 0.14876 · ga - 0.00163 · ga², where ga is gestational age in weeks. Calibrated against published Hadlock anchor weights (15w 110g, 20w 330g, 30w 1500g, 40w 3400g). Percentile band uses ±15% as a simplified reference; clinical charts use sex-specific standards.
About this tool
A baby weight calculator places a weight on the standardised infant growth curve so you can read off the percentile (the percentage of healthy reference children below it at the same age and sex). This page covers the prenatal Hadlock widget and postnatal WHO lookup.
How it works
The widget above estimates fetal weight from gestational age using a quadratic fit to Hadlock anchor points. After birth, the math changes: there is no formula. The child is weighed on a calibrated scale and that weight is looked up in the WHO Child Growth Standards reference table for the child's age in completed months and biological sex.
The WHO tables were built from the Multicentre Growth Reference Study (1997-2003), a longitudinal study of about 8,400 healthy, breastfed infants across Brazil, Ghana, India, Norway, Oman, and the US. They are a prescriptive standard (how children should grow), not a descriptive average. CDC charts, by contrast, are a US national reference and skew higher.
The lookup is mechanical. For a 6-month-old boy weighing 7.9 kg, you read the boys row at 6 months: the 50th percentile is 7.9 kg, so the child is at the median.
Worked example
A 12-month-old girl weighs 9.2 kg at her one-year visit. Where does she sit on the WHO curve?
- Reference row: WHO girls at 12 months: 3rd = 7.0 kg, 50th = 8.9 kg, 97th = 11.5 kg.
- Position: 9.2 kg is just above the 50th and well below the 85th, so roughly the 60th percentile.
- Trajectory: previous visits showed 50th and 55th. Steady tracking near the median.
- Action: no concern. Monitor at the next visit.
WHO weight percentiles at key ages
WHO Child Growth Standards weight-for-age in kilograms at the 3rd, 50th, and 97th percentiles, birth to 24 months.
| Age | Boys 3rd | Boys 50th | Boys 97th | Girls 3rd | Girls 50th | Girls 97th |
|---|---|---|---|---|---|---|
| Birth | 2.5 kg | 3.3 kg | 4.3 kg | 2.4 kg | 3.2 kg | 4.2 kg |
| 1 month | 3.4 kg | 4.5 kg | 5.7 kg | 3.2 kg | 4.2 kg | 5.4 kg |
| 3 months | 5.1 kg | 6.4 kg | 7.9 kg | 4.6 kg | 5.8 kg | 7.4 kg |
| 6 months | 6.4 kg | 7.9 kg | 9.7 kg | 5.8 kg | 7.3 kg | 9.1 kg |
| 12 months | 7.7 kg | 9.6 kg | 11.8 kg | 7.0 kg | 8.9 kg | 11.5 kg |
| 24 months | 9.7 kg | 12.2 kg | 15.3 kg | 9.0 kg | 11.5 kg | 14.8 kg |
Common questions and limitations
- Percentile is population-relative, not health-deterministic. A child on the 5th is simply lighter than 95 percent of the reference group. By design, 5 percent of healthy children sit there.
- Trajectory beats absolute weight. Pediatricians worry about a sudden drop across percentile bands (50th to 10th in months), not the absolute number.
- Calculator is educational, not clinical. For care decisions consult your pediatrician, who measures length and head circumference and reviews the chart trend.
- WHO vs CDC vs national charts. WHO standards apply globally and assume breastfeeding; CDC charts skew higher. UK uses WHO; India tends to use IAP charts.
- Premature infants need a different curve. Babies born before 37 weeks use corrected-age plots or the INTERGROWTH-21st preterm standards until around 24 months corrected age.
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Frequently asked questions
What does the percentile actually mean?
The percentile shows where a child sits versus the WHO reference of healthy, breastfed infants. A boy on the 50th at 6 months weighs the median. The 3rd is lighter than 97 percent of the reference group. It is a position, not a grade.
Is a low percentile dangerous?
Not by itself. By definition 3 percent of healthy children sit below the 3rd. Pediatricians watch trajectory across visits. A baby tracking the 5th steadily is usually fine; one dropping 50th to 5th in three months needs evaluation.
Why are boys and girls measured separately?
Sex-linked hormones and lean-mass growth diverge from the first weeks. At every age from birth to 24 months, boys at the same percentile weigh slightly more than girls. WHO publishes separate tables so neither sex is flagged against a mixed average.
How is this different from a fetal weight estimate?
A fetal estimate uses ultrasound measurements and a Hadlock regression to predict in-utero weight. The widget above is that approximation for 10 to 42 weeks. After birth, WHO percentile lookup replaces the regression: the child is weighed directly.
Should I worry about a single high or low reading?
A single measurement is rarely informative. Scales differ, clothing varies, and feeding times shift the number. Pediatricians use three or more points to draw a curve, and slope matters more than any one dot. If concerned, the next step is your pediatrician.
Sources and further reading
- WHO (2006) WHO Child Growth Standards: weight-for-age, length-for-age, BMI-for-age. Methods and development, Geneva.
- WHO MGRS Group (2006), Acta Paediatrica Supplement 450, on the underlying multicentre reference study.
- CDC (2010) Use of WHO and CDC Growth Charts for Children Aged 0-59 Months in the US, MMWR 59(RR-9).
- Hadlock, F.P. et al. (1991) In utero analysis of fetal growth: a sonographic weight standard, Radiology 181(1): 129-133, for the widget's fetal regression.
- Villar, J. et al. (2015) INTERGROWTH-21st postnatal growth standards for preterm infants, Lancet Global Health 3(11).