The Baby Weight Calculator checks whether your baby's weight (0-24 months) meets WHO growth standards.
Compares weight against the WHO median, -2SD, and +2SD thresholds based on age and sex. Classifies nutritional status as severely underweight, underweight, normal, or overweight.
Disclaimer: For comprehensive growth monitoring, visit your local health center or pediatrician regularly.
Calculator information
📋 How to use this calculator
- Choose the baby's sex (boy or girl) because the WHO growth curves differ for each.
- Enter the baby's age in months, from 0 to 24 months. For babies less than 1 month old, use 0.
- Enter the weight in kilograms to one decimal place, for example 7.4 kg. Weigh the baby without clothing or a diaper for accuracy.
- Click Calculate to compare the result against the WHO median (-2SD, median, +2SD).
- Check the nutrition classification: severely underweight (Z < -3), moderately underweight (-3 to -2 SD), normal (-2 to +2 SD), or risk of overweight (Z > +2).
- Record the result in your baby's well-child visit log and track the trend over time. Tip: consult your pediatrician if results fall outside the normal range on two consecutive measurements.
🧮 WHO Z-score (Weight-for-Age)
Z = (Child's weight - WHO Median) / WHO SD
- Child's weight = actual weight (kg)
- WHO Median = median weight at the same age and sex (kg)
- WHO SD = standard deviation (the difference between -2SD or +2SD and the median, divided by 2)
WHO classification: Z < -3 severely underweight, -3 <= Z < -2 moderately underweight, -2 <= Z <= 2 normal, Z > 2 risk of overweight.
💡 Worked example: Baby boy, 12 months old, weighing 8.5 kg
Given:- Sex = boy
- Age = 12 months
- Weight = 8.5 kg
- WHO median at 12 months = 9.6 kg
- -2SD = 7.7 kg, +2SD = 12.0 kg
Steps:- Compute SD: (12.0 - 9.6) / 2 = 1.2 kg
- Z-score = (8.5 - 9.6) / 1.2 = -0.92
- Because -2 < -0.92 < +2, the baby falls in the normal range.
Result: Normal nutritional status with a Z-score of -0.92; weight is within the WHO standard for a 12-month-old.
❓ Frequently asked questions
What is the difference between the WHO and CDC growth charts?
The WHO charts, released in 2006, are based on a multiethnic study of breastfed infants from 6 countries and represent how children should grow under optimal conditions. The CDC 2000 charts are based on a US population sampled with a mix of breastfed and formula-fed infants. The CDC and the American Academy of Pediatrics recommend using WHO charts for children 0-2 years old and CDC charts for children 2 years and older.
Can this calculator diagnose stunting?
No. Stunting is assessed from length or height for age (LFA or HFA), not weight. This calculator only evaluates weight for age (WFA), which reflects acute nutritional status. To screen for stunting, supine length should be measured for children under 2, and standing height for children 2 and older, then compared against the WHO or CDC length/height-for-age charts.
Why do boys and girls have different standards?
The WHO found that male infants are typically 0.3-0.7 kg heavier than female infants at the same age due to differences in muscle composition and hormones. For that reason the growth curves are separated, which makes nutritional assessment more accurate and avoids misclassifying healthy girls as underweight.
What if the baby was born premature?
For premature infants, use corrected age until 24 months. Calculation: chronological age minus the number of weeks born before 40 weeks. For example, a baby born at 32 weeks who is now 6 months old has a corrected age of 6 - 2 = 4 months. Consult your pediatrician for specialized monitoring, as the American Academy of Pediatrics recommends close follow-up for preterm infants.
How often should babies be weighed?
The American Academy of Pediatrics recommends well-child visits at 1, 2, 4, 6, 9, 12, 15, 18, and 24 months, with weight tracked at each visit. During the critical first 6 months, weighing every 2 weeks at home or at the pediatrician can be helpful. WHO-recommended minimum weight gain: 800 g/month in month 1, 600 g/month in months 2-3, and 400 g/month in months 4-6.
📚 Sources & references
Last updated: May 11, 2026