Baby Weight Calculator: Expected Weight Gain by Age
Tracking a baby's weight gain is one of the most important ways to monitor healthy growth and adequate feeding in the first year of life. Newborns typically lose a small amount of weight in the first few days after birth as they pass meconium and await the establishment of the mother's milk supply. This weight loss, up to 7 to 10 percent of birth weight, is considered normal. Most babies regain their birth weight by 10 to 14 days of age and then begin gaining steadily. The World Health Organization (WHO) Child Growth Standards, based on data from healthy, breastfed babies across six countries, describe expected average weekly weight gain rates that slow progressively as the baby grows: approximately 150 to 200 g per week in the first 3 months, around 100 g per week from 3 to 6 months, and 50 to 100 g per week from 6 to 12 months. This calculator estimates the expected weight range for a baby at a given age in weeks, based on birth weight and these WHO-derived average gain rates. It then compares the actual current weight to the expected range (midpoint plus or minus 15 percent). Results are approximate population estimates. Individual babies grow at different rates, and single-point weight measurements are less informative than growth trends tracked over time. Any concern about your baby's weight must be discussed with a pediatrician or other qualified healthcare provider.
Expected weight range at -- weeks: --.
How expected weight is calculated
The calculator uses the three-phase WHO-derived average weekly gain rates to estimate expected weight at the baby's current age. A tolerance range of plus or minus 15 percent around the midpoint reflects the natural variation in growth rates between healthy babies.
Phase 1 (weeks 0 to 12): gain_g += min(age_weeks, 12) x 175 g/week
Phase 2 (weeks 13 to 26): gain_g += max(0, min(age_weeks - 12, 14)) x 100 g/week
Phase 3 (weeks 27 to 52): gain_g += max(0, min(age_weeks - 26, 26)) x 75 g/week
Expected midpoint = birth_weight_g + gain_g
Expected low = midpoint x 0.85
Expected high = midpoint x 1.15
Note: at 0 to 2 weeks, normal weight loss of up to 10% of birth weight may be observed.
Worked example
Birth weight 3,400 g, current age 8 weeks:
- Phase 1 gain: 8 weeks x 175 g = 1,400 g
- Expected midpoint: 3,400 + 1,400 = 4,800 g (4.80 kg)
- Expected range: 4,080 g (4.08 kg) to 5,520 g (5.52 kg)
WHO average weekly weight gain rates (0 to 12 months)
| Age range | Approximate weekly gain | Notes |
|---|---|---|
| Weeks 0 to 2 | Weight loss expected (up to 10%) | Normal physiological weight loss; birth weight usually regained by 10 to 14 days |
| Weeks 0 to 12 (months 0 to 3) | 150 to 200 g/week | Fastest growth phase; midpoint used in this calculator: 175 g/week |
| Weeks 13 to 26 (months 3 to 6) | Approximately 100 g/week | Growth rate begins to slow |
| Weeks 27 to 52 (months 6 to 12) | 50 to 100 g/week | Further slowing; midpoint used: 75 g/week |
Rules of thumb (WHO/AAP): birth weight approximately doubles by 4 to 5 months; approximately triples by 12 months.
What affects baby weight gain?
Many factors influence how quickly an individual baby gains weight. These include:
- Feeding method: breastfed and formula-fed babies can have different growth patterns, particularly in the first 6 months. WHO growth charts are based on breastfed babies.
- Birth weight and gestational age: premature babies or those with low birth weight may have different growth trajectories and need specialist monitoring.
- Illness: any illness can temporarily reduce appetite and slow weight gain.
- Growth spurts: babies often go through periods of accelerated growth (around 3 weeks, 6 weeks, 3 months, and 6 months) followed by slower periods.
- Feeding effectiveness: for breastfed babies, inadequate latch, low milk supply, or infrequent feeding can reduce weight gain.
A single weight measurement is a snapshot. Your pediatrician will plot growth on a percentile chart over multiple visits to assess the overall growth trend, which is more informative than any single measurement.
Baby weight calculator: frequently asked questions
How much weight should a newborn gain per week?
According to the World Health Organization (WHO) and American Academy of Pediatrics (AAP), most newborns gain approximately 150 to 200 grams per week in the first 3 months of life. Before this gain begins, it is normal for newborns to lose up to 7 to 10 percent of birth weight in the first few days as they pass meconium and before the mother's milk comes in fully. Most babies regain their birth weight by 10 to 14 days of age.
When does a baby's weight gain slow down?
Weight gain gradually slows as babies grow older. From birth to 3 months, the gain is approximately 150 to 200 g per week. From 3 to 6 months (weeks 13 to 26), it slows to around 100 g per week. From 6 to 12 months (weeks 27 to 52), it typically slows further to approximately 50 to 100 g per week. As a general rule of thumb endorsed by the WHO and AAP, birth weight roughly doubles by 4 to 5 months and triples by 12 months.
What is the normal range for baby weight gain?
There is natural variation in how quickly individual babies grow. The WHO Child Growth Standards define expected ranges based on data from healthy, breastfed babies across multiple countries. A weight within roughly plus or minus 15 percent of the expected midpoint for age is generally considered within the normal range, though your pediatrician will assess growth using percentile charts that account for sex and gestational age. Individual feedings, illness, growth spurts, and feeding method all affect week-to-week weight.
Should I use grams or kilograms for this calculator?
You can enter weight in either grams or kilograms using the radio buttons. If you enter kilograms, the calculator converts to grams internally (1 kg = 1,000 g). Grams are more precise for newborns; kilograms become more practical as babies grow larger. Hospital records in the US, UK, and internationally commonly use grams for newborns and kilograms for older infants, so use whichever unit your records show.
What should I do if my baby's weight is outside the expected range?
If your baby's weight is below the expected range, do not adjust feeding without first consulting a pediatrician or lactation consultant. Possible causes include ineffective latch, insufficient milk supply, illness, or a feeding method change. If the weight is above range, this is not necessarily concerning and may simply reflect a healthy baby with a larger build, but your pediatrician can assess growth trajectory. Weight at a single point in time is less informative than the growth trend over multiple measurements. Always discuss concerns with a qualified healthcare provider.
Official sources
- WHO Child Growth Standards: WHO Child Growth Standards (weight-for-age).
- American Academy of Pediatrics: AAP HealthyChildren.org.
- CDC Growth Charts: CDC Clinical Growth Charts.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. General information, not medical advice. Always consult a qualified healthcare provider.