Baby Weight Gain Calculator
Track weekly baby weight gain against healthy ranges by age. Enter values for instant results with step-by-step formulas.
Formula
Weekly Gain = (Current Weight - Birth Weight) / Age in Weeks
The average weekly weight gain is calculated by dividing total weight gained since birth by the baby's age in weeks. This is compared against WHO-recommended healthy gain ranges: 150-240 g/week for 0-4 weeks, 110-200 g/week for 4-16 weeks, and 80-150 g/week for 16+ weeks.
Worked Examples
Example 1: 8-Week-Old Boy Weight Check
Problem: A baby boy was born at 3.4 kg and weighs 5.0 kg at 8 weeks old. Is the weight gain healthy?
Solution: Total weight gain = 5.0 - 3.4 = 1.6 kg\nAverage weekly gain = 1.6 / 8 = 0.200 kg/week = 200 g/week\nHealthy range at 4-16 weeks: 110-200 g/week\nWHO median for 8-week boy: ~5.5 kg\nPercentage gain from birth: (1.6 / 3.4) x 100 = 47.1%\nProjected doubling: at ~17 weeks at current rate
Result: Gain: 200 g/week (Healthy) | Percentile: 30th-70th range
Example 2: 12-Week-Old Girl Growth Assessment
Problem: A baby girl born at 3.0 kg weighs 4.8 kg at 12 weeks. Evaluate the growth pattern.
Solution: Total gain = 4.8 - 3.0 = 1.8 kg\nWeekly average = 1.8 / 12 = 0.150 kg/week = 150 g/week\nHealthy range at 4-16 weeks: 110-200 g/week\nWHO median for 12-week girl: ~5.8 kg\nCurrent is below median but within healthy gain rate\nProjected double weight: ~20 weeks at current rate
Result: Gain: 150 g/week (Healthy) | Below median but normal trajectory
Frequently Asked Questions
How much weight should a baby gain per week?
Baby weight gain varies by age and follows a predictable pattern. In the first few days after birth, babies typically lose 5 to 10 percent of their birth weight, which is considered normal. They usually regain this weight by 10 to 14 days of age. During the first month, healthy babies gain approximately 150 to 240 grams (5 to 8 ounces) per week. From months one to four, the rate is about 110 to 200 grams per week. From four to six months, weight gain slows to approximately 80 to 150 grams per week. By five months, most babies have doubled their birth weight, and by their first birthday, they have typically tripled it. These are general guidelines, and individual babies may vary while still being perfectly healthy.
What factors influence a baby's weight gain?
Multiple factors contribute to how quickly a baby gains weight. Feeding method plays a significant role, as breastfed babies typically gain weight faster in the first few months but then slow down compared to formula-fed babies, who tend to gain more consistently. Feeding frequency and duration affect caloric intake directly. Genetics influence a baby's natural growth pattern, with larger parents tending to have larger babies. Gestational age matters because premature babies often need time to catch up. The baby's overall health, including any underlying medical conditions such as reflux, food intolerances, or metabolic disorders, can impact weight gain. Sleep patterns, activity level, and even environmental factors like temperature can influence caloric expenditure and weight gain rates.
When should parents be concerned about slow weight gain?
Parents should consult their pediatrician if their baby consistently gains less than the healthy range for their age, drops more than two percentile lines on the growth chart, or fails to regain birth weight by two weeks of age. Signs of inadequate weight gain include fewer than six wet diapers per day after day four, infrequent or very small bowel movements, persistent lethargy or irritability, and difficulty feeding. Slow weight gain can sometimes indicate insufficient milk supply in breastfeeding mothers, improper latch, tongue tie, or underlying health conditions. It is important not to compare your baby to other babies, as there is a wide range of normal. Regular pediatric checkups provide the best monitoring, as a single weight measurement is less informative than the overall growth trend over time.
How does feeding method affect baby weight gain patterns?
Breastfed and formula-fed babies follow different weight gain patterns that are both considered normal. Breastfed babies typically gain weight more rapidly in the first two to three months, often outpacing formula-fed babies. After three to four months, breastfed babies tend to gain weight more slowly, and by 12 months they are generally leaner than formula-fed babies. This is one reason the WHO developed separate growth standards based primarily on breastfed infants. Formula-fed babies tend to gain weight more steadily and may be heavier by 12 months. Combination-fed babies fall somewhere in between. Neither pattern is inherently better, and the growth standards reflect the breastfed pattern as the biological norm. Parents should use the WHO charts for breastfed babies and discuss any concerns about growth patterns with their pediatrician.
How is a baby's due date calculated?
Naegele's Rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). Alternatively, subtract 3 months from LMP, add 7 days, and advance the year by 1. Only about 5% of babies are born on the exact due date — the normal range is 37–42 weeks. Ultrasound measurements in the first trimester provide the most accurate dating.
Can I use the results for professional or academic purposes?
You may use the results for reference and educational purposes. For professional reports, academic papers, or critical decisions, we recommend verifying outputs against peer-reviewed sources or consulting a qualified expert in the relevant field.