Baby Weight Calculator — Free WHO Growth Percentile Tool
Check your baby's weight percentile using WHO growth standards. Compare your baby's weight to the median and percentile ranges for boys and girls from birth to 24 months.
Growth Assessment
Weight Percentile
15th - 50th
Your Baby
16.5 lbs
Median (50th)
17.4 lbs
WHO Percentile Range at 6 Months (Boys)
Summary: At 6 months, your boy weighs 16.5 lbs, placing them in the 15th - 50th percentile range according to WHO growth standards. The median weight for this age is 17.4 lbs. Growth should be tracked over time — a single measurement is less informative than a trend. Consult your pediatrician for personalized guidance.
How to Use the Baby Weight Calculator
- Select your unit system: Choose Imperial (pounds) or Metric (kilograms) for entering your baby's weight. Results will display in your chosen unit system.
- Select your baby's sex: Choose Boy or Girl. Growth standards differ between sexes because boys and girls have different average weights at each age. Selecting the correct sex ensures comparison to the right reference data.
- Enter your baby's age: Enter age in months, from 0 (newborn) to 24 months. Use the baby's actual age from birth for full-term babies. For premature babies under 2 years, consider using corrected (adjusted) age — consult your pediatrician about the appropriate age to enter.
- Enter your baby's weight: Enter the current weight. For the most accurate assessment, use a weight taken at your pediatrician's office, as clinical scales are more precise than home scales. If using a home scale, weigh without clothes and diaper for consistency.
The results show your baby's percentile range, how the weight compares to the median (50th percentile), the full percentile breakdown from 3rd to 97th for reference, and the difference from the median weight. A single weight measurement is less informative than a series of measurements over time. Track growth at regular well-child visits for the most meaningful assessment.
Growth Percentile Calculation Method
Percentile Assessment:
Compare Baby Weight to WHO reference values at closest age Reference Percentiles:
P3, P15, P50 (median), P85, P97 Variables Explained
- Percentile (P): Indicates what percentage of children of the same age and sex weigh less than your baby. The 50th percentile (P50) is the median. A child at P75 weighs more than 75% of peers. Percentiles between P3 and P97 are considered within the normal range.
- WHO Growth Standards: Based on the WHO Multicentre Growth Reference Study, which tracked over 8,000 healthy breastfed children from six countries. These standards describe how children should grow under optimal conditions, making them an aspirational reference rather than just a descriptive average.
- Median (P50): The middle value — half of children weigh more, half weigh less. The difference between your baby's weight and the median provides an intuitive measure of where they stand.
- Age Matching: The calculator finds the closest age in the reference data to your baby's age. For ages between standard data points (such as 13 or 14 months), the closest available reference is used.
Step-by-Step Example
Assess a 9-month-old boy weighing 20 lbs (9.07 kg):
- Find WHO reference for 9-month boys: P3=7.1, P15=7.9, P50=8.9, P85=9.9, P97=10.9 kg
- Compare 9.07 kg to reference: above P50 (8.9) but below P85 (9.9)
- Percentile range: 50th to 85th percentile
- Difference from median: 9.07 - 8.9 = +0.17 kg above median
- Status: Average range — healthy growth
Practical Examples
Example 1: Maya's 6-Month Checkup
Maya, a 6-month-old girl, weighs 16.1 lbs (7.3 kg) at her well-child visit. Her parents want to know how she compares to other babies her age.
- WHO reference for 6-month girls: P3=5.7, P15=6.5, P50=7.3, P85=8.2, P97=9.2 kg
- Maya's weight: 7.3 kg = exactly at the 50th percentile (median)
- Percentile range: 50th percentile
- Status: Average range
Maya is right at the median — exactly average for a 6-month-old girl. Her pediatrician notes that she has been consistently tracking along the 50th percentile since birth, which is a positive sign of steady, healthy growth. Maya is breastfed and recently started solid foods, and her growth pattern is typical for breastfed infants.
Example 2: James at 12 Months
James, a 12-month-old boy, weighs 24.5 lbs (11.1 kg). His parents notice he seems larger than other babies at daycare.
- WHO reference for 12-month boys: P3=7.7, P15=8.6, P50=9.6, P85=10.8, P97=11.8 kg
- James's weight: 11.1 kg — above P85 (10.8) but below P97 (11.8)
- Percentile range: 85th to 97th
- Status: Above average
James is on the higher end of the normal range. His pediatrician reviews his height percentile (also around the 85th) and notes that his weight is proportional to his height, indicating he is simply a bigger baby overall rather than overweight. Both parents are taller than average, and James has followed the 85th percentile consistently since 3 months. His growth is healthy and consistent.
Example 3: Lily, a Premature Baby at 4 Months
Lily was born at 34 weeks (6 weeks early). She is now 4 months actual age, which gives her a corrected age of about 2.5 months. She weighs 10 lbs (4.5 kg).
- Using corrected age (2.5 months, closest to 2 months): WHO girls P3=3.9, P15=4.5, P50=5.1, P85=5.8, P97=6.5 kg
- Lily at 4.5 kg: at the 15th percentile for corrected age
- Using actual age (4 months): WHO girls P50=6.4 kg — Lily would appear below P3
This example illustrates why corrected age is crucial for premature babies. Using actual age, Lily would appear severely underweight, but using corrected age, she is at the 15th percentile — within the normal range. Her neonatologist tracks her growth using corrected age until she turns 2, at which point the difference between actual and corrected age becomes negligible. Lily is showing appropriate catch-up growth for a premature baby.
WHO Weight Percentiles (Boys, Birth to 24 Months)
| Age | 3rd (kg) | 15th (kg) | 50th (kg) | 85th (kg) | 97th (kg) |
|---|---|---|---|---|---|
| Birth | 2.5 | 2.9 | 3.3 | 3.9 | 4.3 |
| 3 months | 5.0 | 5.7 | 6.4 | 7.2 | 7.9 |
| 6 months | 6.4 | 7.1 | 7.9 | 8.8 | 9.7 |
| 9 months | 7.1 | 7.9 | 8.9 | 9.9 | 10.9 |
| 12 months | 7.7 | 8.6 | 9.6 | 10.8 | 11.8 |
| 18 months | 8.8 | 9.8 | 10.9 | 12.2 | 13.4 |
| 24 months | 9.7 | 10.8 | 12.2 | 13.6 | 15.0 |
Tips and Complete Guide to Baby Weight Tracking
Why Growth Trends Matter More Than Single Measurements
A single weight measurement provides a snapshot, but the real value of growth monitoring lies in tracking the trend over time. A baby consistently at the 25th percentile is growing well — that is simply their growth pattern. A baby who drops from the 75th to the 25th percentile over several months needs evaluation, even though the 25th percentile is within the normal range. Similarly, a baby at the 3rd percentile who has always been at the 3rd percentile may be perfectly healthy, while a baby who drops to the 3rd from a higher percentile may have an underlying issue. This is why regular well-child visits are essential for meaningful growth assessment.
Feeding Milestones and Weight Gain
Feeding transitions affect weight gain patterns. Exclusive breast milk or formula provides all nutrition for the first 6 months. Around 6 months, introduction of complementary solid foods begins, supplementing (not replacing) milk feeds. Common first foods include iron-fortified cereal, pureed vegetables and fruits, and soft proteins. By 12 months, most babies eat a variety of table foods with continued breast milk or formula. The transition to whole cow's milk occurs at 12 months. Weight gain naturally slows in the second half of the first year as growth velocity decreases and babies become more active. This is normal and expected.
Weighing Your Baby Accurately at Home
If you choose to weigh your baby at home, consistency is key to meaningful tracking. Use the same scale each time, weigh at the same time of day (ideally before a feeding), remove the diaper and clothing for the most accurate measurement, and record the weight to compare over time. Baby scales designed for infants provide more accurate readings than adult bathroom scales. If using an adult scale, weigh yourself first, then weigh yourself holding the baby, and subtract the difference. Keep in mind that home scale readings may differ slightly from your pediatrician's calibrated scale, so note which scale was used for each measurement.
Growth and Developmental Milestones Together
Weight is just one aspect of healthy development. Your pediatrician also monitors length, head circumference, and developmental milestones during well-child visits. A baby who is gaining weight appropriately but not meeting motor or cognitive milestones may need evaluation for other conditions. Conversely, a baby who is slightly below average in weight but meeting all developmental milestones and showing a consistent growth pattern may simply be small. Our baby height calculator can help predict your child's expected adult height based on parental genetics.
Common Mistakes to Avoid
- Weighing too frequently and worrying about daily fluctuations: Baby weight varies by several ounces throughout the day based on feeding, diaper status, and hydration. Weekly or monthly trends are far more meaningful than daily measurements.
- Comparing your baby to other babies directly: Growth charts show population ranges. A baby at the 25th percentile is not "behind" a baby at the 75th — they are both growing normally. Compare your baby to their own growth curve, not to other children.
- Using actual age instead of corrected age for premature babies: Until age 2, premature babies should be assessed using corrected age. Using actual age makes them appear underweight when they may be growing perfectly for their developmental stage.
- Overfeeding to "catch up" to a higher percentile: A baby's natural growth percentile is largely determined by genetics. Forcing extra food to push a baby to a higher percentile can establish unhealthy eating patterns and increase obesity risk.
- Ignoring consistent growth pattern changes: If your baby crosses two or more major percentile lines (either up or down) over several months, discuss this with your pediatrician even if the current percentile is technically within the normal range.
Frequently Asked Questions
WHO growth chart percentiles describe how a child's weight compares to other children of the same age and sex worldwide. The charts are based on the WHO Multicentre Growth Reference Study, which tracked healthy breastfed children from six countries (Brazil, Ghana, India, Norway, Oman, and the United States) from birth to 5 years. A child at the 50th percentile weighs the same as or more than 50% of children their age. A child at the 15th percentile weighs more than 15% of peers but less than 85%. The WHO charts are considered the gold standard for children from birth to 2 years, while CDC charts are often used for children 2 years and older in the United States. Percentiles between the 3rd and 97th are generally considered within the normal range.
During the first 6 months, babies are typically weighed at every well-child visit, which occurs at 2 weeks, 1 month, 2 months, 4 months, and 6 months. From 6 to 12 months, visits occur at 6 months, 9 months, and 12 months. After the first year, visits are at 15, 18, and 24 months. Daily or weekly weighing at home is generally unnecessary and can cause unnecessary anxiety, as normal daily weight fluctuations from feeding, diaper status, and hydration can mask true growth trends. If your pediatrician has concerns about your baby's growth, they may recommend more frequent weight checks. Between well-child visits, focus on whether your baby seems alert, active, has adequate wet diapers (6 or more per day), and is meeting developmental milestones.
A weight below the 3rd percentile means your baby weighs less than 97% of children the same age and sex. This does not automatically indicate a problem — some healthy babies are simply small, especially if their parents are small. However, consistent weight below the 3rd percentile or dropping from a higher percentile to below the 3rd warrants evaluation by your pediatrician. Possible causes include insufficient caloric intake (underfeeding, difficulty breastfeeding, formula preparation errors), malabsorption conditions (celiac disease, cow's milk protein allergy), chronic illness, genetic conditions, or a naturally small body type. Your pediatrician will evaluate feeding patterns, developmental milestones, and overall health to determine whether intervention is needed.
Some percentile shifting is normal, especially during the first 2 years of life. Many babies shift from their birth percentile to their genetically determined growth channel during the first 6 to 18 months. A baby born at the 75th percentile to smaller parents may gradually shift to the 50th or 25th percentile, which is appropriate genetic adjustment rather than failure to thrive. However, crossing two or more major percentile lines (for example, dropping from the 75th to the 15th) over a short period is a concern and should be evaluated. Similarly, rapid upward percentile crossing may warrant attention. The overall trend matters more than any single measurement. Your pediatrician uses serial weights plotted over time to assess growth patterns.
Breastfed and formula-fed babies grow differently. Breastfed babies tend to gain weight faster in the first 3 to 4 months but then slow down compared to formula-fed babies. By 12 months, formula-fed babies tend to weigh slightly more on average. This difference is why the WHO growth charts (based on breastfed babies) are recommended for all infants from birth to 2 years, regardless of feeding method. If your breastfed baby appears to slow their weight gain around 4 to 6 months, this is usually a normal pattern and not cause for concern unless they are crossing multiple percentile lines downward. The introduction of solid foods around 6 months typically supports continued healthy growth regardless of milk source.
Average weight gain follows a predictable pattern: birth to 3 months, babies gain about 5 to 7 ounces (150 to 200 grams) per week. From 3 to 6 months, gain slows to about 4 to 5 ounces (110 to 140 grams) per week. From 6 to 12 months, gain further slows to about 2 to 4 ounces (55 to 110 grams) per week. Most babies double their birth weight by 4 to 5 months and triple it by 12 months. A baby born at 7.5 pounds would typically weigh about 15 pounds at 5 months and 22.5 pounds at 12 months. These are averages — individual babies may gain faster or slower while remaining perfectly healthy, as long as they follow their own growth curve consistently.
Contact your pediatrician if your baby loses more than 7% to 10% of birth weight in the first few days (some loss is normal, but excessive loss needs evaluation), has not regained birth weight by 2 weeks of age, consistently falls below the 3rd percentile or above the 97th, drops across two or more major percentile lines on the growth chart, has fewer than 6 wet diapers per day after the first week, seems lethargic or disinterested in feeding, or shows signs of dehydration (dry mouth, no tears, sunken soft spot). Weight concerns should always be evaluated in context — your pediatrician will consider the baby's overall health, feeding patterns, developmental milestones, and family size patterns before making recommendations.
Premature babies need corrected age (also called adjusted age) for growth chart plotting until age 2. Corrected age is calculated by subtracting the number of weeks premature from the baby's actual age. A baby born 8 weeks early who is now 6 months old has a corrected age of 4 months — so their weight should be compared to 4-month standards on the growth chart. Premature babies often follow different growth trajectories than full-term babies, with many experiencing catch-up growth during the first 2 years. Some remain on lower percentile lines, which is acceptable if they follow their own growth curve consistently. Your pediatrician or neonatologist will guide which growth charts and age corrections are appropriate for your premature baby.
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Disclaimer: This calculator is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional for medical guidance.
Last updated: February 23, 2026
Sources
- World Health Organization — Child Growth Standards: who.int/tools/child-growth-standards
- Centers for Disease Control and Prevention — Growth Charts: cdc.gov/growthcharts
- National Institute of Child Health and Human Development — Child Growth: nichd.nih.gov/health/topics