Baby Weight Percentile Calculator
The baby weight percentile calculator shows where your baby's weight falls compared with the WHO standards for the same sex and age (birth to 24 months). Enter age and weight in pounds and ounces or kilograms to see the percentile, z-score, median weight and the usual 3rd to 97th percentile range.
Type the problems from this lesson and check each step.
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How to use the Baby Weight Percentile Calculator
- Choose your baby's sex.
- Enter age in months and extra days. Use adjusted age for babies born early.
- Enter weight: pounds and ounces, or kilograms.
- Press Calculate to see the percentile, z-score and the median weight for age.
Formula and method
The WHO publishes three numbers for each sex at each age: L (skewness), M (median) and S (coefficient of variation). The calculator interpolates them to your baby's exact age in months and days and computes z = ((measurement / M)L − 1) / (L × S), or ln(measurement / M) / S when L is 0. The percentile is the normal distribution function of z times 100. A z-score of 0 is the 50th percentile, −2 is about the 2nd and +2 is about the 98th.
The WHO tables used here are the monthly LMS values from birth to 24 months. Between whole months the calculator interpolates linearly, so results can differ from the WHO daily tables by a small fraction of a percentile point.
Average baby weight by age: boys
| Age | 5th pct | 25th pct | 50th (median) | 75th pct | 95th pct |
|---|---|---|---|---|---|
| Birth | 2.60 kg | 3.03 kg | 3.35 kg (7.4 lb) | 3.69 kg | 4.21 kg |
| 1 months | 3.57 kg | 4.08 kg | 4.47 kg (9.9 lb) | 4.89 kg | 5.54 kg |
| 2 months | 4.52 kg | 5.12 kg | 5.57 kg (12.3 lb) | 6.05 kg | 6.80 kg |
| 3 months | 5.24 kg | 5.89 kg | 6.38 kg (14.1 lb) | 6.90 kg | 7.71 kg |
| 4 months | 5.80 kg | 6.48 kg | 7.00 kg (15.4 lb) | 7.55 kg | 8.41 kg |
| 5 months | 6.24 kg | 6.97 kg | 7.51 kg (16.6 lb) | 8.09 kg | 8.99 kg |
| 6 months | 6.61 kg | 7.37 kg | 7.93 kg (17.5 lb) | 8.54 kg | 9.48 kg |
| 7 months | 6.92 kg | 7.71 kg | 8.30 kg (18.3 lb) | 8.93 kg | 9.91 kg |
| 8 months | 7.19 kg | 8.00 kg | 8.61 kg (19.0 lb) | 9.27 kg | 10.29 kg |
| 9 months | 7.43 kg | 8.27 kg | 8.90 kg (19.6 lb) | 9.58 kg | 10.63 kg |
| 10 months | 7.65 kg | 8.51 kg | 9.16 kg (20.2 lb) | 9.86 kg | 10.95 kg |
| 11 months | 7.86 kg | 8.74 kg | 9.41 kg (20.8 lb) | 10.13 kg | 11.25 kg |
| 12 months | 8.05 kg | 8.96 kg | 9.65 kg (21.3 lb) | 10.38 kg | 11.54 kg |
| 15 months | 8.60 kg | 9.57 kg | 10.31 kg (22.7 lb) | 11.10 kg | 12.35 kg |
| 18 months | 9.11 kg | 10.15 kg | 10.94 kg (24.1 lb) | 11.79 kg | 13.13 kg |
| 21 months | 9.60 kg | 10.70 kg | 11.55 kg (25.5 lb) | 12.46 kg | 13.90 kg |
| 24 months | 10.07 kg | 11.25 kg | 12.15 kg (26.8 lb) | 13.12 kg | 14.66 kg |
Average baby weight by age: girls
| Age | 5th pct | 25th pct | 50th (median) | 75th pct | 95th pct |
|---|---|---|---|---|---|
| Birth | 2.53 kg | 2.93 kg | 3.23 kg (7.1 lb) | 3.55 kg | 4.04 kg |
| 1 months | 3.33 kg | 3.81 kg | 4.19 kg (9.2 lb) | 4.59 kg | 5.22 kg |
| 2 months | 4.13 kg | 4.69 kg | 5.13 kg (11.3 lb) | 5.59 kg | 6.34 kg |
| 3 months | 4.75 kg | 5.37 kg | 5.85 kg (12.9 lb) | 6.36 kg | 7.19 kg |
| 4 months | 5.24 kg | 5.91 kg | 6.42 kg (14.2 lb) | 6.98 kg | 7.88 kg |
| 5 months | 5.64 kg | 6.35 kg | 6.90 kg (15.2 lb) | 7.49 kg | 8.45 kg |
| 6 months | 5.98 kg | 6.72 kg | 7.30 kg (16.1 lb) | 7.92 kg | 8.93 kg |
| 7 months | 6.27 kg | 7.04 kg | 7.64 kg (16.8 lb) | 8.30 kg | 9.36 kg |
| 8 months | 6.52 kg | 7.32 kg | 7.95 kg (17.5 lb) | 8.63 kg | 9.74 kg |
| 9 months | 6.75 kg | 7.58 kg | 8.23 kg (18.1 lb) | 8.94 kg | 10.08 kg |
| 10 months | 6.96 kg | 7.81 kg | 8.48 kg (18.7 lb) | 9.21 kg | 10.40 kg |
| 11 months | 7.15 kg | 8.03 kg | 8.72 kg (19.2 lb) | 9.48 kg | 10.71 kg |
| 12 months | 7.34 kg | 8.24 kg | 8.95 kg (19.7 lb) | 9.73 kg | 11.00 kg |
| 15 months | 7.88 kg | 8.84 kg | 9.60 kg (21.2 lb) | 10.44 kg | 11.81 kg |
| 18 months | 8.40 kg | 9.42 kg | 10.23 kg (22.6 lb) | 11.13 kg | 12.60 kg |
| 21 months | 8.91 kg | 10.00 kg | 10.85 kg (23.9 lb) | 11.81 kg | 13.38 kg |
| 24 months | 9.41 kg | 10.56 kg | 11.47 kg (25.3 lb) | 12.49 kg | 14.16 kg |
How much weight do babies gain each month?
Typical monthly gain (change in the WHO median from one month to the next) is largest in the first three months and slows over the first year. Individual babies gain in bursts, so a single month tells you little.
| Month | Boys: typical gain (median to median) | Girls: typical gain (median to median) |
|---|---|---|
| 0 to 1 months | 1124 g | 955 g |
| 1 to 2 months | 1095 g | 940 g |
| 2 to 3 months | 809 g | 718 g |
| 3 to 4 months | 627 g | 578 g |
| 4 to 5 months | 508 g | 475 g |
| 5 to 6 months | 424 g | 399 g |
| 6 to 7 months | 363 g | 345 g |
| 7 to 8 months | 318 g | 306 g |
| 8 to 9 months | 286 g | 277 g |
| 9 to 10 months | 264 g | 255 g |
| 10 to 11 months | 247 g | 239 g |
| 11 to 12 months | 236 g | 229 g |
Newborn weight loss and regaining birth weight
Most newborns lose some weight in the first days and regain birth weight by about 10 to 14 days. Your baby's care team checks that trend. The percentile on this page is meant for later checks, after the early newborn period.
What a baby weight percentile means
A baby at the 40th percentile weighs the same as or more than 40 out of 100 babies of the same age and sex. It is a comparison, not a score. Healthy babies are found across the 3rd to 97th percentile range.
Low and high weight percentiles
A baby below the 3rd percentile or above the 97th deserves a closer look from the doctor, including feeding, medical history and growth over time. Many small or large babies are simply following their family patterns. A drop across two major percentile lines is more concerning than one low reading.
Breastfed and formula-fed babies
The WHO standards are based on mostly breastfed babies. Formula-fed babies often gain a little faster in the first year. Use the Baby Formula Calculator to estimate daily formula amounts, and rely on your baby's doctor for feeding concerns.
Premature babies and adjusted age
Babies born before 37 weeks are usually plotted by adjusted (corrected) age until about 24 months. Adjusted age is the age since birth minus the number of weeks early. Enter the adjusted age in the age boxes and ask your baby's doctor how they interpret the result, because very preterm babies often follow their own curves. The Adjusted Age Calculator works out the adjusted age for you.
How to weigh a baby at home
Use the same scale each time, with the baby undressed or in a clean dry diaper (weigh the diaper separately if needed). Home scales are less accurate than clinic scales, so compare readings from the same scale over time and treat small differences with caution.
When to call the doctor
Call your baby's doctor or nurse if a measurement is below the 3rd or above the 97th percentile, if a percentile has crossed two major lines (for example from the 50th to the 10th), if your baby is not regaining birth weight by about two weeks, is feeding poorly, has fewer wet diapers than usual, seems unusually sleepy or fussy, or if you are worried for any reason. Do not wait for the next routine visit if something seems wrong.
Frequently asked questions
What is a normal weight for a 6-month-old baby?
The WHO median is about 7.9 kg (17.5 lb) for boys and 7.3 kg (16.1 lb) for girls; the 5th to 95th percentile range is wider, as the tables show.
Is the 10th percentile for weight too low?
No. About one in ten healthy babies is at or below it. Doctors watch for sustained drops across lines.
How often should a baby be weighed?
At routine well-baby visits and as your doctor advises. More frequent checks are common in the first weeks.
Can I use pounds and ounces?
Yes. Choose pounds and put the ounces in the ounces box.
Does this work for premature babies?
Use adjusted age and talk to your doctor, because very preterm babies may follow their own curves.
Why does my baby's percentile change?
Babies grow in spurts, and feeding, illness and sleep can move weight for a few weeks. Persistent changes are worth discussing with a doctor.
Sources
- WHO Child Growth Standards
- CDC: WHO Growth Charts for children from birth to 24 months
- CDC: Growth chart percentile data files with LMS values
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