Child growth
Weight-for-Age Percentile Calculator
Enter sex, age, and weight. Under 24 months the tool uses WHO Child Growth Standards. From 24 months it uses CDC 2000 weight-for-age. The highlighted result is the percentile, with a z-score beside it.
Reviewed September 2026
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What weight-for-age is for, and what it is not
Weight-for-age asks how a child's weight compares with children of the same sex and age. It does not say whether that weight is high or low for the child's length. A short heavy infant and a tall light infant can share a weight percentile and need opposite follow-up.
From birth through 23 completed months this page uses WHO Child Growth Standards. At 24 months it switches to CDC 2000 weight-for-age through 20 years. That is the US clinic split CDC publishes: WHO to age 2, CDC after.
WHO calls underweight a weight-for-age z-score below -2, and severely underweight below -3. High weight-for-age above +2 is a flag to open weight-for-length for infants, not a WHO overweight diagnosis on this chart alone.
How to use the weight-for-age calculator
Choose units and sex. Enter completed years and extra months. A 14-month-old is 1 year and 2 months. Enter the weight from today's scale, not last month's well visit if the child has been ill or on a catch-up plan.
The highlighted result is the percentile. Z-score and a short reading sit under it. The chart name on the last line tells you whether WHO or CDC supplied L, M, and S.
Naked or dry-diaper weights are what clinics want for infants. A soaked diaper can add 0.2 to 0.4 lb, enough to nudge a small infant across a z-score line. School-age kids can keep underwear and a shirt.
If two caregivers weigh on different bathroom scales, pick one scale and stick with it for home tracking. Percentile jumps from scale shopping are not growth. The clinic scale is still the one that belongs on the medical record.
Do not enter birth weight unless the child is still a newborn. A 3.4 kg birth weight typed in at 9 months will print a severe underweight reading that is about the keyboard, not the baby.
LMS formula and the 24-month switch
Convert pounds to kilograms (lb x 0.45359237) when you use US units. Z = ((kg / M)^L - 1) / (L x S) at that sex and age. Percentile is the normal CDF of Z.
WHO rows in this tool are monthly from 0 through 24. CDC rows run 24 to 240 months, looked up at the CDC half-month convention. A child at exactly 24 months uses CDC, matching the "2 years and older" CDC charts.
Interpolation is linear in L, M, and S between neighboring rows. That is the method CDC describes when you need a finer age than the published month.
Which weight-for-age table this calculator loads
| Completed age | Table | Overweight screen lives on | Underweight flag on this page |
|---|---|---|---|
| 0 to 23 months | WHO weight-for-age | Weight-for-length | Z below -2 (severe below -3) |
| 24 months to 20 years | CDC 2000 weight-for-age | BMI-for-age from age 2 | Low percentile band, especially below 3rd |
| Past 20 years | None | Adult tools | This page stops |
Worked example: a 12-month-old boy at 21 lb
Boy, 1 year, 0 extra months, 21 lb. That is 9.53 kg. WHO boys' weight-for-age median at 12 months is near 9.6 kg on the CDC-hosted WHO file. Expect a percentile near the 50th and a z-score near 0.
If the same boy weighs 16 lb (7.26 kg), Z falls well below -2 on WHO tables. The reading prints underweight or severely underweight. That is a clinic call the same week, along with length-for-age, not a "wait for the next checkup" shrug.
At 8 years 0 months, 56 lb is 25.4 kg. CDC weight-for-age for boys near 96 months has a median in the mid-25 kg range. The percentile should land near the middle, independent of whether 56 lb sounds "small" to an adult relative.
A 15-year-old who still uses the default 21 lb infant weight will plot at an extreme low CDC percentile. Change the years field first. Extra months still matter in adolescence because CDC tables are monthly, but a wrong year dwarfs a wrong month.
Underweight here versus wasting on weight-for-length
Wasting is low weight for length. Underweight is low weight for age. A stunted infant can be underweight on this chart and still have a normal weight-for-length. Treating that child as "too skinny to eat more" because WFL looks fine misses the length problem.
CDC's WHO training page puts the infant body-size cutoffs on weight-for-length (2nd and 98th percentiles on the CDC-adapted WHO plots, which match about +/-2 SD). Use this page for the age comparison, then run weight-for-length before you decide an infant is overweight.
Failure to thrive is a clinical pattern, not a single percentile. Some teams still use downward crossing of two major percentile lines plus intake and exam. This calculator prints one point. It cannot see the last four well visits.
Refeeding after gastroenteritis can jump weight-for-age in a week. Plot after the child is hydrated and eating usual food, not on the afternoon of the emergency visit if you can wait for the follow-up scale.
Scale habits that keep the percentile honest
Infant scales should read to 0.01 kg or 0.5 oz if you have the choice. Place the scale on a hard floor. A rug under an infant scale is a common home error.
If you weigh yourself holding the baby, subtract a self-weight taken immediately after, same clothes. Two bathroom-scale readings have more noise than a clinic infant scale. Treat home percentiles as a draft for the next visit.
Zero the infant scale with the blanket on it if you will weigh the baby in that blanket. Forgetting the tare is a classic 200 g error, about half a pound, which is a real z-score move in a 4 kg infant.
Using the percentile without turning it into a diet
A child tracking the 15th percentile for two years with a matching height percentile is often a small kid, not a feeding emergency. A child who was at the 60th and is now at the 10th in four months is a different story even if 10th is still "on the chart."
Juice and snack sugar do not belong in this formula. They belong on the juice-limit and lunchbox tools if intake is the question after a clinician looks at growth.
A high weight-for-age with a matching high length-for-age is often a large child. Open weight-for-length anyway if the child is under 2. That is the chart that answers "too heavy for this length."
School-age CDC weight-for-age does not replace BMI-for-age. A 10-year-old can sit on the 90th for weight and the 90th for height and still have a healthy BMI, or not. Run BMI-for-age after this page for anyone 2 or older.
Accuracy limits, edge cases, and safety
Premature infants are often plotted on corrected-age charts in clinic until about 2 years. This tool uses calendar age. If the child was born 8 weeks early, ask the pediatrician which age they plot before you compare home numbers with the well-visit paper.
Edema, casts, and heavy braces add weight the LMS table cannot see. Fluid-overloaded weights are not "healthy weight-for-age."
WHO weight-for-age is a standard for how children should grow under good conditions, including a breastfeeding-heavy sample. CDC weight-for-age is a US reference of how children did grow. Crossing 24 months can move the percentile even if grams did not change that week.
Casts, wheelchair scales, and amputations need a clinic protocol. A bathroom scale that only caught one foot is not a WHO data point.
Do not add rice cereal to bottles to "fix" a low percentile without a clinician. That pattern raises choking and overweight risk and does not treat the cause.
Same 21 lb, different questions
| Child | Weight-for-age job | Do not skip | Wrong conclusion to avoid |
|---|---|---|---|
| 12-month boy, 21 lb, average length | Near-median WHO weight-for-age | Weight-for-length still, once | Calling 21 lb "huge" because an adult cousin was 21 lb at 6 months |
| 12-month boy, 21 lb, very short | May look "fine" on weight-for-age | Length-for-age and weight-for-length | Praising weight while missing stunting |
| 8-year-old, 21 lb | Far below CDC weight-for-age | Urgent clinical care | Using the infant default weight on a school-age child |
WHO standard, CDC reference, and why the percentile can jump at 2
WHO Child Growth Standards describe how children should grow in supportive conditions. The Multicentre Growth Reference Study included breastfed infants in six countries. CDC 2000 weight-for-age describes how US children did grow in surveys from the 1960s through the early 1990s, a mix of feeding types.
That is why a child can sit on different percentiles on the two charts even if grams are unchanged the week they turn 2. CDC training says to interpret classification changes at that visit with caution. Bring both printouts if you have them. Do not declare a growth failure from the switch alone.
MMWR guidance from 2010 is the US policy paper for using WHO charts from 0 to 24 months and CDC charts after. This calculator follows that split. It does not blend the tables in a 23-to-25-month fade.
If the well-visit software still plots CDC infant charts from 2000 for a 6-month-old, your home WHO number will not match the paper in the folder. Ask which file they use. The clinical recommendation is WHO for that age.
How it works
Z = ((weight_kg/M)^L - 1) / (L x S) at the child's sex and age. WHO tables cover completed months 0 to 24. CDC tables cover 24 to 240 months. Overweight under age 2 is read on weight-for-length, not this chart.
Frequently asked questions
Why did the chart name change at age 2?
CDC recommends WHO standards from birth to 2 years and CDC charts from 2 to 20 years in US clinics.
Does a low percentile mean the child is underfed?
One point is a screen. WHO flags underweight below -2 SD. A clinician looks at length, trend, and health together.
Related calculators
References
- WHO. Weight-for-age
- CDC NCHS. WHO Growth Charts Data files
- CDC. Using WHO Growth Standard Charts
- CDC NCHS. CDC Growth Charts Data Files (LMS values)
- CDC MMWR. Use of World Health Organization and CDC Growth Charts for Children Aged 0-59 Months in the United States
- WHO. Malnutrition in children
- AAP. Bright Futures
- CDC. Growth Charts
Kitchen tools are estimates for home cooking. Nutrition tools are screening aids, not a diagnosis. They are not a substitute for a pediatrician or registered dietitian. Food-safety temperatures follow USDA FSIS. Do not reuse leftover meat brine.