Weight-for-Length Percentile Calculator

The infant weight-status chart US clinics actually use

Weight-for-length compares an infant's weight with other infants of the same sex and recumbent length. Age is not in the LMS lookup. A long 7-month-old and a short 11-month-old who share a length share a reference, which is the point: this is body proportion, not weight-for-age.

CDC and AAP tell US clinicians to track weight status under age 2 on WHO weight-for-length, not BMI-for-age. BMI for infants uses length the same way, but the recommended plot in US practice is this chart.

WHO wasting is a weight-for-length z-score below -2 (severe below -3). Overweight is above +2. Obesity on WHO weight-for-length is above +3. CDC-adapted WHO paper charts label the outer lines as the 2nd and 98th percentiles, which sit at about +/-2 SD.

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How to use the weight-for-length calculator

Enter sex, recumbent length, and weight. Age is only used to remind you that after 24 months BMI-for-age is the usual CDC category tool. The math still runs if length is between 45 and 110 cm, which is the WHO table span.

Length must be lying down. Standing "height" under age 2 is usually shorter and makes the child look heavier for length, which can fake a high weight-for-length.

Read the highlighted percentile, then the WHO reading line. A z-score of +2.3 is overweight language. A z-score of -2.1 is wasting language. Those words are screens for a clinician, not names you put on the child.

Age defaults to 9 months because that is a common well-visit. Change it if your child is 4 months. The LMS lookup still ignores age, but the note about switching to BMI-for-age is wrong if you leave a 9-month age on a 3-year-old.

Length of 28 in is 71.12 cm. If you flip to metric and leave the default 71 cm, you are about a millimeter off, which is noise. If you flip to metric and forget to change 28 cm, you have entered a newborn length on a 9-month-old and the table may still run while the z-score becomes fiction.

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Formula: length is x, weight is X

Convert length to centimeters and weight to kilograms. Look up L, M, and S for that sex at that length (interpolated between 0.5 cm rows when needed). Z = ((kg / M)^L - 1) / (L x S). Percentile is the normal CDF.

Because M is the median weight at that length, a child right on the median has Z near 0 regardless of whether they are 6 months or 14 months old, as long as length matches.

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Worked example: 9 months, 71 cm, 8.6 kg

Default inputs are a 9-month-old boy, 28 in (71.1 cm), 19 lb (8.62 kg). WHO boys' weight-for-length median near 71 cm is about 8.6 kg on the CDC-hosted file. Expect a percentile near 50th and a z-score near 0.

Keep length 71 cm and raise weight to 24 lb (10.9 kg). Z climbs past +2. The reading should move into overweight. That is a length-matched problem, not "he is just tall." If he were much longer, the same 24 lb might still sit near the median.

Keep 71 cm and drop weight to 14 lb (6.35 kg). Z falls below -2. Wasting language prints. Recheck the diaper and the length board before you drive, then still call. True wasting is not a cereal-in-the-bottle fix.

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Measure length and weight as a pair

Do both the same hour. A length from last month and a weight from this morning plots a child who grew in between as heavier for length than they are.

Two-person length technique matters more here than on weight-for-age, because a 1 cm short length raises weight-for-length the same way a short stature raises BMI. If the percentile looks shocking, repeat length twice before repeating weight.

A wet cloth diaper can add 100 to 200 g. On an 8 kg infant that is 1 to 2 percent of body weight, enough to wiggle a z-score by a few tenths. Dry diaper or naked is the clinic habit for a reason.

Do not subtract "bone weight" or add a clothing fudge factor you invented. Enter what the scale showed after you undressed the child the way the nurse does.

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The messy jump to BMI-for-age at age 2

CDC warns that classification can change at 2 years because you switch charts, switch from length to stature, switch from a breastfed-heavy standard to a US reference, and switch cutoffs. A child a little above the WHO 98th on weight-for-length may land lower on CDC BMI-for-age. Interpret that visit with the clinician, not as proof the child "got healthier overnight."

This page will still compute WHO weight-for-length after age 2 if you enter a length in range. The result card notes that BMI-for-age is the usual tool. Do not collect a stack of conflicting labels from both pages and pick the nicer one.

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Food, juice, and this chart

High weight-for-length is not a license to put an infant on a diet you found in a comment thread. Low weight-for-length is not a license to add sugar for calories. Both are measurement-plus-history problems.

Juice still has an AAP ounce cap that does not care about this z-score. If you came here because the 18-month-old looks "solid" and drinks two juice boxes, run juice-limit-by-age after you plot length and weight.

Breastfeeding is the WHO reference pattern, not a problem to solve when weight-for-length sits on the 40th. Formula-fed infants can plot higher on WHO standards than they did on older CDC infant charts. That shift is documented in CDC training. It is not proof the formula "made the baby obese" by itself.

Rapid upward crossing on this chart in the second six months, especially with juice and cereals added early, is a feeding-history talk. Rapid downward crossing with diarrhea is a hydration and illness talk. The same z-score can be two stories.

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Accuracy limits, edge cases, and safety

WHO tables start at 45 cm. Newborns shorter than that, including many preterm infants, are outside this file. Use the neonatal team's chart.

Above 110 cm of length the table ends. That is usually a toddler who belongs on BMI-for-age anyway.

Edema from kidney or heart disease inflates weight-for-length. So does a soaked cloth diaper you forgot to subtract.

Down syndrome and other conditions have their own weight-for-length style charts. A WHO z-score still computes. It may not be the chart the specialty clinic uses.

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Complementary foods, juice, and a high or low WFL

Complementary foods start around 6 months for most term infants, not because a weight-for-length percentile demanded rice cereal at 4 months. AAP juice policy still bans juice under 12 months. A high WFL is not a reason to add juice "so they eat." A low WFL is not a reason to add juice for calories.

Force-feeding past satiety because a relative said the 25th percentile looked "small" is how mealtimes go wrong. The 25th percentile is a normal channel. Wasting language starts near -2 SD, about the 2nd percentile on CDC-adapted WHO plots, not at the 25th.

If WFL is high and the child drinks 8 ounces of juice plus two pouches, the juice-limit tool and this chart belong on the same afternoon. The z-score will not drop the day you swap juice for water. Length still grows. Weight gain can slow. Recheck in weeks with the clinician, not tomorrow on a different bathroom scale.

Allergic elimination diets that remove large food groups can drop weight-for-length if calories are not replaced. Tell the allergist the z-score. Do not restart a food you were told to avoid because a calculator looked low.

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How it works

Age is collected for the note on the result card. The LMS lookup uses length in centimeters, not age. Z = ((kg/M)^L - 1) / (L x S) from the WHO weight-for-length table for that sex.

Frequently asked questions

• Do I use height or length?

  Recumbent length on a board. Standing height under age 2 is usually shorter and can mis-plot this chart.

• Why not BMI for a 9-month-old?

  AAP and CDC track weight status under age 2 with WHO weight-for-length, not BMI-for-age.

Related calculators

References

1. WHO. Weight-for-length/height
2. CDC NCHS. WHO Growth Charts Data files
3. CDC. Using WHO Growth Standard Charts
4. CDC. WHO Child Growth Standards training (CDC)
5. CDC MMWR. Use of World Health Organization and CDC Growth Charts for Children Aged 0-59 Months in the United States
6. Hampl et al., AAP. Clinical Practice Guideline for the Evaluation and Treatment of Children and Adolescents With Obesity
7. Heyman & Abrams, AAP. Fruit Juice in Infants, Children, and Adolescents
8. CDC. Growth Charts