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The selected reference table changes with sex.
Changing units converts the displayed value without changing the physical weight.
Supported adjusted lengths are 45–110 cm for WHO and 45–103.5 cm for CDC.
Choose the method used for this measurement.
Keep the source name with any screenshot, copy, or export.
months
Leave blank for neutral behavior; accepted context is 0–60 months.
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The chart renderer is unavailable. The same lane weights remain listed in the reference ledger.

Measure Value Interpretation Copy
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Weight-for-Age Percentile Calculator Calculate a child's weight-for-age percentile from WHO or CDC reference data with corrected-age handling and clear screening limits.
Growth Screening Flag Checker Screen serial child growth measurements for percentile drift and low height velocity, with age-routed adiposity checks and data-quality flags.
BMI-for-Age Percentile Calculator Calculate a child’s BMI-for-age percentile from measurements or known BMI and interpret it with WHO or CDC curves, z-scores and P95 checks.
Body Metrics Calculator Compare adult BMI and waist ratios with three body-fat estimates, derived fat and lean mass plus BSA and method-spread warnings.

Two children can have the same weight and very different body proportions. Weight-for-length places measured weight against recumbent length for children of the same sex, so it describes relative weight for body size rather than weight by age.

This comparison is most often used in infancy and early toddlerhood. In U.S. clinical practice, CDC recommends World Health Organization (WHO) growth standards from birth to age 2, followed by CDC growth charts for children age 2 and older. That transition also changes from recumbent length to standing height and from weight-for-length to body mass index (BMI)-for-age, so an apparent percentile shift may reflect a new chart and measurement method rather than sudden growth.

MeasureQuestion it addressesImportant distinction
Weight-for-lengthHow does weight compare at this body length?Does not use age in the percentile calculation
Length-for-ageHow does linear growth compare at this age?Needs accurate age and recumbent length
Weight-for-ageHow does weight compare at this age?Does not account for body length
BMI-for-ageHow does BMI compare by age and sex?Used with older-child chart pathways

A percentile is a position in a reference distribution. It is not a percent of ideal weight, a diagnosis, or a target that every child should move toward. The 50th percentile is the reference median. A value near the 3rd or 97th percentile means the weight-for-length pair is near an outer part of that reference, but trend, feeding, health history, and measurement quality determine what it means for one child.

Length technique can move the result. Recumbent length is measured lying down on a length board and is typically about 0.8 cm greater than standing height. A bent knee, lifted heel, bulky clothing, unit error, or movement on the scale may shift the plotted point enough to cross a nearby percentile lane.

Weight-for-length is screening information, not a diagnosis or care plan. Repeat an unexpected measurement carefully and review growth over time with a qualified health professional, especially when there are feeding concerns, illness, dehydration, swelling, weight loss, or other symptoms.

How to Use This Tool:

Enter the measured pair and preserve the reference source and measurement method with the result.

  1. Choose the recorded Sex, then enter Body weight in kg or lb.
  2. Enter Length in cm or in and select how it was measured. Standing height receives a +0.8 cm correction before lookup.
  3. Select Reference view. Compare mode uses WHO as the primary result and also reports CDC when the adjusted length is within the CDC table.
  4. Optionally enter Age review in Advanced. Age changes only the source-fit note; it never changes the percentile calculation.
  5. Read the primary source, percentile, z-score, length used, and reference lane together. Keep the source name and measurement basis with any copied result.
  6. If a value is rejected, check units and table coverage. Adjusted length must be 45 to 110 cm for WHO, while CDC-only lookup stops at 103.5 cm.

Interpreting Results:

The Primary percentile and z-score belong to the named source, sex, and adjusted length. Compare mode deliberately makes WHO primary. The Source gap is the absolute difference between WHO and CDC percentiles when both tables cover the length; it shows that references can classify the same measurement differently.

LaneBoundary ruleReading
Below P3Percentile < 3Outer lower reference lane
P3–P153 ≤ percentile < 15Lower reference lane
P15–P8515 ≤ percentile < 85Middle reference lane
P85–P9785 ≤ percentile < 97Upper reference lane
Above P97Percentile ≥ 97Outer upper reference lane

A middle lane does not prove that growth is healthy, and an outer lane does not establish a diagnosis. Check the measurement, then interpret it as part of a longitudinal growth record with feeding and clinical context.

Technical Details:

WHO and CDC tables store sex-specific LMS parameters at each supported length. L controls skew, M is the median weight, and S is a coefficient of variation. When a length lies between stored rows, each LMS value is linearly interpolated at that length; the calculation does not extrapolate beyond table coverage.

Lookup Core:

ReferenceStored length coverageSelection behavior
WHO Child Growth Standards45.0 to 110.0 cm at 0.1 cm rowsPrimary for WHO-only and compare modes
CDC 2000 infant reference45.0 to 103.5 cm, mainly 1 cm spacing after 45.5 cmPrimary only in CDC-only mode; secondary in compare mode

Weight is converted with 1 lb = 0.45359237 kg and length with 1 in = 2.54 cm. Standing height receives exactly +0.8 cm before table selection. Age review accepts blank or 0 to 60 months and affects only the explanatory source-fit message.

Formula Core:

The LMS equation converts weight at the selected length into a z-score.

z=WML−1LS

When L is effectively zero, the logarithmic limit is used instead.

z=lnWMS

The z-score is converted to a percentile with the standard normal cumulative distribution, using a deterministic error-function approximation. Displayed percentiles are bounded from 0.1 to 99.9. The reverse LMS equation calculates the weights at P3, P15, P50, P85, and P97 for the percentile map.

W=M1+LSz1L

The result uses full precision internally. Percentile, z-score, source gap, and target weights are rounded only when displayed. Small differences from another charting system can arise from reference choice, interpolation, percentile approximation, measurement correction, or display rounding.

Limitations and Accuracy Notes:

One point cannot show growth velocity or confirm a feeding or medical problem. The stored tables cover only the stated length ranges, and CDC-only results above 103.5 cm are deliberately rejected.

  • Use a calibrated infant scale and proper recumbent-length technique when possible.
  • Repeat an unexpected value before acting, especially near a lane boundary.
  • Keep reference source, sex, units, and measurement basis consistent when comparing visits.
  • Use the age-appropriate chart pathway chosen by the child's health professional.

Worked Examples:

Same measurement, different references

A female child weighing 8.1 kg at 68 cm recumbent length is about the 68.76th percentile on the WHO table and about the 65.86th percentile on the CDC table, a gap of about 2.90 percentage points. Compare mode reports WHO as primary and places the result in P15–P85. The difference is a reference effect, not a change in the child.

References: