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Published standards. Clear calculations.

Baby Growth is an independent reference utility. It is not affiliated with or endorsed by WHO or CDC.

Which reference is used?

The calculator uses WHO Child Growth Standards before the second birthday, then CDC 2000 weight-for-age, stature-for-age and BMI-for-age references from the second through the 18th birthday. Both girls and boys are supported. CDC publishes its child tables through age 20; this utility stops at age 18. Head circumference is optional for infants and not calculated from age two. When both weight and length are entered, infants also receive a WHO weight-for-length result; children aged two and older receive a CDC BMI-for-age result.

Age and reference selection

WHO calculations use expanded daily L, M and S tables. Age is the calendar-day difference between dates, computed in UTC; birth is day zero. Reference selection follows calendar birthdays (29 February anniversaries use 28 February in non-leap years). CDC age in months is elapsed days divided by 30.4375, bounded to 24–216 months at the supported endpoints. CDC L, M and S are linearly interpolated between surrounding table ages. Core age-reference files were retrieved on 27 September 2026; weight-for-length and BMI reference files were retrieved on 29 September 2026.

The LMS calculation

L represents the Box–Cox power, M the median and S the generalized coefficient of variation. For measurement X, the standard score is:

z = ((X / M)^L − 1) / (L × S)

If L is zero, the logarithmic form is used: z = ln(X / M) / S. The standard normal cumulative distribution converts z to a percentile. The displayed result is rounded to one decimal; tails below 0.1 or above 99.9 are shown as bounds. This tool does not report extreme WHO z-scores or implement WHO’s adjusted extreme-score calculation.

Extended BMI for high values

For children aged two and older, BMI values through the CDC 2000 95th percentile use the LMS calculation above. Above that threshold, the calculator uses the CDC 2022 extended BMI method: the age-and-sex-specific 95th percentile anchors a half-normal distribution and CDC’s published sigma equations calculate the extended percentile. This avoids extrapolating the original LMS curve where it was not designed to be used.

Units and the chart

Pounds are converted to kilograms with 1 lb = 0.45359237 kg. Inches are converted with 1 in = 2.54 cm. Charts and stored history use kilograms and centimetres. The curves show the 3rd, 25th, 50th, 75th and 97th percentiles. WHO chart months are approximate; observations use exact days. CDC charts use fractional age in years derived from the month calculation. WHO and CDC observations are shown on separate charts. Historical infant measurements saved by the earlier version retain their original WHO reference and lying-down method, even at day 730.

Limits that matter

Under age two, measure length lying down. From age two, measure standing height without shoes. No automatic adjustment between methods is made.
Premature or corrected-age calculations are not supported.
Weight-for-length and BMI-for-age are calculated only when both weight and length or height are entered. They are reference comparisons, not diagnoses or growth forecasts.
Each chart is a reference comparison, not a clinical assessment. Interpretation needs a healthcare professional and broader context.

Original sources

Reference data is provided for educational tracking. If you are concerned about your baby’s growth, ask your healthcare professional.

Review status: Maintained by Baby Growth and technically reviewed on 29 September 2026. Calculations and product ranges are checked against the cited WHO, CDC and manufacturer sources. This independent tool has not received clinical review and does not replace professional care.