Child Growth Percentile Calculator (LMS Method)

When a pediatrician says a child is "in the 60th percentile" for height or weight, they mean a statistical position on that measurement's population distribution for the child's exact age and sex — not a grade or a target. This calculator computes that position with the real technique used to build official growth charts, called the LMS method, entirely in your browser: nothing you enter is uploaded anywhere.

LMS stands for the three parameters that describe the distribution of a measurement at a given age: L (a Box–Cox power that captures the skewness of the distribution), M (the median) and S (the coefficient of variation, roughly how spread out values are). From these, a measurement is converted into a z-score with a closed-form formula: z = ((value / M)^L − 1) / (L × S) when L is not zero, or z = ln(value / M) / S in the special case where L = 0. That z-score is then converted into a percentile with the standard normal cumulative distribution function, which this tool implements as a genuine numerical approximation (Abramowitz & Stegun's rational formula), not a lookup table.

Honesty matters more than polish here: authoring the complete, byte-exact official WHO or CDC LMS reference tables — which list separate L, M and S values for every month of age — by hand is not something this tool can responsibly claim to have done. Instead it bundles a small, internally consistent set of illustrative L/M/S rows, one every 12 months from birth to 20 years, for boys and girls, for height-for-age and weight-for-age, shaped to follow realistic pediatric growth curves (a decelerating infant growth rate, a visible adolescent growth spurt, a median that plateaus near typical adult height and weight). The calculator linearly interpolates between the two nearest bundled ages for whatever exact age you enter — the same interpolation approach real growth-chart software uses — but the underlying numbers are a teaching approximation, not the official chart. For a clinically precise percentile, use your pediatrician's official WHO or CDC chart.

Use this tool to understand how the LMS method actually works and to get a reasonable, illustrative sense of where a measurement sits — not as a medical result. It is not medical advice and cannot replace your pediatrician's growth assessment, which uses validated reference data and considers your child's full growth history, not a single measurement. Everything here runs locally: no age, sex or measurement is ever sent to a server or stored.