Methodology

Height percentiles

What it calculates: where a child's height falls within a published growth reference for their exact age and sex.

Inputs: sex, age (months or years + months), measured height, and the growth reference (CDC, WHO, or UK-WHO).

Formula — the LMS method, exactly as published by CDC/WHO:

• z = ((height ÷ M)^L − 1) ÷ (L × S), where L, M, S are the reference parameters for that age and sex
• percentile = standard normal CDF(z) × 100

Between tabulated ages, L, M and S are linearly interpolated — the approach CDC documents. Interpolation is never applied across the WHO length/height measurement boundary at 24 months (there is a real ~0.7 cm step there, preserved as published).

Rounding: percentile to 1 decimal, z-score to 3 decimals, reference median to 2 decimals. Percentiles are reported within 0.1–99.9: beyond that the normal model implies more precision than a growth reference can support.

No extrapolation: ages outside a reference's published range return no result — for example, CDC covers 2–20 years and UK-WHO covers 2 weeks–4 years. The calculator says so honestly instead of inventing a number.

Child height prediction

What it calculates: an estimate of a child's likely adult height from both parents' adult heights.

Formula — the classic Tanner mid-parental height formula:

• Boy: (father's height + mother's height + 13 cm) ÷ 2
• Girl: (father's height + mother's height − 13 cm) ÷ 2

The ±13 cm adjustment reflects the average adult height difference between men and women. The result is always shown with a ±8.5 cm likely range (roughly ±2 standard deviations of prediction error) — the range is part of the answer, not a footnote. No method predicts adult height exactly.

Rounding: prediction and range bounds to 1 decimal.

Unit conversions

All conversions use the exact international definition: 1 inch = 2.54 cm.

• cm → feet + inches: inches rounded to 1 decimal (e.g. 180 cm → 5 ft 10.9 in)
• feet + inches → cm: rounded to 2 decimals

Heights are stored in centimetres internally and converted on the fly, so switching units never loses precision.

Height differences

On the comparison tool, the height difference between two figures is the absolute difference of their heights. If they are within 0.1 cm, they are reported as the same height; otherwise the tool states how much taller or shorter one is than the other.

Data sources

• CDC 2000 Growth Charts, stature-for-age (National Center for Health Statistics), ages 2–20 — official data files
• WHO Child Growth Standards, length/height-for-age (WHO Multicentre Growth Reference Study, 2006), birth–5 years — official standards
• UK-WHO charts (RCPCH), 2 weeks–4 years — computed from the WHO tables above (identical values in that window). Known gap: from age 4 the UK charts use the British 1990 reference, whose tables are under an MRC licence and not open data, so no UK result is given above 4 years.