📏 Height Converter and Child Height Predictor

Two tools in one: 175 cm converts to 5' 8.9", and a boy with a 178 cm father and 165 cm mother has a predicted adult height of 178 cm (range ~169.5-186.5). Switch modes above for conversion or prediction.

How to Use This Tool

Mode one converts between metric and imperial height. Mode two predicts a child's adult height from parents' heights with the mid-parental (Tanner) method used by pediatricians.

The Mid-Parental Height Method

The 13 cm (5 inch) adjustment is the average adult sex difference; averaging the parents estimates the family's genetic height potential. The ±8.5 cm band reflects everything else: the other parent's relatives, nutrition, health, and the ~20% of height variation not captured by parents' heights. Pediatricians plot children against this target on growth charts: a child tracking toward the target is reassuring even at the 10th percentile, while a child falling away from a high target deserves evaluation. Bone-age x-rays refine predictions in late bloomers or early developers by revealing remaining growth.

Worked Example

Father 178, mother 165: boy target = (178+165+13)/2 = 178.0 cm, range 169.5-186.5. Girl target = (178+165−13)/2 = 165.0 cm, range 156.5-173.5. Conversion check: 178 cm / 2.54 = 70.1 inches = 5' 10.1". I verified the tool reproduces these. Note the symmetry: the boy's target equals the father's height here only by arithmetic coincidence of these particular numbers, not by rule. Real data: children of these parents cluster around the target with the stated spread, and about 5% land outside it entirely.

What Moves Height Off Target

The clinical use is trend-watching: crossing percentile lines downward on the growth chart triggers the workup (thyroid, celiac screen, IGF-1, bone age), not the absolute percentile. A child at the 5th percentile tracking the 5th toward a 5th-percentile target is fine; a child falling from the 60th to the 25th is not. For growth tracking, our head circumference percentile covers infants, our adjusted age calculator corrects preterm measurements, and our BMI calculator interprets weight against the height.

Conversion Details

The inch is defined as exactly 2.54 cm, so conversions are exact, not approximate: 5'9" is precisely 175.26 cm. Medical records in the US use feet/inches while most of the world uses cm; errors creep in at handoffs (5'9" misheard as 5'7"), which matters for BMI, drug dosing, and ventilator settings. When precision matters, use total inches or cm rather than feet-and-inches shorthand.

Limits of Prediction

Mid-parental height explains ~80% of height variation; the rest is nutrition, health, and chance. It assumes average timing of puberty: very early or late developers deviate temporarily. Secular trends add noise: each generation averages slightly taller with better nutrition. And the formula needs accurate parental heights: self-reported heights run ~2 cm high on average, so measure rather than ask when it matters.

Bring prior measurements to every visit: the chart needs history to show velocity, and parent-kept logs fill gaps when records transfer between practices, and velocity needs at least two points to exist. Growth velocity below 4 cm/year before puberty, or a drop across percentile lines, is the trigger most pediatric endocrinologists act on. Early referral beats watchful waiting when velocity truly slows.

Growth Charts: Reading Your Child's Curve

Pediatricians track height on standardized growth charts, and the curve's shape matters more than any point. A child consistently at the 25th percentile is normal; a child falling from the 75th to the 30th across two years needs evaluation regardless of where they land. The mid-parental target translates to a percentile: plot the target height at age 18-20 on the chart, and the child's current percentile should roughly match it. Crossing two major percentile lines (e.g. 75th to 25th) is the standard trigger for workup: thyroid function, celiac screening, IGF-1/IGFBP-3, and bone age x-ray. Constitutional growth delay (the "late bloomer") shows delayed bone age with normal growth velocity; pathologic causes show slowing velocity. Velocity is the key number: normal prepubertal growth is 5-7 cm/year, pubertal peak 8-12 cm/year. A child growing 3 cm/year at age 8 is abnormal even at a normal percentile.

For informational purposes only, not medical advice. Consult a healthcare professional.