Children's ideal weight is most often estimated with age-based formulas — for ages 2 to 12, a common clinical rule of thumb is standard weight in kilograms equal to age in years times 2 plus 8 — and then read against a BMI-for-age or weight-for-age growth curve, not against the height-only adult formulas used by the Ideal Weight Calculator. Infants and toddlers use different formulas again: 1 to 6 months is often estimated as birth weight plus 0.7 kg per month, and 7 to 12 months as 6 kg plus 0.25 kg per month. These pediatric rules exist because children are still growing in both height and body composition at the same time, so any single "ideal" number needs an age attached to it to be meaningful. Adult ideal-weight formulas, by contrast, treat height and sex as the only inputs and assume the body has finished growing — which is exactly why the Devine, Robinson, Miller and Hamwi equations inside the calculator on this site do not apply to children. This article walks through the pediatric formulas, where they came from, and the situations where the adult Ideal Weight Calculator is — and isn't — the right tool for the job.
Common Formulas Used for Children's Ideal Weight
Several age-segmented formulas appear in pediatric textbooks and on Chinese pediatric reference sites such as the 9-city child growth survey. They are screening estimates, not clinical diagnoses, but they are useful when you only have age and a rough sense of build. The table below lists the formulas themselves, not computed results — plug your own numbers in once you have picked the right row, then check the answer against an age-appropriate growth curve.
| Age Range | Standard Weight Formula (kg) | When It Applies |
|---|---|---|
| 1 to 6 months | birth weight + 0.7 × age in months | Infancy, when monthly weight gain matters most |
| 7 to 12 months | 6 + 0.25 × age in months | Late infancy through the first birthday |
| 1 to 2 years | approximately 3 × birth weight | Toddler years as a rough check |
| 2 to 12 years | age (years) × 2 + 8 | The most widely cited quick estimate |
| Adolescents | Use BMI-for-age percentiles | Adult formulas start to apply once growth plates close |
The 2-to-12 rule (age × 2 + 8) appears in several pediatric references as the quick screening number. For a 7-year-old it lands near 22 kg and for a 10-year-old near 28 kg, with normal variation around those points. These are starting points only — the next step is to plot the result against a percentile curve for the same age and sex, not to treat it as a target on its own.
How to Get an Ideal Weight Estimate in Three Steps
For an adult height, the calculator on this site runs the four classic equations and the BMI band at once. Here is the exact sequence the tool follows, which mirrors the verification contract it ships with.
- Decide whether the person is a child or an adult. For adults, the Ideal Weight Calculator applies directly; for anyone still growing, use the age-based formulas above plus a growth curve instead.
- Select your sex, then pick a height unit — centimeters, inches, or feet-plus-inches. In feet-plus-inches mode, fill in both the feet box and the inches box; the tool reads both together.
- Read your ideal weight from all four formulas, their average, and the healthy BMI range below. The numbers update instantly as you type and are shown in both kilograms and pounds so you can read whichever unit you prefer. There is no button to press and nothing is uploaded.
To see how this looks in practice, take a 175 cm (5 ft 9 in) man. He is roughly 8.9 inches above the 5 ft baseline that all four formulas share, so Devine gives 50 + 2.3 × 8.9 ≈ 70.5 kg, Robinson 52 + 1.9 × 8.9 ≈ 68.9 kg, Miller 56.2 + 1.41 × 8.9 ≈ 68.7 kg, and Hamwi 48 + 2.7 × 8.9 ≈ 72.0 kg. The healthy BMI range at the same height, using 18.5 and 25 (the band the WHO treats as normal for adults), works out to about 56.7 kg up to 76.6 kg — a noticeably wider and more useful band than any single formula number.
Why the Adult Formulas Don't Apply to Children
Every formula inside the Ideal Weight Calculator — Devine (1974), Robinson (1983), Miller (1983) and Hamwi (1964) — was fitted to data from adults of 5 feet (152 cm) and above. Below that height the equations collapse to their baseline value and stop tracking the real relationship between height and weight, which is why the tool itself flags low-height results and points you to the BMI range. The product documentation is explicit that the calculator "is not suitable for children" and should be treated as "a rough, informational estimate, not a diagnosis."
For a child, the same two inputs (height and sex) ignore the dominant variable in pediatric growth — age — so the result will mislead you in both directions. A seven-year-old of average height can sit well above "their" Devine number and still be on the 75th weight-for-age percentile, while a tall but lean adolescent can sit below every formula and still be perfectly healthy. Pediatricians therefore use BMI-for-age or weight-for-age percentiles published by the WHO or national bodies, which compare a child to a reference population of the same age and sex rather than to a static adult target. The age × 2 + 8 rule works as a rough screen, but the percentile chart decides whether the screen result is healthy.
Reading the BMI Range vs. a Single "Ideal" Number
Even for adults, no single "ideal" weight exists — that is exactly why the calculator shows four numbers instead of one. Devine (1974), built by Dr. B. J. Devine to calculate medication doses, is the most quoted formula in clinical settings. Hamwi (1964), created by Dr. G. J. Hamwi to estimate nutrition needs, gives the highest figures. Robinson (1983) and Miller (1983) reworked Devine's numbers against fresh data and generally sit a little lower. Because their slopes and starting points differ, a tall adult sees a wider spread between the formulas than a short adult does.
Alongside the formulas, the calculator also reports a healthy weight range based on the body mass index band that the WHO treats as normal for adults: BMI 18.5 to 25. Multiplying each end of that band by your height in metres squared converts it into an actual weight range, which is often the most useful figure on the page — it is a range rather than a single target, and it is grounded in population health data rather than a mid-20th-century rule of thumb.
| BMI | WHO Adult Category | Where It Falls on the Calculator |
|---|---|---|
| Below 18.5 | Underweight | Below the healthy range shown |
| 18.5 to 24.9 | Normal weight | The range the tool reports |
| 25.0 to 29.9 | Overweight | Above the healthy range shown |
| 30.0 and above | Obese | Above the healthy range shown |
These cut-offs do not apply to children. Pediatric BMI assessment uses age- and sex-specific percentiles instead, with "overweight" usually defined as above the 85th percentile and "obese" as at or above the 95th, based on the same WHO reference data. Once a child is old enough that growth has essentially stopped, the calculator's adult bands start to apply — but until then, the percentile chart is the right yardstick.
Choosing the Right Method for the Right Person
- For a child, use an age-based formula or a percentile growth curve, not the Ideal Weight Calculator.
- For an adult of 5 ft or taller with an average frame, the calculator gives a useful starting reference across all four formulas.
- For an athlete or muscular adult, expect the formula numbers to underestimate healthy weight because the equations cannot see muscle, bone or frame size.
- For a small-framed adult, the formula numbers can overestimate healthy weight.
- For a quick sanity check at any adult height, the healthy BMI range is more informative than any single "ideal" number.
What These Formulas Don't Account For
All four formulas inside the calculator use only height and sex, so they cannot see muscle mass, bone size, frame, ethnicity, age or medical history. The product documentation is clear that the tool is for general information only, is not a diagnosis or a treatment recommendation, and is not suitable for children. Treat the result as a rough reference point, pair it with body composition context where you have it, and — especially when the question is about a child — speak with a doctor or a registered dietitian, who can read a growth curve, weigh the family history, and set targets that an online calculator cannot. For a deeper adult-focused walkthrough, see our guide to calculating your ideal body weight step by step. To put the BMI band in context, the BMI Calculator reports the WHO weight category for the same height and weight.