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Health and nutrition · Weight and composition

Calculate adult BMI (WHO bands)

Body mass index is weight in kg divided by height in meters squared. Labels use adult WHO cut-offs. That is not an obesity diagnosis by itself, and it is not a child BMI.

Calculate

Mass in kilograms. BMI does not separate muscle and fat. Convert lb on the weight converter.

Height in centimeters, converted to meters in the formula. Convert feet and inches first if needed.

How the calculator works

Two fields suffice. Height becomes meters (cm / 100), then BMI = weight / height². The class follows fixed cut-offs: under 18.5; 18.5 to under 25; 25 to under 30; then 30, 35, and 40 for obesity classes I, II, and III as the tool labels them from the cited WHO fact sheet. These cuts are public-health conventions for adults, not a clinical exam. A “normal” BMI does not assert that you are healthy; a BMI of 26 does not assert a disease. Children and adolescents have other growth references, not implemented. CDC adult categories use the same 18.5 / 25 / 30 cuts.

Formula and method

IMC = poids(kg) / [taille(m)]²

Height(m) = height(cm) / 100. Result usually left as kg/m².

WHO publishes lifestyle guidance that includes these adult BMI classes. We use them as labels, pointing to the source sheet, without adding “Asian” or other cut-offs we have not cited. BMI was designed as a simple indicator at population scale; for one person it ignores fat distribution, muscle, bone, edema. Hence the link to body fat (another estimator, other limits) and to reference weight (the 18.5–24.9 kg band on the same definition).

Worked example

65 kg at 170 cm (about 143 lb at 5 ft 7 in)

65 kg, 170 cm.

  1. Height = 1.70 m.
  2. BMI = 65 / 1.70² = 65 / 2.89 ≈ 22.5.
  3. 22.5 is in 18.5–24.9 → adult WHO “normal weight” class in this tool.

BMI ≈ 22.5, adult WHO class 18.5–24.9. That is not a health certificate.

90 kg at 175 cm (about 198 lb at 5 ft 9 in)

90 kg, 175 cm.

  1. BMI = 90 / 1.75² = 90 / 3.0625 ≈ 29.4.
  2. 29.4 is in 25–29.9 → “overweight” label on the adult grid.

BMI ≈ 29.4. The “overweight” label is the grid’s, not a diagnosis issued by this site.

Input notes

Weight
A morning scale, bladder empty, is enough for the math. BMI does not ask for waist; clinic uses other measures too, off this form.
Height
Height overstated by 3 cm lowers BMI artificially. Measure standing, without shoes.

Assumptions and limits

Assumptions

  • Adult, a build for which kg/m² has population-screening meaning.
  • Weight and height measured, not guessed.
  • Cut-offs: 18.5 / 25 / 30 / 35 / 40 as this tool’s breaks.

Limits

  • Not for children (no z-scores, no growth charts).
  • Misleading in many muscular athletes, older people with sarcopenia, pregnant women.
  • Does not locate visceral fat.
  • Not an obesity diagnosis in the sense of a visit.

How to read the result

Use BMI as a first arithmetic sort, the same one public-health campaigns use. If the number and the mirror diverge (a lot of muscle, or central fat with a “normal” BMI), BMI has hit its limit: other tools, and especially a professional, take over. The reference-weight page shows the kg interval that matches 18.5–24.9 for your height — an interval, not a personal target.

Common mistakes

  • Using it for a 10-year-old.

    Adult classes 18.5 / 25 / 30 do not apply. This tool refuses to be a pediatric BMI: it does not have those references.

  • Equating “BMI 30” with an obesity diagnosis from a visit.

    The class label is population-level. Clinical diagnosis integrates other elements. We issue none.

  • Comparing two people with the same BMI as “equally fit.”

    Same kg/m², different composition. BMI is silent there.

Comparison

Classes displayed (adults, from WHO cut-offs used by the tool)
BMILabel in the result
< 18.5Underweight
18.5 – 24.9WHO “normal” weight
25 – 29.9Overweight
30 – 34.9Obesity class I
35 – 39.9Obesity class II
≥ 40Obesity class III

Methodology · Sources

Related calculations

Frequently asked questions

Does BMI diagnose obesity?

No. It is a kg/m² indicator with adult WHO cut-offs, not a diagnosis issued by HexaCalc.

WHO uses it as a population marker. A person can have a high BMI with a lot of muscle, or a “normal” BMI with other risk factors. Only a professional interprets the full file. CDC adult categories use the same numeric cuts.

Why not children?

Because 18.5 and 25 are adult classes, not growth references.

Pediatric BMIs are read as percentiles or z-scores by age and sex. We did not include them, so we do not stick an adult label on a child.

My BMI is “normal”: nothing to watch?

BMI does not say that. It sees neither blood pressure, nor glucose, nor habits.

A result in 18.5–24.9 means only that weight/height² falls in that band. It is neither a workup nor permission to ignore the rest.

What is the link with the calorie calculator?

None in the formula. BMI does not enter Mifflin.

You can chain the pages out of curiosity (build, then energy), but BMI does not set TDEE. Mifflin uses weight, height, age, and sex, not the WHO class.

Author and update

Written by Rédaction HexaCalc (editorial team). Content last updated: August 25, 2026. No third-party medical or financial review is claimed.

See the 18.5–24.9 weight range

For a given height, the reference-weight page converts the same adult WHO cuts into kilograms, beside historical formulas that are not health targets.

Open reference weight

BMI is a weight/height² ratio labeled with adult WHO cut-offs. Not a diagnosis, not a child BMI, not a body-fat measurement. Figures on this page are estimates from published formulas, not a diagnosis, not medical advice, and not a personalized eating plan. They do not replace a visit with a clinician. The tool is not designed for children, pregnancy, breastfeeding, eating disorders, or many illnesses and medications. If you are unsure, ask a health professional.

Category: Health and nutrition