Body Mass Index (BMI)
BMI is the ratio of weight to height used to roughly gauge whether body weight is underweight, normal or excessive. It's a convenient screening metric, not a diagnosis: it doesn't tell muscle from fat and doesn't show where fat is actually stored. Calculate your BMI, BMI Prime and waist-based risk — and find out how the number connects to your biological age.
What body mass index is
Body mass index (BMI) is a calculated measure of weight relative to height. It lets you quickly estimate how well weight and height match and indirectly judge whether weight is underweight, normal or excessive. It's precisely this simplicity that made BMI the worldwide screening standard.
It's important to grasp the main limitation: BMI is only a preliminary tool. It doesn't account for individual build, the ratio of muscle to fat, or fat distribution. Assessing body composition accurately needs additional methods. So BMI is best considered alongside other indicators — waist circumference, how you feel and lab results — rather than as a final verdict.
The formula and how to measure correctly
No shoes, back straight, heels together. Measure under the same conditions each time.
On the same scale, ideally in the morning, on an empty stomach, in light clothing.
Per the WHO protocol — midway between the lowest rib and the top of the hip bone, tape parallel to the floor.
Why add the waist? BMI doesn't tell muscle from fat and doesn't show where fat is stored. And it's precisely visceral fat around the organs that is the most dangerous. Waist circumference is a simple marker of abdominal obesity and is one of the criteria for metabolic syndrome: entering it is optional, but if you do, we'll additionally assess risk from fat distribution.
BMI Calculator
All calculations run locally in your browser. BMI, BMI Prime, WHO category, healthy weight for your height and waist-based risk — with no data sent.
WHO BMI categories and health risk
A single scale applies to adults aged 20–65. Color reflects the level of health risk:
BMI Prime — how close you are to normal
BMI Prime is the ratio of your BMI to the upper limit of the optimal range (24.9 kg/m²). It's a dimensionless number, independent of the units used. Its main strength is clarity:
- < 1.0 — weight is below the upper limit of normal;
- = 1.0 — you are at the upper limit of normal;
- > 1.0 — overweight (for example, 1.30 means exceeding the optimum by 30%).
Because the optimal BMI can differ between populations, BMI Prime is handy for comparison. But, like ordinary BMI, it doesn't account for body composition — so it's more reliable for sedentary people than for athletes.
Smart BMI (SBMI) — adjusted for age and sex
The smart body mass index (SBMI) differs from ordinary BMI in three important ways. First, it takes age and sex into account, not just weight and height. Second, it's a purely comparative figure on a 70-point scale. Third, the health significance of weight can be read straight from the SBMI: the risk levels (green → yellow → orange → red) are always 10 of 70 points wide. The ideal range is 30/70–39/70.
The first version, SBMI 1.0, was released in February 2014; the current 2.7 came out in February 2019. The scale was calibrated against many risk curves: the width of the “green” zone was fitted to the WHO definition of normal weight at about age 20, and the borders of the “red” zones to the definitions of extreme obesity and underweight. The risk curves for men and women differ, meanwhile: studies show that women tolerate more excess and insufficient weight than men do.
| SBMI, points | Category | Risk |
|---|---|---|
| 0–9 / 70 | extreme anorexia | Very high |
| 10–19 / 70 | anorexia → underweight | High |
| 20–29 / 70 | mild to moderate underweight | Moderate |
| 30–39 / 70 | normal body weight | Low |
| 40–49 / 70 | slight to moderate excess | Moderate |
| 50–59 / 70 | from excess toward obesity | High |
| 60–70 / 70 | morbid obesity | Very high |
Waist circumference and abdominal risk
BMI can be normal while fat accumulates around the organs. So waist circumference rounds out the picture and helps catch metabolic risk even when the weight number looks “normal.”
| Waist circumference | Men | Women |
|---|---|---|
| Low risk | ≤ 90 cm | ≤ 80 cm |
| Elevated risk | 90–102 cm | 80–88 cm |
| High risk | > 102 cm | > 88 cm |
BMI and biological age
Weight affects the pace of aging. Excess fat tissue sustains chronic inflammation, insulin resistance and oxidative stress, accelerating cellular aging. Being underweight is dangerous in a different way — deficiencies and muscle loss (sarcopenia). To estimate how many years your metabolism is “worth,” try metabolic age from your metabolism.
| BMI category | Effect on biological age | Key mechanism | Reversibility |
|---|---|---|---|
| Underweight (< 18.5) | + 1–2 years | Nutrient deficiency, muscle loss | High |
| Normal (18.5–24.9) | baseline | Optimal metabolic function | — |
| Overweight (25.0–29.9) | + 2–3 years | Chronic inflammation, insulin resistance | High |
| Obesity I (30.0–34.9) | + 3–5 years | Cellular aging, oxidative stress | Moderate |
| Obesity II (≥ 35.0) | + 5–8 years | Multi-organ stress, hormonal disruption | Moderate to low |
How BMI changes with age
BMI naturally changes over a lifetime and passes through four phases:
There are also differences between the sexes: men have a higher BMI due to greater muscle mass, which peaks at 25–30 years; the male curve then plateaus, while the female curve keeps rising longer — partly because of weight gain after pregnancy and a larger share of fat tissue.
When BMI misleads: limitations
- Athletes and muscular people. Muscle is heavier than fat, so BMI may wrongly place them in the “overweight” range.
- Age. At the same BMI, older adults usually have more fat and less muscle than younger people.
- Sex. At the same BMI, women on average have a higher fat share than men.
- Ethnicity. The link between BMI, body-fat percentage and risk differs between populations; for Asian groups the thresholds are lower.
Common mistakes in interpretation
How often to check BMI
For most adults, checking BMI every 3–6 months is enough. If you're deliberately changing your weight, it helps to measure your waist once a week and your BMI every 2–4 weeks. Sudden weight changes or health problems are a reason for more frequent monitoring and a doctor's visit.
