BMI calculator — orientation, not diagnosis

What does the BMI calculator do?

The calculator computes the Body-Mass-Index and assigns it the right classification for your situation. Inputs are weight in kilograms, height in centimetres, sex and age. From age the tool auto-derives the mode — Adults with the WHO classes, Children and Adolescents with the Kromeyer-Hauschild percentiles, or Seniors aged 65+ with the DGE band 22–27. You can override the auto-pick manually, for instance when a 17-year-old elite athlete wants the adult bands.

Three differentiators other online BMI calculators do not combine: a visible “No data storage” trust signal directly in the UI (health values fall under Article 9 GDPR as a special category of personal data), a multi-mode switcher inside a single page instead of three separate URLs for adults, children and seniors, and companion cards with waist-to-height ratio and Body-Roundness-Index — both 2026 research trends that capture fat distribution instead of pure weight.

Everything runs in the browser. Closing the page wipes the inputs; opening it again gives you an empty form.

How is BMI computed?

The formula has been unchanged since the 1830s (Lambert Adolphe Jacques Quetelet, hence the historical name “Quetelet index”):

BMI = weight_kg / (height_m)²

Worked example: weight 70 kg, height 175 cm. 175 cm becomes 1.75 m. Squared: 3.0625 m². 70 divided by 3.0625 is 22.86 kg/m². For a 30-year-old adult that lands in WHO “normal weight” (18.5–24.9), for a 70-year-old senior it sits inside the DGE band 22–27. For a child with the same BMI the assignment depends on age and sex — via the Kromeyer-Hauschild percentiles.

Classification is therefore not a function of BMI alone but of BMI together with age, sex and mode.

Which classes does the calculator know?

Three separate classification systems — each with the correct cut-offs:

Mode	Source	Classes
Adults 18–64	WHO	Underweight < 18.5 · Normal 18.5–24.9 · Overweight 25–29.9 · Obesity I 30–34.9 · II 35–39.9 · III ≥ 40
Children / adolescents 2–17	Kromeyer-Hauschild (S3 guideline AGA / DGKJ)	≤ P3 underweight · P3–P10 low-normal · P10–P90 normal · P90–P97 overweight · > P97 obesity
Seniors 65+	DGE (Nutrition Report)	< 22 underweight · 22–27 normal · 27–29.9 overweight · 30+ obesity I–III per WHO

The seniors correction is not marketing. A series of observational studies shows that older adults with BMI between 22 and 27 have the lowest all-cause mortality. Underweight is one of the strongest risks at advanced age; the adult 18.5 cut-off would catch it too late.

What does the range visualisation show?

A simple monochrome bar with fine markers at the four class boundaries (18.5 / 25 / 30 / 35). A dark slider shows your current BMI on a 12–40 kg/m² scale. No gradient, no heatmap, no emotional colouring — the class information sits in the card next to the number. Visual restraint here is not stylistic minimalism but a deliberate decision: a red/yellow/green gradient stigmatises in an already sensitive subject.

Below the bar a card surfaces the healthy weight range. We deliberately call it a range, not an “ideal” — a single-number recommendation like “you should weigh 73 kg” is neither clinically nor psychologically supportable.

How does children mode work?

Between 2 and 17 the calculator auto-switches to children mode. For each integer year 2 to 18 an LMS reference (L, M, S — the three LMS parameters of Cole’s method) is stored for boys and girls. The calculator interpolates linearly between the years. From BMI, age and sex it derives a z-score and then a percentile via the standard-normal distribution.

The result appears as a percentile with one decimal plus a class label per the S3 guideline. Right below sits a calm “Note for parents” card pointing to the German preventive check-ups U10, U11 and J1/J2 — all of which already include a BMI check. Deliberately omitted: diet, sport and nutrition tips. For children that would simply be irresponsible — the liability belongs in a pediatric or adolescent-medicine practice.

What do WHtR and BRI do?

As soon as you enter a waist circumference the calculator unlocks two extra cards:

• Waist-to-Height-Ratio (WHtR) — waist divided by height. Ashwell (2012) defined four bands: below 0.40 below reference, 0.40–0.50 healthy, 0.50–0.60 elevated, 0.60+ markedly elevated. Multiple meta-analyses show that WHtR captures cardiovascular risk better than BMI alone. In 2026 the US Department of Defense moved its annual fitness tests from BMI to WHtR.
• Body-Roundness-Index (BRI) — Thomas et al. (2013) mapped the geometry of an elliptical cylinder onto humans and barrels. BRI = 364.2 − 365.5 · √(1 − (waist / (π · height))²). In 2026 BRI is being discussed in the research community as a complementary marker, particularly for children and adolescents.

Both stay companion metrics. If you don’t know or don’t want to measure the waist circumference you simply get the BMI card without the bonus cards — the calculator does not overload the display.

When does the athlete card appear?

As soon as BMI is 25 or above in adult / senior mode the tool surfaces a small card: “Do you train intensely?”. Background: BMI does not account for muscle mass. A 90 kg person at 178 cm with 14 % body-fat lands at BMI 28.4 — the tool would classify them as overweight even though body composition is lean.

The card optionally opens an input for body-fat percentage. From weight and body-fat the calculator derives fat-free mass and computes FFMI (fat-free-mass index per Kouri 1995) plus a height-normalised variant. If body-fat sits clearly below 20 % (men) or 30 % (women) the tool flags that the BMI overweight classification is likely a false positive. Importantly the pointer stays a pointer. The tool does not recommend a training plan, a diet or a therapy — FFMI is a passive marker, not a diary.

What the calculator deliberately does NOT do

Three lines are drawn in the sand so the tool stays an orientation — and not a source of legal liability:

• No diet, calorie, training, supplement or therapy advice. BMI is decision support for clinical conversations, not a driver of self-medication. The calculator tells you where you sit on the WHO scale — what happens next belongs in a consultation.
• No single-point ideal-weight target. Instead a range, plus Broca as a comparison value. Anchoring on a single number is psychologically risky and we don’t amplify that.
• No severity score. Lines like “obesity class III means X years shorter life expectancy” are statistically defensible but psychologically risky for individual readers. We name the class — that’s it.

Also deliberately omitted: persistent history (no localStorage, no cookies, no account), affiliate links to diet programmes or fitness apps, smart-scale integration, a pregnancy mode (phase 2 once the DGGG tables are maintained), Imperial units (phase 2 for the US launch).

Which related calculators exist?

For other measurements around bodies, families and everyday life:

• Dog age calculator — the “seven dog years” myth is wrong; breed-aware dog-to-human conversion.
• Pregnancy week calculator — gestational week, due date and trimester.
• Hourly to annual salary — fast conversion for freelancers and employees.
• Gross-to-net calculator — 2026 net salary with all German tax and social security deductions.
• MPG calculator — fuel consumption, cost per kilometre and CO₂ per litre.

Phase-B candidates: a pregnancy-BMI mode with trimester tables, Imperial units for the US launch, an inverse “What does BMI say for X kg muscle mass?” computation. What gets built depends on Google Search Console data after four to six weeks of impressions.

What else do people ask about BMI?

The frontmatter of this page contains eight FAQ entries with schema.org/FAQPage markup, sourced from top Google “People also ask” patterns around BMI, child percentiles, the seniors band and athlete adjustments. The answers address each question in the first 10 words — voice-search optimised for smart speakers and AI search assistants.

Important note: BMI is a screening number. Muscle mass, age, sex, fat distribution and individual history are not captured. For a concrete assessment — whether clearly overweight or underweight — talk to your GP, your pediatric or adolescent-medicine practice. This tool deliberately offers no diet, sport or therapy recommendations.