BMI calculator
Updated on 18 September 2026
Your BMI = weight (kg) ÷ height² (m). A BMI between 18.5 and 25 is considered normal for an adult. Important limitation: BMI does not distinguish muscle from fat and often misleads muscular trainees.
BMI (body mass index) is a simple marker relating your weight to your height. Useful for placing a population, it has real limits for a strength trainee. This tool gives you your BMI and your 'healthy' weight range, while explaining how to read it honestly.
Enter your height and weight.
How to read your BMI
BMI is calculated by dividing weight (kg) by height squared (m). Reference thresholds: below 18.5 = underweight, 18.5 to 25 = normal, 25 to 30 = overweight, above 30 = obesity. The 'healthy' weight range shown corresponds to a BMI between 18.5 and 25 for your height.
These thresholds are statistical markers set on large populations. They give an initial orientation but do not replace medical advice or a body-composition measurement.
Why BMI misleads athletes
BMI does not distinguish muscle from fat. Yet muscle is denser than fat: a muscular person weighs more at equal volume, and can therefore show an 'overweight' BMI while being lean and perfectly healthy.
For a strength trainee, markers like waist circumference, progress photos or a body-fat estimate are far more telling than BMI alone. Use BMI as a rough marker, nothing more.
Beyond BMI: better ways to gauge your shape
Since BMI ignores body composition, complement it with finer markers: waist circumference (a good indicator of belly fat), the trend of your photos and measurements, or a body-fat estimate. Together they tell your condition far better than a single number.
For a trainee, the scale alone is misleading: building muscle can raise your weight — and BMI — while your figure gets leaner. Track several signals at once rather than just one.
BMI and health risk
Across large populations, a high BMI is statistically associated with an increased risk of type 2 diabetes, high blood pressure and cardiovascular disease. It is this population-level correlation that justifies using BMI as a screening tool in general medicine.
At the individual level, this link remains probabilistic, not deterministic: someone overweight by BMI can be in better metabolic health than someone at a 'normal' weight but sedentary. BMI opens a conversation with a health professional, it does not replace one.
Calculating your BMI step by step
The calculation takes three steps: convert your height to metres, square it, then divide your weight in kilos by that result. For 68 kg and 1.65 m: 1.65 × 1.65 = 2.72; 68 ÷ 2.72 ≈ 25.0, right on the line between normal and overweight.
This sensitivity to height explains why two people of the same weight can show very different BMIs: at 68 kg, someone 1.55 m tall will have a markedly higher BMI than someone 1.80 m tall. The tool above does this calculation instantly to avoid any rounding error.
BMI and losing or gaining weight
If your BMI falls outside the normal range, that's not a verdict: it's a starting point to assess whether a weight change would serve your health. Losing or gaining weight is built gradually, with a moderate calorie deficit or surplus, not extreme measures.
Track your BMI trend over several weeks rather than day to day: weight naturally fluctuates with hydration, meals and hormonal cycles. A trend over a month is far more telling than a single isolated measurement.
Frequently asked questions
Is a high BMI necessarily bad?expand_more
Not necessarily. In a muscular person, a BMI above 25 can reflect muscle mass, not excess fat. That is precisely the index's big limitation. Always cross-check it with other markers before concluding.
Does BMI work for everyone?expand_more
It is less relevant for very muscular athletes, older adults, pregnant women and children (who have their own charts). For general adult use, it remains a handy first marker, to be interpreted with common sense.
What BMI counts as normal?expand_more
A BMI between 18.5 and 25 is considered normal for an adult. Below that is underweight; above it, overweight then obesity. These thresholds are population markers, not an individual diagnosis.
Does BMI depend on sex?expand_more
The calculation is identical for women and men, and the standard thresholds are the same. However, at equal BMI, body composition often differs — which is why cross-checking with other measures helps.
How do I calculate my BMI?expand_more
Divide your weight in kilos by the square of your height in metres. For example, 75 kg at 1.80 m gives 75 ÷ (1.80 × 1.80) ≈ 23.1. The tool above does the calculation and places the result on the scale.
Is BMI valid for children?expand_more
No: children and teenagers use growth charts specific to their age and sex, not this tool's adult thresholds. A paediatrician or health record remains the reference for that age group.
How was BMI invented?expand_more
It was devised in the 19th century by Belgian statistician Adolphe Quetelet to study populations, not to assess an individual. Its individual medical use is a later adaptation of this statistical tool.
Should I aim for a precise BMI to be healthy?expand_more
No: targeting an isolated number doesn't make sense on its own. It's better to follow a consistent path (eating habits, regular physical activity) and use BMI as just one marker among others, not a goal in itself.
Does BMI change with age?expand_more
The calculation stays the same, but its interpretation shifts: in older adults, a BMI slightly above the standard adult range is sometimes linked to a better reserve in case of illness. Here too, a professional's judgment outweighs the raw number.
Does water weight throw off BMI?expand_more
Yes, temporarily: water retention, a heavy meal or a hormonal cycle can shift weight by 1 to 2 kg in a day, and therefore the BMI slightly. Weigh yourself under the same conditions (fasted, in the morning) to limit these swings.
What's the link between BMI and body fat?expand_more
No direct, reliable link: BMI doesn't measure body fat, only the weight-to-height ratio. Two people with the same BMI can have very different body-fat percentages depending on their muscle mass.
Is a low BMI always a good sign?expand_more
No: a BMI below 18.5 can also signal insufficient muscle mass or nutritional deficiencies. Being underweight isn't automatically a sign of good health, just as being overweight isn't automatically a sign of poor health.
Sources
- menu_bookBMI reference thresholds (WHO)
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