What it reads
Two numbers, and nothing else. Neither age, nor sex, nor body composition enters the formula, whatever the calculators that ask for those fields without using them may suggest.
The tool
BMI calculator
Body mass index is weight divided by height squared. The calculation returns the value, the WHO category it belongs to (all eight, not the usual four) and the weight range that matches a normal build for that height.
No account, no sign-up, no cap. Measurements go in as pounds and feet or as kilos and centimeters, and they stay in the browser: nothing is sent, nothing is stored.
The measurements, then the result
The calculation
One division, and two centuries of use
The index is weight in kilograms divided by height in meters squared. For 70 kg and 1.75 m, 154 lb at 5 ft 9 in: 70 ÷ 3.0625 = 22.9. Pounds and inches reach the same number through 703 × lb ÷ in², which is that division with the conversion folded in.
Where it comes from
Adolphe Quetelet published it in 1835, to describe a population, not an individual. The WHO took it up in 2000 as a screening tool at population scale, and that is the standing it has kept.
What it ignores
The share of muscle and fat, and where that fat sits. A very muscular person lands in overweight with no excess fat; an older person who has lost muscle keeps a normal index with a high fat mass.
The classification
The eight WHO categories
Most calculators show four: underweight, normal, overweight, obese. The WHO distinguishes eight, and the two distinctions that get dropped are the ones that matter most: the three degrees of thinness on one side, the three obesity classes on the other.
| Index | Category |
|---|---|
| Under 16.0 | Severe thinness |
| 16.0 to 17.0 | Moderate thinness |
| 17.0 to 18.5 | Mild thinness |
| 18.5 to 25.0 | Normal range |
| 25.0 to 30.0 | Overweight |
| 30.0 to 35.0 | Obese class I |
| 35.0 to 40.0 | Obese class II |
| 40.0 and over | Obese class III |
These bounds hold for adults. Before eighteen, build is read on reference curves by age and sex, in percentiles, and the thresholds above mean nothing there.
The complement
Waist circumference says what the index ignores
The index does not know where the mass sits, and that is its most documented limit. Waist circumference corrects part of it: at equal index, an abdominal distribution carries a higher cardiometabolic risk. The WHO publishes two thresholds per sex, and this calculator applies them.
| Sex | Increased risk | Substantially increased risk |
|---|---|---|
| Male | 94 cm, 37.0 in, and over | 102 cm, 40.2 in, and over |
| Female | 80 cm, 31.5 in, and over | 88 cm, 34.6 in, and over |
The measurement is taken midway between the lowest rib and the top of the hip bone, at the end of a normal breath out, without pulling tight. A tailor's tape measure is enough, and the value is noted to the nearest half centimeter or quarter inch.
The so-called ideal weight
There is no ideal weight, there is a range
The question comes up in that shape, and the usual answer is a formula that returns a single number from height alone: Lorentz, or Broca, published in 1871. Neither carries established clinical value, and both display a precision to the kilo that physiology does not have.
The calculation above therefore shows a range: the interval that puts the index between 18.5 and 25 for the height entered. It is commonly twenty kilos wide, around forty-four pounds, and that is the honest answer. What a day actually weighs is the next question, and the free calorie calculator computes it, with the Mifflin-St Jeor equation and the split into macronutrients.
A ratio between two numbers does not describe a body.
The index was built to compare populations, and it does that well. Applied to one person, it adds muscle to fat, ignores frame, age and geographic ancestry, and gets it wrong in predictable ways on the builds that sit away from the average.
What really gets measured is what goes in, and what the weight does over several weeks. That is the work Zemiam does, and it is why this number opens a question rather than closing one.
Questions
What comes up most often
How is BMI calculated?
Body mass index is weight in kilograms divided by height in meters squared. For 70 kg and 1.75 m: 70 ÷ 3.0625 = 22.9. Pounds and inches reach the same number through 703 × lb ÷ in², which is that same division with the conversion folded in.
The formula reads neither age, nor sex, nor body composition. That is what makes it simple to compute, and it is also the source of every one of its limits.
What counts as a normal BMI?
The WHO places the normal range between 18.5 and 25 in adults. Below 18.5 it distinguishes three degrees of thinness; above 25, overweight, then three obesity classes from 30 up.
These bounds are population markers, drawn on statistical risk observed at scale. They do not describe an individual state of health, and an index inside the normal range is not a check-up.
Does BMI work for a child?
Not with these thresholds. The formula is the same, the reading is not: before eighteen, build is read on reference curves by age and sex, in percentiles, because it moves with growth. An index of 17 is unremarkable at seven and says something else at seventeen.
Growth charts are published by national health authorities and printed in child health records. Applying adult thresholds to a child produces a false verdict wearing the look of a result: this calculator does not offer it.
Why is BMI criticized?
Because it only measures a ratio between two numbers. It does not separate muscle from fat, ignores where that fat sits, and accounts for neither age, nor frame, nor geographic ancestry.
The cases where it gets things wrong are known and common: a very muscular person lands in overweight with no excess fat; an older person who has lost muscle keeps a normal index with a high fat mass. It is a screening indicator at the scale of a population, not a diagnosis at the scale of a person.
What does waist circumference add?
It corrects the main limit of the index by saying where the mass sits. At equal index, an abdominal distribution carries a higher cardiometabolic risk, and the WHO publishes thresholds for it: 94 cm (37 in) and 102 cm (40.2 in) in men, 80 cm (31.5 in) and 88 cm (34.6 in) in women.
The measurement is taken midway between the lowest rib and the top of the hip bone, at the end of a normal breath out, without pulling tight. Those thresholds hold for European populations; they are lower for South and East Asian populations.
What is the ideal weight for my height?
There is no ideal weight, there is a range. This page shows it: the interval that puts the index between 18.5 and 25 for the height entered, and it is commonly twenty kilos wide, around forty-four pounds.
The formulas that return a single number, Lorentz or Broca, read height alone, date from the nineteenth century for the second, and carry no established clinical value. A range drawn from the WHO classification answers the same question without inventing a precision that does not exist.
Is this calculation medical advice?
No. It applies a published formula to two typed measurements. Pregnancy, breastfeeding, a chronic condition, intense training or an eating disorder change how this number reads, and no calculator knows about them.
The index is a way into a conversation with a doctor, not a conclusion.
Is the calculation free?
Yes, with no account, no sign-up and no cap. Nothing is stored: what gets typed stays in the browser and goes nowhere.
Sources for the classification
- The eight-category classification this page applies. World Health Organization. “Obesity: preventing and managing the global epidemic. Report of a WHO consultation”. WHO Technical Report Series 894, Geneva, 2000.
- The waist thresholds, and how they shift between populations. World Health Organization. “Waist circumference and waist-hip ratio: report of a WHO expert consultation”. Geneva, December 8 to 11, 2008.
- The origin of the index, built to describe a population and not a person. Quetelet A. “Sur l'homme et le développement de ses facultés, ou Essai de physique sociale”. Bachelier, Paris, 1835.
This page applies a published classification to typed measurements. It replaces neither a doctor nor a dietitian, and it does not suit pregnancy, breastfeeding, a chronic condition, or an eating disorder.
Next
The number is set. The next question is elsewhere.
How much to eat in a day
The index says where a build sits, never what a day asks for. Calorie needs come out of Mifflin-St Jeor, from height, weight, age and activity level, and they split into protein, carbs and fat.
Free calorie calculatorWhat to fill the day with
The recipes in the catalogue carry their own calories and macros, recalculated from the Anses Ciqual table rather than estimated. Enough to attach a number to a real plate.
See the recipesAn index gets estimated. A day gets measured.
In Zemiam, a photo of the meal is enough to log the day, and weight is followed as a trend rather than as isolated weigh-ins.