BMI calculator

Body mass index and recommended range. Free, no upload. 1 GB max · Up to 1 GB · Processed locally, never sent to a server.

Body mass index relates a weight to a height. Two fields are enough, weight in kilograms and height in centimetres, and the page returns the index to one decimal, the World Health Organization category it falls into, and the weight range that matches the so-called normal band for that height.

The calculation is a single division, but reading the result calls for care that few pages bother to spell out. The index was built as a statistical instrument for describing populations. Applied to one person it knows nothing about body composition, age, sex or medical history, which sharply limits what the number can honestly be used for.

Two practical points before you start. The fields are metric only, kilograms and centimetres, with no unit switch: if you weigh in pounds and measure in feet, convert first, and the conversions are set out below. And nothing here is a diagnosis. This page recommends no diet and replaces no consultation; the figures you type stay in the page, are neither stored nor transmitted, and disappear when the tab is closed.

How to use it

  1. Enter your weight in kilograms The field takes decimals in steps of 0.1, so 68.4 is a valid entry and a reading off the scales does not have to be rounded. Working in pounds, multiply by 0.45359237 first: 154 lb is 69.9 kg.
  2. Enter your height in centimetres The field expects 175, not 1.75. A height typed in metres produces an index tens of thousands of times too large, an error impossible to miss. From feet and inches, multiply the total inches by 2.54: 5 ft 9 in is 69 in, so 175.3 cm.
  3. Read the index The first box gives the index to one decimal, in the conventional unit of kilograms per square metre, which is not repeated on screen.
  4. Read the interpretation The second box turns the index into a category, using five labels: underweight, normal weight, overweight, moderate obesity, and severe obesity above 35.
  5. Look at the weight range The third box reverses the formula and gives, for the height you entered, the two weights that correspond to an index of 18.5 and an index of 24.9.

The formula, and what to do with pounds and inches

The index is the weight in kilograms divided by the square of the height in metres. The page converts the centimetres you type into metres first, then divides. For 70 kg and 175 cm: 1.75 × 1.75 = 3.0625, then 70 ÷ 3.0625 = 22.857, displayed as 22.9.

The weight range comes from the same relationship read backwards, multiplying the square of the height by the two boundaries of the normal band. Still at 175 cm, 18.5 × 3.0625 = 56.7 kg and 24.9 × 3.0625 = 76.3 kg, which is the interval shown in the third box.

Height enters squared, which makes the result sensitive to how it was measured. The same 70 kg gives 23.4 at 173 cm and 22.3 at 177 cm: four centimetres of difference move the index by more than a full point. Measuring with shoes on, or from memory, is enough to push a value sitting near a boundary into the next category.

Imperial units have to be converted before they go in, since the fields offer no switch: pounds × 0.45359237 gives kilograms, inches × 2.54 gives centimetres. The familiar shortcut, weight in pounds × 703 divided by height in inches squared, returns the same index because 703 is those two conversion factors folded together: 154 lb at 69 in gives 22.7 either way.

The World Health Organization thresholds

The classification used is the WHO's for adults: below 18.5, underweight; from 18.5 to 24.9, normal weight; from 25 to 29.9, overweight; from 30 upwards, obesity. This page labels the 30 to 34.9 band moderate obesity and anything above 35 severe obesity.

The international classification splits obesity into three classes, the third beginning at 40; the display stops before that subdivision. These boundaries are statistical conventions anchored in mortality and morbidity studies across large adult populations, not biological frontiers. Nothing changes in a body between 24.9 and 25.0, and a single reading close to a boundary carries very little information on its own.

The WHO has also noted that cardiometabolic risk appears at lower indices in several Asian populations, which led it to propose additional public health action points around 23 and 27.5. The same number therefore does not read the same way everywhere.

What the index does not measure

The formula knows two numbers. It cannot separate fat mass from muscle mass, and it says nothing about where that fat sits. A rugby player, a weightlifter or anyone training seriously with weights routinely passes 25 with no excess fat at all, while a sedentary person can post a perfectly normal index on very little muscle.

Location matters at least as much as quantity: it is abdominal fat that is associated with cardiovascular and metabolic risk, and the index is blind to it. Health professionals therefore pair it with other measurements, waist circumference first among them, along with how the weight has moved over time and what the blood work says.

The index is finally indifferent to anything that changes weight without changing build: oedema, ascites, pregnancy, an amputation. In those situations the value calculated stays arithmetically correct and practically meaningless.

Who these thresholds do not apply to

For children and adolescents, the index is read against growth charts by age and sex rather than against the fixed boundaries shown here. Build changes a great deal during growth, and the same figure does not mean the same thing at six and at fifteen. Paediatric practice works in centiles, not in the adult bands above.

In older adults, muscle loss and vertebral compression distort both sides of the division at once, so an identical index describes a different body. Several national guidelines use a higher undernutrition threshold after seventy, around 22, where 18.5 applies to younger adults. During pregnancy, weight gain is expected and it is the pre-pregnancy index that clinical follow-up works from.

Athletes, anyone carrying a large muscle mass and anyone recovering from illness sit outside the assumptions behind the bands. An index above or below them is neither a verdict nor a diagnosis. At most it suggests a conversation with a doctor or a dietitian, with an examination, a personal history and measurements that two input fields will never replace.

Frequently asked questions

Should I enter my height in metres?

No, in centimetres. The page divides by a hundred before squaring, so 1.75 typed as it stands produces an index of around two hundred thousand, which cannot be mistaken for a plausible value. Type 175.

Can I enter pounds and inches?

Not directly: the two fields are kilograms and centimetres, with no unit switch. Convert first, pounds × 0.45359237 and inches × 2.54. The imperial shortcut, pounds × 703 divided by inches squared, gives the same index if you would rather check the figure that way.

Why is a muscular athlete classed as overweight?

Because muscle is denser than fat and the formula cannot tell them apart. A high index alongside a normal waist measurement and regular training does not mean the same thing as the same index with very little muscle behind it.

What should I do with the category shown?

Read it as a statistical marker, not as a test result. A health professional will put it back into a wider picture: how the weight has moved, waist circumference, family and personal history, current medication and daily habits. On its own the label decides nothing.

Does the index work for a child?

Not with these thresholds. Below eighteen, the index is plotted on growth charts by age and sex, which take the stage of growth into account. The fixed adult bands used on this page would misclassify a large share of children.

Is the weight range shown a target?

It is an arithmetic consequence of applying the 18.5 and 24.9 boundaries to your height, and nothing more. It takes no account of your build, your age, your muscle mass or your state of health, and it is not a recommendation to reach any particular weight.

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