What Is BMI? How to Calculate It and Read the Result

By Nesim Razon, independent Android developer · 2026-11-10

Body mass index, or BMI, is a single number that relates your weight to your height. Doctors and public health agencies use it as a quick screening tool. This article explains the formula, the official categories, how to work it out by hand and what the number cannot tell you. It is general information, not medical advice.

What BMI actually is

BMI stands for body mass index. It was designed to describe, for groups of adults, how weight compares with height. It is not a measurement of body fat and it does not look inside the body. Two people with the same height and weight get exactly the same BMI, even if one of them is very muscular and the other is not.

That is why health professionals call it a screening measure. It is cheap, fast and needs only a scale and a tape measure, so it is a handy first look. A high or low result is a reason to ask questions, not a diagnosis.

The formula

In metric units the formula is weight in kilograms divided by height in metres squared: BMI = kg / m2. Squaring the height means multiplying it by itself. A person who is 1.75 m tall has a height squared of 1.75 x 1.75 = 3.0625.

If you use pounds and inches, the formula is BMI = 703 x pounds / inches squared. The number 703 is only a unit conversion factor, so both versions give the same result for the same person. Do not mix the two: use 703 only with pounds and inches, never with kilograms.

Worked examples

• 70 kg and 175 cm: height in metres is 1.75, squared 3.0625. 70 / 3.0625 = 22.86, shown as 22.9. This falls in the normal range.
• 5 ft 9 in and 154 lb: the height is 69 inches, squared 4761. 703 x 154 = 108,262. Divided by 4761 that is 22.74, shown as 22.7. The same person as above, give or take rounding.
• 50 kg and 170 cm: 1.70 squared is 2.89. 50 / 2.89 = 17.3, which is in the mild thinness band.
• 100 kg and 175 cm: 100 / 3.0625 = 32.65, which is 32.7 when rounded and falls in obesity class I.

Try a few of your own numbers on paper first; once you have done it twice, the steps become automatic.

The WHO categories for adults

The World Health Organization groups adult BMI values into bands. The thresholds are 16, 17, 18.5, 25, 30, 35 and 40. The BMI Calculator app uses these bands and shows the result with one decimal place, picking the category from that rounded value.

• Below 16: severe thinness
• 16 to 16.9: moderate thinness
• 17 to 18.4: mild thinness
• 18.5 to 24.9: normal weight
• 25 to 29.9: overweight
• 30 to 34.9: obesity class I
• 35 to 39.9: obesity class II
• 40 and above: obesity class III

Because the app rounds to one decimal, a raw result of 24.96 is shown as 25.0 and counted as overweight. Edges like this are arbitrary lines drawn through a smooth scale, so a person at 24.9 and a person at 25.0 are not really different.

How a healthy weight range is found

The normal band runs from 18.5 to 24.9. If you turn the formula around, weight = BMI x height squared, you can find the weights that correspond to those two limits for your own height. For a person who is 1.70 m tall, height squared is 2.89. Then 18.5 x 2.89 = 53.5 kg and 24.9 x 2.89 = 72.0 kg. So the normal range for that height is roughly 53.5 to 72 kg.

This is how the app shows a healthy weight range for your height and how many kilograms separate you from it. Treat it as a reference band, not a personal target. A wide range exists on purpose, because bodies differ.

What BMI cannot tell you

• Body fat and muscle: the formula only sees total weight. Muscle is dense, so a strong, lean person can land in the overweight band.
• Where fat is stored: fat around the waist is treated differently by doctors than fat elsewhere. Waist circumference and other measures add information that BMI lacks.
• Age and sex: the same BMI can mean different things for a young adult and an older person.
• Ethnic background: the WHO has noted that in some populations health risks appear at lower BMI values than the standard cut-offs.
• Children and teenagers: the adult bands do not apply. For under-18s a BMI is compared with percentile charts for age and sex, and a doctor should interpret it.

How small errors move the number

It helps to know how sensitive the result is. At a height of 1.70 m, one kilogram changes BMI by about 0.35, because 1 divided by 2.89 is about 0.35. At 1.75 m the same kilogram changes it by about 0.33. So a swing of two or three kilograms from water, food or the time of day can shift the result by around one point.

Height errors matter too, and they work in the opposite direction. Take a person who weighs 70 kg. At 170 cm the BMI is 24.2, but if the height is entered as 171 cm it becomes 23.9. One centimetre is enough to move the result by about 0.3. Measure your height carefully, barefoot and standing straight against a wall, before you trust a result near a category limit.

A simple by-hand routine: first convert your height to metres, second multiply it by itself, third divide your weight by that number. Write down the result to one decimal place and compare it with the list above. If you do this once a few months apart, you will see the trend, which says more than any single value.

A sensible way to use the number

Use BMI as one piece of information alongside how you feel, your energy, your blood pressure and blood tests, and what your doctor knows about your history. If the result surprises you, do not panic and do not start drastic changes on your own. Talk to a doctor or another qualified health professional, who can look at the whole picture.

This article and the BMI Calculator app give general information only. They do not diagnose, treat or replace a medical consultation. In the app, your height and weight stay on your phone, and it gives you the BMI, the category and the healthy weight range in centimetres and kilograms or in feet and pounds.