Underweight, healthy weight, overweight and obesity: what they mean and what they do not

What does BMI measure, and what does it not measure?

Body mass index (BMI) is calculated by dividing weight in kilograms by height in metres squared. According to the World Health Organization, it is an indirect marker of body fat and does not measure it directly [1]. Two people with the same weight and height have the same BMI, but their body composition can differ: one may have more muscle and the other more body fat.

For example, take 70 kilograms and 1.70 m. 1.70 times 1.70 is 2.89, and 70 divided by 2.89 is 24.2. The calculation does not tell you what that weight is made of.

Other measurements, such as waist circumference, help put the result into context [1]. BMI classifies weight; diagnosis requires more information.

What are the BMI cut-offs for underweight, healthy weight, overweight and obesity?

In adults, overweight starts at a BMI of 25 and obesity at 30 [1]. Underweight means a BMI below 18.5 from the age of 18 [2]. A BMI between 18.5 and 24.9 is considered a healthy weight [3], the range often called normal weight.

Obesity is divided into three classes: 30 to 34.9, 35 to 39.9, and 40 or above [3]. The Spanish obesity guideline caps the third class at 49.9 and adds a fourth from 50 upwards [4].

If you have seen charts that define underweight as below 20, they do not follow the current cut-off of 18.5 [2].

Do these cut-offs apply at every age, with any amount of muscle, and across all backgrounds?

Not entirely. These are adult cut-offs and should be interpreted with three points in mind.

Age. These charts do not apply to children and teenagers; BMI charts adjusted for age and sex are used instead [3]. From age 65 onwards, the UK obesity guideline recommends interpreting BMI with care [3]. Other health conditions and what a person can do in day-to-day life also matter. A slightly higher BMI may be protective at this age [3].

Muscle. With a lot of muscle mass, BMI gives a less accurate picture of body fat [3]. The reverse can also happen: with little muscle mass, a BMI between 18.5 and 24.9 may hide excess body fat [4].

Background. The thresholds are lower for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds. Overweight is a BMI of 23 to 27.4, and obesity starts at 27.5 [3].

BMI was developed using a Caucasian population and is less accurate for people from other backgrounds [4]. It is also less accurate during pregnancy, after menopause, in people under 18, and when there is substantial fluid retention [4]. BMI cut-offs are the same for men and women, so BMI should be interpreted alongside waist circumference and in light of sex, age and background [4].

How to measure your waist and what it adds

Waist circumference adds to the BMI assessment. Measure standing up, with the tape parallel to the floor, halfway between the lowest rib and the top of the hip bone (iliac crest), after breathing out [4].

For people of Caucasian background, risk increases from 94 cm for men and 80 cm for women. It is higher from 102 cm and 88 cm, respectively [4]. This measurement is more useful when BMI is below 35, because above that it is harder to distinguish fat inside the abdomen [4].

The UK obesity guideline suggests dividing waist circumference by height when BMI is below 35 [3]. The aim is for your waist to be less than half your height, a ratio below 0.5 [3]. This applies to men and women, across backgrounds, and also to people with a lot of muscle [3]. Between 0.5 and 0.59, the risk of type 2 diabetes, high blood pressure and cardiovascular disease is higher; from 0.6 upwards, it is higher still [3].

If you are 170 cm tall, half your height is 85 cm. If your waist is over 85 cm, that matters even if your BMI is in the middle range.

Are weight and health the same thing?

No. The Spanish obesity guideline describes BMI as a measure of size, not health [4]. An overweight BMI does not, by itself, mean that you are ill. And normal blood test results do not erase distress around food.

That is why an initial assessment includes blood pressure in both arms, fasting blood glucose, glycated haemoglobin (a summary of blood glucose over recent months) and cholesterol [4]. Looking at BMI alone can also lead people to judge someone by their size [4].

When a full assessment confirms it, obesity is a chronic disease linked to excess body fat that harms health [4]. Our obesity guide explains what this means and what support can look like. If your goal is to lose body fat gradually while preserving muscle, start with our weight-loss guide.

When should you seek advice about underweight or overweight?

A BMI below 18.5 warrants a medical appointment if you have not been trying to lose weight [2]. If food has also become a source of fear or rules, start with the assessment in the next section before following any plan to gain weight. If gaining weight is appropriate and your doctor has ruled out another cause, see our guide to gaining weight.

A BMI between 25 and 29.9 is not an emergency because of the number alone. Consider it alongside waist circumference while BMI is below 35 [3], and alongside blood pressure, blood glucose and blood lipids [4]. With a BMI of 30 or above, the relevant framework is the obesity guide linked above.

At either end of the range, if your weight goes down or up without any change in what you eat, contact your doctor first. Food advice cannot identify the cause.

What do we see in appointments when someone arrives with a label about their weight?

Three situations come up almost every week.

• Taking the category as a diagnosis. A calculator classifies weight; an assessment is made by someone who considers the whole picture.
• Weighing yourself all the time after seeing the label. The number becomes a daily worry and tells you little more than it did the first time.
• Wanting to gain weight without knowing why it was lost. If weight has gone down without you trying, your doctor should look for the cause first.

If you are afraid of eating

Some behaviours cannot be addressed with a meal plan. The UK guideline on eating disorders says they should be taken into account [5].

• A very low or very high BMI for your age.
• Rapid weight loss.
• Restricting food when you are already underweight.
• Avoiding meals with other people.
• Worry about weight or body shape taking over your day.
• Trying to compensate for eating by vomiting, using laxatives or exercising.

The risk of an eating disorder is higher between the ages of 13 and 17 [5]. If an eating disorder is suspected, the guideline says not to decide what help is needed based on BMI alone and to refer promptly to an age-appropriate specialist team [5]. At Alimmenta, this means starting with specialist psychological support rather than a meal plan. If you recognise yourself here, do not wait until the number “deserves” it.