How to calculate BMI? Calculator and interpretation

28 Mar

How to calculate BMI? The formula is simple — weight in kilograms divided by length in meters squared — but interpreting the result requires more thought than most people think. BMI (Body Mass Index) is used by doctors, insurance companies and researchers worldwide as a quick screening tool to assess whether a person is in a healthy weight range. At the same time, the measure has obvious limitations that affect how you should interpret your own results. Here we go through BMI calculation step by step, present the table that doctors use as a starting point, and discuss the options for those who want a more complete picture of their body composition.

BMI calculation — the formula and a calculation example

The BMI formula looks like this:

BMI = body weight (kg) ÷ (height in meters × height in meters)

A concrete example: a person who weighs 82 kg and is 1.76 m tall calculates his BMI as follows: 82 ÷ (1.76 × 1.76) = 82 ÷ 3.0976 = 26.5. According to the WHO classification, it means overweight (BMI 25.0–29.9).

How do you interpret your BMI result using the BMI table?

The BMI scale used internationally looks like this:

|BMI range|Classification|Health risk (general)|

|Under 18.5|Underweight|Increased risk of nutritional deficiency, osteoporosis|

| 18,5–24,9 |Normal weight|Lowest general health risk|

| 25,0–29,9 |Overweight|Slightly increased risk|

| 30,0–34,9 |Obesity grade I|Increased risk|

| 35,0–39,9 |Obesity grade II|Clearly increased risk|

|40.0 and above|Obesity grade III|Very high risk|

The table applies to adults (18+ years). For children and young people, separate growth curves are used that take age and gender into account. Older people (65+) often have a different risk profile: a BMI of 25–27 has been shown in several studies to be linked to lower mortality in the elderly, which has led to discussions about raising the limit of “normal weight” for the elderly population.

One thing that the table does not capture: where on the body the fat is located affects the risk at least as much as the total amount of fat. Visceral fat (around the organs in the abdominal cavity) is metabolically active and produces inflammatory markers, while subcutaneous fat (under the skin, for example on the thighs and hips) has a lower health risk per kilogram. It is possible to have a BMI of 26 and excellent metabolic health — but also to have a BMI of 24 with a high percentage of visceral fat and hidden risk factors.

A common question concerns normal weight BMI and what is actually “ideal” from a health point of view. Research published in The Lancet (2016) using data from 10.6 million participants worldwide showed that the lowest mortality risk was at a BMI of 20–25 for non-smokers. Each increase of 5 BMI units above 25 was linked to approximately 31% increased mortality risk. Interestingly, being underweight (BMI below 18.5) was also linked to increased risk of mortality — nutritional deficiencies, reduced bone mass and compromised immune systems are well-known risks at the lower end of the scale.

What exactly is BMI — and what doesn’t it measure?

BMI was developed by the Belgian mathematician Adolphe Quetelet in the 1830s as a statistical tool to describe the body composition of populations at large. It was never intended as an individual diagnostic measure — a nuance that is often forgotten in today’s debate.

What BMI measures is the ratio between weight and height. What BMI does not measure is body composition: what proportion of weight is made up of muscle mass, fat mass, bone and water. Two people with an identical BMI can have radically different bodies. A person who strength trains 5 times per week with 12% body fat may have the same BMI as a sedentary office worker with 30% body fat — but their health risks are vastly different.

This limitation makes BMI particularly unreliable for three groups:

• Fit individuals with a lot of muscle mass — muscle weighs more per unit volume than fat, giving a falsely elevated BMI. Professional rugby players often have a BMI of 28–32 without being morbidly obese.

• Older people with age-related muscle loss (sarcopenia) — may have a “normal” BMI but a high percentage of body fat and low muscle mass, which is a hidden health risk.

• Individuals of certain ethnic backgrounds — studies show that people of Southeast Asian background develop metabolic complications at lower BMI (as early as 23-25) compared to Europeans, while people of Polynesian background may be metabolically healthier at higher BMI.

Despite the limitations, BMI is valuable as a population-level screening tool and as a first indication of individual risk. The problem arises when it’s used as the sole measure — and when individuals make decisions about diet, exercise, or medical treatment based solely on a number that doesn’t take into account their unique body composition.

There are active discussions in medical research about replacing or supplementing BMI with more nuanced measures. The Edmonton Obesity Staging System (EOSS) classifies obesity based on functional impairment and metabolic disease rather than weight alone, and is already used at some Swedish specialist clinics. The system divides patients into stages 0–4, where a person with a BMI of 32 but no metabolic disorders falls into stage 0, while a person with a BMI of 31 and type 2 diabetes falls into stage 2 — a distinction that BMI alone cannot make.

Alternatives to BMI — which measurements give a more complete picture?

Several additional measures can give a fairer picture of health risk linked to body weight:

Waist measurement is the easiest and most accessible option. Measure around the waist at the height of the navel. Limit values ​​(WHO): men >94 cm mean increased risk, >102 cm clearly increased risk. Women >80 cm mean increased risk, >88 cm clearly elevated. Waist measurement correlates better with visceral fat than BMI does.

Waist-to-hip ratio (WHR) captures the distribution of fat between the torso and hips. Men with WHR >0.90 and women with WHR >0.85 have increased cardiovascular risk. This measurement is also easy to measure at home with an ordinary tape measure.

Waist-length ratio (WHtR) has been shown in meta-analyses to be the best single anthropometric measure for predicting cardiometabolic risk. The rule of thumb is simple: if your waist measurement exceeds half of your height, you should act. A person who is 180 cm tall should therefore aim for a waist measurement below 90 cm.

DEXA scanning (Dual-energy X-ray Absorptiometry) is the gold standard for measuring body composition — fat, muscle and bone are quantified separately with high precision. The cost is SEK 500–1,500 per scan in Sweden, and it is offered by some hospitals and private clinics. Bioimpedance analysis (BIA) is a cheaper alternative that is built into some personal scales and is used in gyms and health centers. BIA provides a rough estimate of body fat percentage by measuring electrical resistance — but accuracy varies greatly depending on hydration, time of day, and equipment quality. As a home tool for following trends over time, BIA works acceptably, but for single measurements, DEXA is superior.

How can you use your BMI result in practice?

A BMI result is a starting point, not a final verdict. If your BMI lands in the 25-30 range, it doesn’t automatically mean you need to lose weight — but it does warrant a deeper investigation. Check your waist measurement, think about your physical activity level and feel free to take blood tests (fasting insulin, HbA1c, blood fats) to see what your metabolic health actually looks like.

If your BMI lands in the 18.5-24.9 range and your waist measurement is within the limits, there’s probably no immediate cause for concern — but measuring regularly (for example, every quarter) will help you catch trends before they become problems. Sudden weight gain without an obvious explanation can be an early sign of hormonal changes, medication side effects or lifestyle changes that deserve attention.

If your BMI is 30 or higher, there are strong reasons to seek medical advice. Since 2024, the National Board of Health and Welfare recommends that pharmacological treatment can already be offered at a BMI of 27 if there are weight-related sequelae. Read more about what options are available in our article on effective weight loss injections.

Regardless of where you fall on the scale, a combination of measurements — BMI plus waist measurement plus metabolic markers — is always better than a single number. Your body is more complex than a single formula can capture, and decisions about diet, exercise or medical treatment should never rest solely on the number from a BMI calculator. Do you want to start taking concrete measures? Our guide on how to lose weight fast provides an evidence-based plan to get you started.

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