Body mass index (BMI) is calculated by dividing weight in kilograms by height in metres squared: BMI = weight (kg) ÷ height² (m²). In adults, 18.5 to 24.9 is classed as normal, 25 to 29.9 as overweight and 30 and above as obese. However, BMI cannot tell fat from muscle, so it should be interpreted not on its own but together with waist circumference and other findings.
BMI is one of the most commonly used measurements in health screening; it is quick, free and needs only height and weight. In this article I explain step by step how to calculate your BMI, what the result means and in which situations it can be misleading.
- Formula: BMI = weight (kg) ÷ [height (m) × height (m)].
- For adults: below 18.5 is underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight, 30 and above obese.
- BMI does not tell muscle from fat; it can be misleading in athletes, older people and some ethnic groups.
- A waist measurement of less than half your height is a practical measure for assessing fat around the abdomen.
- The adult BMI table is not used for children, teenagers or pregnant women.
How Is Body Mass Index Calculated?
To calculate BMI, just divide your weight (kg) by your height in metres squared. Remember to write your height in metres rather than centimetres: 165 cm is 1.65 m.
- Convert your height to metres (for example 165 cm = 1.65 m).
- Square your height: 1.65 × 1.65 = 2.72.
- Divide your weight by this number: 70 kg ÷ 2.72 ≈ 25.7.
In this example, an adult who is 1.65 m tall and weighs 70 kg has a BMI of about 25.7, which falls just at the start of the "overweight" range. Someone of the same height who weighs 58 kg has a BMI of about 21.3, which is in the normal range. A calculator is enough for the calculation; because the time you weigh yourself and what you are wearing can make small differences, repeating the measurement in similar conditions gives more accurate tracking.
What Do BMI Results Mean?
In adults, BMI results are interpreted using the ranges used by the World Health Organization (WHO). These ranges are not an individual diagnosis, but a screening indicator in terms of health risks.
| BMI (kg/m²) | Classification | What to do |
|---|---|---|
| Below 18.5 | Underweight | Assessment for undernutrition and underlying causes |
| 18.5 – 24.9 | Normal | Keep up balanced eating and regular movement |
| 25 – 29.9 | Overweight | Assessment together with waist circumference and other risk factors |
| 30 – 34.9 | Obesity (class 1) | Planned weight management with a doctor and dietitian |
| 35 – 39.9 | Obesity (class 2) | Medical assessment for related conditions |
| 40 and above | Obesity (class 3) | Comprehensive medical assessment and multidisciplinary follow-up |
The NHS notes that in people of South Asian, Chinese and other Asian, Middle Eastern, Black African or African-Caribbean origin, health risks can begin at lower BMI values; in these groups 23 to 27.4 is considered overweight and 27.5 and above obese. This is also a reminder that BMI limits do not show the same risk for everyone.
How Can You Find the Weight Range for Your Height?
By turning the formula around, you can find the weight in kilograms that corresponds to a BMI of 18.5 to 24.9 for your height: calculate 18.5 × height² for the lower limit and 24.9 × height² for the upper limit. For example, for someone who is 1.65 m tall, this range is about 50.4 to 67.8 kg.
| Height | Weight corresponding to a BMI of 18.5 to 24.9 (approximate) |
|---|---|
| 1.55 m | 44.4 – 59.8 kg |
| 1.60 m | 47.4 – 63.7 kg |
| 1.65 m | 50.4 – 67.8 kg |
| 1.70 m | 53.5 – 72.0 kg |
| 1.75 m | 56.7 – 76.3 kg |
| 1.80 m | 59.9 – 80.7 kg |
| 1.85 m | 63.3 – 85.2 kg |
This range is quite wide, and not every weight within it means the same thing for everyone. Your age, muscle mass and health determine which part of the range is realistic and sustainable for you. The same calculation gives another practical piece of information: for someone who is 1.65 m tall, 1 BMI point corresponds to about 2.7 kg. So even small changes in BMI can correspond to a difference of a few kilograms on the scales; dividing goals into small, achievable steps helps to keep motivation up.
In Which Situations Can BMI Be Misleading?
BMI cannot tell how much of your weight is muscle and how much is fat, so in some people it can show a picture of health that differs from the real one. A review published in 2024 also stresses that BMI is valuable for population screening but insufficient on its own for predicting individual risk and assessing body fat.
- Muscular people and athletes: Because muscle tissue is heavy, BMI can come out high even though body fat percentage may be low.
- Older people: Muscle mass falls with age; body fat percentage can be high despite a normal BMI.
- Fat around the abdomen: Of two people with the same BMI, the one with the larger waist may have a higher metabolic risk.
- Swelling and fluid retention: Fluid build-up linked to heart, kidney or liver conditions can raise weight and make BMI misleading.
You can also take a look at the article in which I discuss these limits of BMI in more detail, is BMI an accurate predictor of health.
Which Measurements Should You Look at Alongside BMI?
What makes BMI most meaningful is looking at it together with measurements such as waist circumference and body composition. In this way you can see the picture behind the number on the scales more clearly.
Waist-to-height ratio
The NHS recommends that your waist measurement should be less than half your height. For example, for someone who is 1.65 m tall, the aim is for a waist of less than 82.5 cm. Waist circumference is measured at the midpoint between the lower edge of the ribs and the top of the hip bone, after breathing out normally.
Body composition analysis
Body analysis devices used in a dietitian's consultation give an approximate idea of fat, muscle and fluid percentages. Because these measurements can also be affected by factors such as the device, the time of day, fluid intake and exercise, it is more meaningful to look at the trend of measurements repeated in the same conditions than at a single value.
Laboratory and clinical findings
Findings such as blood sugar, cholesterol values, blood pressure, fatty liver and family history show how weight is affecting your health. This assessment should be done together with your doctor.
How Is BMI Assessed in Children and Pregnant Women?
The adult BMI ranges are not used in children and teenagers; BMI is interpreted on growth charts prepared according to age and sex. If you have a concern about your child's weight, the assessment should be made with your child's doctor.
In pregnancy, weight naturally rises with the baby's growth and the increase in fluid volume, so BMI measured during pregnancy is not interpreted against the adult table. BMI before pregnancy, on the other hand, guides the planning of recommended weight gain during pregnancy. For weight monitoring and nutrition in pregnancy, in line with your obstetrician's advice, you can look at my page on nutrition in pregnancy.
What Should You Do If Your BMI Is High or Low?
If your BMI is outside the normal range, the first step is not panic but a thorough assessment. The result is a prompt to start a conversation about your health; it is not a diagnosis on its own.
If your BMI is high, it is healthier to start with sustainable small changes rather than crash diets that promise quick results: a meal routine, portion awareness, a plate based on vegetables and protein, fewer sugary drinks and daily movement. If your BMI is below 18.5, or if you are losing weight without meaning to, it is important to look into the cause with your doctor; I have touched on this in my article on the risks of being very underweight.
What your personal goal should be depends not only on BMI, but on your age, your health, your body composition and your circumstances. If you want to estimate your daily energy needs, you can take a look at my article on calculating basal metabolic rate. For a realistic goal and a sustainable plan that suit you, we can work together through weight management counselling.
Sources
- WHO – Obesity and overweight
- NHS – Overweight and obesity in adults
- Wu Y, Li D, Vermund SH. Advantages and Limitations of the Body Mass Index (BMI) to Assess Adult Obesity. Int J Environ Res Public Health, 2024 (PubMed)
- NHS – BMI calculator
This article is for general information only; please consult your doctor and dietitian about your own situation.
Frequently Asked Questions
What should my BMI be?
For adults, a range of 18.5 to 24.9 is generally considered healthy. However, the right target for a person varies with factors such as age, muscle mass, waist measurement and health, so a single number is not right for everyone.
Is BMI calculated differently for women and men?
In adults, the formula and the classification ranges are the same for women and men. However, at the same BMI, women usually have a higher body fat percentage than men, so additional measurements such as waist circumference strengthen the assessment.
Can someone with a normal BMI be unhealthy?
Yes. Some people with a normal BMI have low muscle mass, a lot of fat around the abdomen or abnormal blood values. This is why BMI needs to be considered together with waist circumference, blood tests and lifestyle.
How often should BMI be measured?
If you are managing your weight, weighing yourself weekly or every two weeks is enough; daily fluctuations are mostly down to fluid. Weighing yourself in similar conditions each time, for example in the morning on an empty stomach, makes tracking more meaningful.
How is BMI interpreted in older people?
Because muscle mass falls with age, BMI can make body fat look lower than it is. In older people, unintentional weight loss, muscle strength and nutritional status can give more important information than BMI; the assessment should be made together with a doctor.
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This content is for information only. For a personalised nutrition programme, please consult a dietitian.
