Obesity is measured by body mass index (BMI), which for an adult is a ratio of weight in kilograms by height in metres squared. The NHS has an online calculator. It does not work for children who are still growing and it is often regarded as a blunt instrument for individuals, because some very fit people have bulky muscle tissue that will make them heavier. But for most people and for looking at populations as a whole, it is a useful indicator of healthy or unhealthy weight.
Normal weight is a BMI of 18.5 to 25. Below that you are underweight, which is also unhealthy. Between 25 and 30, an adult is considered overweight. BMI of 30 to 40 is obese. Anyone with a BMI higher than that is considered severely or morbidly obese and in need of medical intervention, such as bariatric surgery.
The categories are not exactly the same for people from black, Asian and other minority ethnic groups, who have a raised risk of obesity-related diseases such as type 2 diabetes with a BMI of 23 and a high risk at 27.5.
You can also find out whether you are obese with a tape measure. Carrying fat around your stomach is a significant risk for heart disease, type 2 diabetes and stroke. To measure your waist, put the tape around the midpoint between the bottom of the ribs and top of the hips and breathe out naturally.
If a woman’s waist is 80cm (31.5ins) or more or a man’s is 94cm or more, they would be advised to lose weight. A waist of 88cm or more for women or 102cm or more for men puts you at very high risk and you should speak to your GP.
You can also use the tape measure to check whether you have a healthy waist to hip ratio. The higher the ratio, says the British Heart Foundation, the greater the risk. You can pretty much tell by looking in the mirror. A pear shaped body, with larger hips than waist, is healthier than an apple shape. You should measure the widest part of your hips. High risk is defined as a waist-hip ratio above 0.90 for males and above 0.85 for females.
The BHF also describes the “string challenge”. Measure your height with a piece of string. Then fold it in half and see if it fits around your waist. If it doesn’t, you are at risk of obesity-related disease.
• The main image used for this article was changed on 28 July 2020 as the original did not meet the Guardian’s guidelines for imagery used to illustrate obesity. This article was further amended on 29 July 2020 to change the reference in the subheading to “how can you tell if you are unhealthily” overweight, to “how can you tell if you are obese”.