According to the NHS, your Body Mass Index (BMI) should sit somewhere between 18.5 and 24.9. Anything over that, and you're overweight.
So something's gone seriously wrong when you're just 14 years old and your BMI is a whopping 100.
That was the case for Indian teen Mihir Jain, who has shed more than a third of his body weight since having gastric bypass surgery back in April.
He's dropped 10 stone from his pre-surgery weight of 37 stone, and can breathe properly for the first time. It's also the first time he's been able to walk unaided.
The boy from Delhi was labelled the world's heaviest teen before the life-saving surgery.
Mihir was born a normal, healthy weight - 5lb 5oz - but weighed 12 stone by the time he was just five years old. That's about four times heavier than healthy children that age.
He was too young to have surgery then, so was put on medication instead. His mother has claimed that that medication left her son bed-bound and resulted in him actually gaining more weight.
His weight until now has affected his mental health as well as his physical health. "I used to get very angry and I was aggressive all the time, I had many problems," he said.
Mihir had to be home-tutored and lost touch with all his friends, even though his sister carried on with her life as normal.
His previous vegetarian diet consisted of high amounts of carbohydrates - full-fat milk, ice cream, fried potato, rice, and fizzy drinks.
His parents started the process of getting Mihir the gastric bypass operation five years ago, but he was too large to leave the house for appointments.
Once he made it to the hospital - at this stage weighing 37 stone despite being just 5'2" - he was instructed to lose a bit of weight to make him safe for surgery.
He spent four days in hospital post-op, surviving on a dull diet of soup and juice, which he'll have to stick to for most of his life. But he's able to walk more now, and is, for the first time in his life, looking forward to his future.
"I couldn't do normal everyday things," he said. "But I used to say to myself: 'don't worry, you'll do it some day'. I have reached that some day.
"For the first time in ages I am looking forward to the future."