BMI is simply a measure of risk. If you're white, risk of health problems (type 2 diabetes, cardiovascular disease, various cancers) increase at a BMI of over 25, increase more at over 30 and even more at over 35. However, some people will deal better with excess weight than others. Another, more accurate, assessment of risk is waist circumference.
You have a higher risk of health problems if your waist size is:
more than 94cm (37 inches) if you're a man
more than 80cm (31.5 inches) if you're a woman
Your risk of health problems is even higher if your waist size is:
more than 102cm (40 inches) if you're a man
more than 88cm (34.5 inches) if you're a woman
The BHF tells you how to measure it: Waist circumference (Don't just use trouser sizes - you're probably wearing them under your waist.)
If you are Asian your risk increases at lower weights /waist circumferences.
The BMI is widely used because it's pretty simple. If you are a professional athlete chances are your BMI will put you in the higher risk category but your waist circumference will be lower. It's not a hard test though - is my BMI >25? Am I an athlete? If I wear my trousers on my hips does my stomach overhang them?If the answers are Yes, No, Yes then you're at higher risk.
The ideal BMI is around 21-22 but you can go down to 18.5 before anyone gets really concerned. 60% of the population have a BMI of over 25 so people with lower, but still normal, BMIs look skinny to modern eyes (unless they've been airbrushed).