The British Dietetic Association: Definition of BMI.

• A BMI of less than 18.5 kg/m2 indicates you are underweight. You may need to gain weight.
• If your BMI is 19 to 24.9 kg/m2, you're are a healthy weight, and should aim to stay that way.
• A BMI of 25 to 29 kg/m2 is defined as overweight. It's a good idea to lose some weight for your health's sake, or at least aim to prevent further weight gain.
• A BMI of over 30 kg/m2 is defined as fat and means your health is at risk. Losing weight will improve your health.
• A BMI of over 35 kg/m2 is defined as severely obese and means your health is at risk. Losing weight will improve your health.