Found some information - I think most doctors use what I've been taught, which is 20-25 is healthy, but the WHO (world health organisation guidelines) say:
* Underweight — BMI < 18. 5 kg/m2
* Normal weight — BMI greater than or equal to 18.5 to 24.9 kg/m2
* Overweight — BMI greater than or equal to 25.0 to 29.9 kg/m2
* Class I obesity — BMI = 30.0 to 34.9 kg/m2
* Class II obesity — BMI = 35.0 to 39.9 kg/m2
* Class III obesity — BMI greater than or equal to 40 kg/m2. This type of obesity is also referred to as severe, extreme, or morbid obesity.
The definition of overweight and obesity varies by race. The WHO and NIH Guidelines are currently applied to whites, hispanics and blacks. For Asians, overweight is a BMI between 23 and 29.9 kg/m2 and obesity a BMI >30 kg/m2.
I've also found a good table comparing body fat percentage and BMI: Females Males
Age 20 to 39 Percent fat
18.5 20% 25% 21% 8% 13% 8%
25 32% 35% 33% 20% 23% 21%
30 38% 40% 39% 26% 28% 26%
Age 40 to 59 Percent fat
18.5 21% 25% 23% 9% 13% 11%
25 34% 36% 35% 22% 24% 23%
30 39% 41% 41% 27% 29% 29%
A little confusing! If you go across the top of the numbers, the first column is BMI, the next six are all percentage body fat. Starts with females (first three), then males (second three), and the ethnic groups are African-American, Asian and Caucasian in order (1,2,3 then 1,2,3 for men). Make sense?
WAIST CIRCUMFERENCE: In adults with a BMI of 25 to 34.9 kg/m2, a waist circumference greater than 102 cm (40 in) for men and 88 cm (35 in) for women is associated with a greater risk of high blood pressure, type 2 diabetes, and high cholesterol, and heart disease.
Hope this helps! I'll try to summarise : Basically, underweight is considered below 18.5, but it depends on your ethnic group. The body fat percentages related to BMI are above. Women always have a higher body fat than men. If you have an underweight BMI, you are very unlikely to have dangerously high levels of body fat - anywhere up to and often over 30% is still okay as long as BMI and waist circumference and within normal limits.