A healthy body mass index, or BMI, is generally considered to range between 18.5 and 24.9. However, this is far too basic and frequently fails to accurately assess a person's actual level of health. When calculating BMI, if the calculations are based solely on weight and height, a perfectly healthy and fit person might be called underweight. One the other hand, obesity might be declared when someone is really only slightly overweight.
Body Shape and Body Mass Index
Your body shape, which most BMI calculators fail to take into account, plays a very important part in determining whether or not your weight is healthy for your height. However, more detailed calculations involving BMI can be accurate in determining your level of health. Finding your waist circumference can help you know more precisely what to make of your BMI. Also, by calculating your level of exercise and body mass index, the calories consumed per day to remain at a constant weight can be estimated.
It's difficult to pinpoint a specific healthy body mass index for various body types since everyone's body shape is different. Muscle mass is heavier than fat, so a body builder may be considered obese based on BMI alone (or an extremely out-of-shape person considered healthy). Because of this, and other inaccuracies, it may be a better and more informative use of your time to instead calculate your body fat percentage.
Like BMI, calculating your percent body fat is easy to do online, but unlike BMI, your body fat percentage takes many different measurements into account. It's a much more accurate indication of your health overall.
This isn't to say that BMI is useless, however. It can provide a rough guideline for ideal height-weight proportions and, when combined with more specific measurements such as waist circumference, provide warning of potential health risks. It can be useful to calculate your BMI to use with other measurements, but calling any BMI "healthy" is inaccurate. BMI in and of itself cannot be an accurate indication of health, not without having additional measurements to calculate body fat percentage (or warn of possible future health issues).
A Healthy BMI
To answer the question posed in the title: A "healthy" BMI for your shape doesn't necessarily mean "healthy" at all. The standard BMI range of 18.5 to 24.9 for adults may be used as a reference, but shouldn't be taken as a measure of health. Actual health can be determined by calculating body fat percentage, but really has little to do with your BMI.