How accurate is bmi in relation to your body fat?

Both are needed : Bmi ( body mass index) sex specific and age determined. Fat density determined with calipers, immersion and or ultrasound methods affects this index as does bone structure.

It doesn't care: The Body Mass Index (BMI) is basically a relationship of the mass of a persons body divided by their height. It is best used in population studies, rather than individual assessments, though it often is. The "normal" assumes an average proportion of muscle mass/ bone structure and fat. However, you could be all muscle or all fat and with the same bone structure and the BMI would be the same.