BMI is off because the formula is based on a two-dimensional model.  BMI = kg/m2 People are three dimensional, but we still don't scale up as perfectly as geometric figures, so a nice clean formula is a little more difficult.  There are a lot of adjusted formulas that generally aren't used simply because the old one is so simple and prevalent.  Under the current formula, this means that BMI is accurate for people that are about 5'6" tall.  Anyone that's too far from that needs to adjust their numbers.  For people that are short, around 5' or so, need to add a full point to their score.  People that are around 6' tall need to subtract one. A new formula that's a bit more accurate is BMI = 1.3*kg/m2.5   which I've linked since absolutely no one wants to try to do that on their phones. However, I will say this about BMI.  Even though the popular formula is slightly off, it's still an excellent indication for at least the general weight class for most people.  It may not be perfect down to the tiniest body fat measurement like a DXA scan (extremely accurate, bonus points for knowing about it), but it's good enough for general assessment.