By WHO standards, 18.5–24.9 is normal; 25+ is overweight; 30+ is obese. In Korea, 23+ is sometimes classified as overweight.
Not necessarily. BMI cannot distinguish fat from muscle. Athletes with high muscle mass can have a high BMI. Use it as a general reference only.
Ideal weight (kg) = height (m)² × 22. For example, for 170 cm: 1.7² × 22 ≈ 63.6 kg.