Use the simple rule "Keep your waist to less than half your height" — a stronger predictor of risk than BMI.
Body Mass Index treats a muscular athlete and an overweight desk worker identically if their BMI is the same — but they carry very different cardiometabolic risks. Waist-to-Height Ratio (WHtR) captures abdominal fat, which is more metabolically active and dangerous than peripheral fat. Multiple meta-analyses (Ashwell 2012; Ross 2020) show WHtR predicts diabetes, hypertension, and cardiovascular events better than BMI.
Stand relaxed, exhale normally, measure at the midpoint between the bottom of the rib cage and the top of the iliac crest (usually at the navel). The tape should be snug but not compressing skin.