Tape measure and formulas is better than both weight and BMI. And while the errors can be large for some, you won't mistakenly think you are fit at 35% nor fat

Tape measure and formulas is better than both weight and BMI. And while the errors can be large for some, you won't mistakenly think you are fit at 35% nor fat at 10% using them. There's actually not very much good knowledge out there on exactly what levels are optimal for health. It's mainly based on epidemiological research. However, as long as you aren't below 10-12 % BF (ca. 84 cm waist) there is probably no danger going down. If you are going towards 25% BF and above(ca 98 cm waist) you probably should see over your habits. If you want to keep it simple, just set some random measurement in-between those numbers as a limit and start dieting when above. I don't think the big issue for regular people is that they have tried measuring themselves but by random chance got an inaccurately low estimate, rather that they haven't really tried measuring or does not succeed in losing fat. The latter is sad as losing fat is mostly a solved issue, but the misinformation is so high that it's hard to get on the right track. If someone could solve the information problem they'd be a public health hero! For people into body-building or serious fitness, the issue of estimating fat is a bigger one. Some people go for DEXA scans, but they are quite expensive and takes time. Something doable in your home, but more accurate than tape measure or callipers, especially for tracking variations, would be the ideal.