They measure different things
Weight is a scalar. Measurements describe shape. Neither substitutes for the other, because they answer different questions.
Why weight fails for clothing
Two people at the same weight can have entirely different chest, waist and hip measurements. Fit depends on the second, not the first.
A 2025 survey found 91% of shoppers say their size depends on the brand, which weight tells you nothing about.
Why measurements fail for health
Waist circumference carries independent health significance in ways weight does not, because it reflects visceral fat rather than total mass.
Which makes accurate waist measurement a health question, not just a shopping one. Unfortunately it is also the least reliably measured value.
Inter-observer correlation for waist was 0.92 and for waist-to-hip ratio only 0.51 in one study of twelve clinicians. The number that matters most for health is the one we measure worst.
The evidence that shape drifts independently
A study of 24,979 adults tracked from 1986 to 2021 found waist circumference and waist-to-hip ratio rose in women at flat BMI. Male change was driven by mass, with weight up 21%.
Shoulder breadth stayed essentially unchanged throughout.
So in that data, women's shape changed without weight changing, while height and shoulders were stable.
What each is for
- Weight — fitness tracking, health monitoring, dosing.
- Measurements — shopping, tailoring, brand comparison.
- Body fat percentage — composition, but not shape, and useless for fit.
Practically
Track weight for what it tells you. Store measurements for what it tells you. Recheck measurements after weight changes, since girth tends to move first.
Further reading in the full comparison and how often to remeasure.