Why your waist matters more than your weight
Put two people side by side. Same height. Same age. The same number on the scale. Every calorie app you have ever used would hand them an identical target, because the equations behind almost all of them take height, weight, age and sex, and stop there.
Kriterion gives them different numbers. The thing that separates them is a tape measure.
Muscle is the engine. Fat is what the van is carrying.
Two vans can weigh exactly the same at the weighbridge while one is mostly engine and the other is mostly load. The one with more engine burns more fuel doing nothing at all, because an engine costs something to keep running and a crate in the back does not.
Bodies work the same way. Muscle is metabolically active tissue: it costs energy to maintain around the clock, whether you train that day or not. Stored fat costs very little by comparison. So two people at the same weight, one carrying more muscle and less fat, do not have the same daily energy requirement, and giving them the same target guarantees it is wrong for at least one of them.
This is the single biggest thing a scale cannot tell you. It reports one number for everything you are made of, and then the equation treats that number as though the proportions were the same for everybody.
So why the waist, and not something more precise?
Because the precise options are not available to you at home, every week, for nothing.
| Method | What it costs you |
|---|---|
| DEXA scan | A clinic appointment and a fee, for a snapshot you will not repeat monthly |
| Skinfold calipers | Cheap, but the result moves with whoever is holding them, and self-measuring your own back is a losing game |
| Bioimpedance scales | Convenient and heavily influenced by how hydrated you are, which is exactly what changes week to week |
| A tape measure at the waist | A few seconds, no equipment worth the name, and repeatable by the same person in the same way every time |
Waist circumference is not the most accurate measurement of body composition that exists. It is the most accurate one that you will actually take, consistently, for years. A method you use beats a method you admire, and an estimate that gets repeated is worth more than a precise figure taken once in January.
It also carries information weight alone does not. Where a body stores fat differs from person to person, and the waist is the site that tracks most closely with how much of your mass is not muscle. That is the part the calculation needs.
What it actually changes
Your body burns energy in rough proportion to how much living tissue it has to keep running, not how heavy you are. Kriterion uses your waist alongside your weight to estimate how much of you is lean, and works your requirement from the lean figure rather than the total.
For someone carrying more muscle than average, that is the difference between a target that quietly under-feeds them for months and one that does not. For someone carrying less, it is the difference between a target that was never going to work and one that might.
It is the smallest thing the app asks you for, and it changes the answer more than anything else on the form.
How to measure it so the number means something
Consistency matters more than precision. A measurement taken slightly wrong in the same way every time is more useful than one taken perfectly once, because what the app reads is the change.
- Same time of day, every time. First thing in the morning is easiest to repeat, before you have eaten or drunk anything.
- Bare skin, not over clothing. A jumper is worth more centimetres than any change you are trying to detect.
- Halfway between your lowest rib and the top of your hip bone. Find both with your fingers rather than guessing from your navel, which sits in a different place on different people.
- Breathe out normally and then measure. Do not hold it in, and do not push it out. The tape should sit flat against the skin without denting it.
- Stand relaxed. Arms down, weight even on both feet.
Do not measure daily. It moves with food volume, salt, hydration and, for women, the point in your cycle, none of which is the thing you are trying to see. Every few weeks is plenty.
What this does not claim
Estimating lean mass from a tape measure is an estimate, and Kriterion says so rather than presenting it as a measurement. It is a better starting point than height and weight alone, which is a low bar that almost every other tracker fails to clear.
It is also only the starting point. From your third weigh-in onwards the app stops leaning on any formula and starts working from what your own weight actually did against what you actually ate. The waist improves the guess it begins with; your own data replaces it.
Kriterion is a wellness and training tracker, not a medical device. Waist measurement here is used for one purpose, estimating your energy requirement, and nothing in the app is medical advice or an assessment of your health.