Which body numbers are worth tracking, and which waste your time

There are four numbers people commonly track about their bodies, and they are not equally useful. One of them is close to worthless on its own, one is excellent and nobody uses it, and the most popular one is the noisiest thing you could possibly measure daily.

Here is what each one tells you, and the measuring frequency that makes it informative rather than annoying.

Weight: useful, and measured wrongly by almost everyone

Weight is a strong signal that people ruin by reading it daily and responding to it daily. Day to day, your weight moves one to two kilograms on water, food in transit, salt, and where you are in a cycle. None of that is fat.

The signal you want is buried under that noise, and it is small: half a kilogram a week is a good rate of loss, which is less than the daily swing. So a single morning reading cannot distinguish progress from a salty dinner.

BMI: fine for populations, poor for you

BMI divides weight by height squared. That is the entire formula, and it was designed in the 1830s to describe groups rather than individuals.

It has no way of knowing whether a kilogram is muscle or fat, so anyone who lifts seriously reads high. It squares height while bodies scale closer to a cube, so tall people read high and short people low for the same build. And it says nothing about where fat sits, which is the part associated with risk.

None of that makes it useless. It makes it a rough screen that should never be the only thing you look at, and should never be read as a verdict on a single day.

Waist to height ratio: the one nobody uses

This is the quiet winner. Measure your waist, divide by your height, aim for under 0.5. Keep your waist under half your height and you are in the healthy band.

It beats BMI at BMI's own job because it knows something BMI does not: where the mass sits. Central fat behaves differently from fat on hips and legs, and it is the distribution most associated with metabolic risk. The threshold is also the same for men and women, which makes it easier to remember and harder to argue with.

It responds faster than the scale too. Waist often moves before weight does when a diet starts working, which makes it more encouraging in exactly the first few weeks when people quit.

Body fat: useful for one specific question

A tape-based estimate lands within about three or four percentage points of a proper scan. That is not precise enough to argue over a single point, and it is precise enough to answer one question no other number here can: is the weight you are losing fat?

Someone whose weight holds steady for a month while body fat drops has gained muscle and lost fat. The scale alone would have reported nothing happening, and plenty of people quit during exactly that month.

What I would track

1. Weight, weekly, read as a four week trend and never as a single reading.
2. Waist, monthly, same time of day, same tape.
3. That is it, unless you have a specific reason for more.

Two numbers. Body fat is a useful third if you are training seriously and want to separate fat loss from muscle loss. BMI you can calculate once a year out of curiosity and otherwise ignore.

The cost of tracking more than this is not the measuring time, which is trivial. It is that every extra number is another chance to respond to noise, and responding to noise is how people end up changing a plan that was working.

A measurement you take monthly and read honestly beats four you take daily and interpret badly.

The thing none of these measure

Whether you did the work. Body numbers are outcomes, and outcomes lag by weeks and get pushed around by things you do not control. A month of good training can show up as no movement at all on every number in this article.

Whether you trained four times this week is a fact available the same day, entirely within your control, and a better predictor of where the outcome numbers land in three months. Track the behaviour daily and the body numbers monthly, not the other way round.

The short version

• Daily weight swings of one to two kilograms are water and food, not fat. Read the weekly average.
• BMI cannot tell muscle from fat or see where it sits. Rough screen only.
• Waist to height ratio is the best value here: under 0.5, measured monthly, needs only a tape.
• Body fat answers one question well: is the weight you are losing actually fat?
• Two numbers is enough for most people. Every extra one is another chance to respond to noise.
• Track behaviour daily, body numbers monthly.

If you want to check where you currently sit, the calculators for each of these are free and run entirely in your browser. Then put them away for a month, which is the part that makes them useful.