The number on the scale moves every morning for reasons that have little to do with fat. This check reads your weigh-ins the way a statistician would: a 7-day average, a smoothed trend, a weekly rate with an honest range, and a plain answer to "is it really a plateau?". It never mentions calories or targets, and nothing you paste leaves your browser.

Why the daily number is noisy

A salty dinner, a day of more carbohydrate than usual, or a late meal that hasn't been digested yet can each move the scale by several hundred grams overnight, and then undo it within days. None of that is fat. The fat or water check shows how large those swings can be. Over a few weeks they cancel out, which is why an average tells you more than any single morning.

The 7-day average and the smoothed trend

The 7-day average is the mean of your weigh-ins over the last seven calendar days, counted only when there are at least three of them. The Avocalo app shows the same kind of average on its Weight tab.

The smoothed trend is the one John Walker describes in The Hacker's Diet: each new weigh-in moves the trend 10% of the way from where it was. A single odd morning barely shifts it. On a day with no weigh-in, the trend simply stays where it was; no weight is invented.

How the plateau check works

The check fits a straight line through your weigh-ins from the last 28 days and reports its slope as a weekly rate, with a 95% range from Student's t distribution. Then:

a range wholly below zero means your weight is still going down;
a range wholly above zero means it is going up;
a range that holds zero and is narrower than ±0.2 kg a week means it is flat: by the numbers, a plateau;
a range that holds zero but is wider means the swings still hide any change of that size, so it is too soon to tell.

Before two weeks of weigh-ins it gives no verdict at all. The width of the range depends on how much your weight swings and how many days you have. With a typical swing of 0.5 kg, 14 daily weigh-ins can't separate a change smaller than about ±0.51 kg a week from noise; 28 daily weigh-ins narrow that to about ±0.17 kg a week.

A worked example

The example has 35 mornings, from 17 August to 20 September 2026. After a salty weekend the scale read 80.0 kg on two mornings in a row, higher than any day in the week before. The 7-day average still fell from 79.5 kg to 78.8 kg. Over the last 28 days the rate is about 0.3 kg a week down, with a 95% range from −0.41 to −0.18 kg a week, so the verdict is "still going down". The typical day-to-day swing is ±0.33 kg, and with these 28 weigh-ins any change smaller than ±0.11 kg a week would be lost in it.

What it can't tell you

It describes your numbers; it can't say whether a change is fat, water or muscle, or why it happened. It assumes each weigh-in was taken the same way, ideally at the same time of day. It is not medical advice.

What counts as a weight loss plateau?

In a review of weight-loss trials with at least a year of follow-up, weight loss levelled off at about six months on average (Franz et al., 2007). Here the word has a narrower meaning: a 95% range for your weekly rate that holds zero and is no wider than ±0.2 kg a week.

Why is my weight higher on Mondays?

A European study of adults weighing themselves at home found a within-week fluctuation of about 0.35% of body weight: higher after the weekend and lower through the week (Turicchi et al., 2020). Comparing one Monday with one Friday mostly measures that rhythm.

How many weigh-ins do I need?

Three on different days to see any numbers, two weeks for a verdict, and about four weeks of daily weigh-ins for the narrowest range. Missing days are fine: the averages and the line use the days you have.

Sources

John Walker, The Hacker's Diet: Signal and Noise, the source of the smoothed trend.
Franz MJ et al. Weight-loss outcomes: a systematic review and meta-analysis of weight-loss clinical trials with a minimum 1-year follow-up. J Am Diet Assoc, 2007.
Turicchi J et al. Weekly, seasonal and holiday body weight fluctuation patterns among individuals engaged in a European multi-centre behavioural weight loss maintenance intervention. PLoS One, 2020.

When a single morning worries you, the fat or water check shows what else can move the scale.