The scale · Decision 14 of 14

Why the weight chart draws a line you did not measure

The heavy line on the weight chart is not your weighings. It is a weighted average of them, where recent days count for more. The kilos-per-week figure beside it is read off that line, not off the scale.

Last updated 1 October 2026.

What the app does: Moves the line a tenth of the way towards each new reading, and shows a rate measured over whichever range you are looking at. The dots stay on the chart.
How sure we are Fair , Real, but small, indirect, or thinly measured.: The noise figure comes from two small studies that measure different kinds of variation. The smoothing itself is an engineering choice, not a research finding. It runs about nine days behind.

The evidence for it

Two studies describe daily variation, each measured in a different way. Reading the rate off the range you selected is our engineering choice. It is not a tested guarantee of precision.

0.54% of body weight: the average spread around each person’s own trend, not a limit on your own error Khosla & Billewicz (1964) · 19 people, daily, range 0.39–0.78%

0.53% standard deviation of day-to-day weight differences in ONE man over 9,521 days Schneditz et al. (2023) · rising to 0.69% across a week

7, 30 or 90 days the rate is read off the range you have selected; it is not a margin of error Product setting. A shorter range is a noisier answer. The real uncertainty depends on how often you weigh and how much your body varies

We are showing you a calculated figure in the place where a measurement would go, so here is the case for it.

Twelve weeks of a person losing 0.63 kg a week Simulated: the app's own smoothing, on a scale a little noisier than the studies below report, with one morning in five missed.

What the scale said, 67 mornings What the app draws The true weight, which nobody can see

The dashed line is the true weight the simulation was given, which nobody in real life gets to see. The dots are what the scale said. The heavy line is what the app draws, and it is the argument: it smooths out the short-term scatter, but it lags behind the simulated change. The lag varies during this run. The line uses the same filter as the app, including how it handles missed days. The random numbers are fixed, so this is the same picture every time the page loads. It is a simulation of our own smoothing, not a finding about people.

Why a single weighing tells you little

Khosla & Billewicz (1964) weighed nineteen people daily under standard conditions for several weeks and measured how far each morning was from that person's own trend. The day-to-day variation ranged from 0.39% to 0.78% of body weight, with a mean of 0.54%. They concluded that day-to-day changes rarely go above 1.5% of body weight. Schneditz et al. (2023) studied one man over 9,521 days of standardised measurements. The standard deviation was 0.53% between one day and the next, rising to 0.69% across a week. The older study measured how far readings sat from a person’s trend. This one measures the difference between two readings. The percentages look alike, but they do not measure the same thing.

The noise is six times the size of a day's progress At 82 kg, losing 0.5 kg a week. Both bars are kilograms.

Example spread around a person’s trend ≈0.4 kg

0.54% × 82 kg = 0.44 kg, rounded. Not a limit for one person

What losing 0.5 kg a week is, per day 0.07 kg

The thing you are trying to measure, underneath the bar above

0 0.5 kg

Taking the last weighing of a month minus the first means subtracting two numbers. Each one carries about five days' worth of real progress as error. So it can show weight going up through two weeks of real loss. That is the obvious way to do it, and it is why the app does not.

Khosla & Billewicz did more than describe the noise. They set out the sums for estimating a change from several readings at each end rather than one. They also noted that readings on nearby days tend to be off in the same direction. So readings spread further apart tell you more than readings bunched together. Averaging the ends is exactly what a smoothed line does, all the time.

The numbers behind our two settings

The smoothing moves the line a tenth of the way towards each new day's reading. So a change takes about a week to show half of itself. The weekly rate is measured over the range the chart is showing. So the 7, 30 and 90 day views give three different numbers. The short one is about this week and is the noisiest. The long one is about the whole stretch. After a gap of d days, the next reading gets a weight of 1 − 0.9ᵈ. So a long gap is not treated like a single day. The chart above uses the same WeightTrend calculation as the app. Its noise is random, in a bell curve shape, with an SD of 0.6% of the simulated weight. Each day has a 20% chance of being missed. These are example assumptions.

How quickly the filter responds to a lasting change Calculated response with a reading every day: 1 − 0.9ᵈ. No body noise in this example.

After 1 day 10%

After 7 days 52%

After 14 days 77%

After 28 days 95%

0 100% of a lasting change

After seven daily readings, about half of a lasting change has reached the line. These percentages describe the filter, not how accurate it is.

The evidence against

Both studies are small. Khosla & Billewicz is nineteen people in 1964, including two babies and pregnant participants, weighed on household scales. Schneditz et al. is a single man. Its purpose was setting target weights for dialysis, not dieting. They measure different things. They cannot give a noise allowance that fits everyone.

The smoothing itself is not a research finding. A weighted average that takes about a week to show half of a change is an engineering choice with a long history in weight tracking. A Kalman filter, a more advanced kind of smoothing, could draw a band of uncertainty around the line. But whether that band was right would depend on its assumptions and how it was tuned. We chose the simpler one because you can check it by hand, and because we cannot yet show a band we would trust.

A smoothed line runs behind reality by design. With a steady change and a reading every day, the lag settles at about nine days. Gaps and changing trends change that. If you change something today, the line will not agree with you for a week or two. During that time it is wrong in a direction we can predict. That is the cost of a line that does not react to water weight.

None of this makes the scale a measure of fat. Everything above is about separating a trend from noise in body weight. Glycogen, salt, the menstrual cycle and creatine all change weight without changing fat. And the 30 day range is uncomfortably close to the length of a cycle. The dots stay on the chart on purpose. You can see the scatter the line is drawn through, rather than being handed a confident line and asked to trust it.