What your weight is actually doing
A scale reading is the weight you care about plus two kilos of water, food and glycogen that you do not. This separates the two, works out what you actually burn from your own history rather than a formula, and tells you when a flat fortnight is a plateau and when it is noise. Everything stays in this browser.
This is arithmetic, not medical advice. It works out what your own numbers imply and says how sure it is. It cannot know anything about your health, and it is written for adults. Nothing you type is uploaded — it stays in this browser, and you can clear it in one click.
Your weigh-ins
One line a day: date, weight, and the calories if you track them. Daily is best; every other day still works.
Or paste a history — one line each, date, weight, calories — from a spreadsheet or an app export.
The trend, and whether it is real
The line is a ten-day weighted average. The dots are what the scale said.
What you actually burn
Not a formula: your own intake and your own weight change, run backwards.
No intake logged? Start from the formula instead
Mifflin–St Jeor times an activity factor. For any one person it is routinely 15% out in either direction, which is why the figure above it — from your own history — is worth far more once you have a fortnight of logging.
Where this is going
At the trend you are on now — with the range that trend actually supports.
Why the trend and not the reading
Bodyweight swings by one to two kilos over a day and across a week, from water, sodium, glycogen and what is still in the gut. None of that is fat, and all of it is on the scale. A ten-day exponentially weighted average keeps the part that persists and drops the part that does not, which is why a trend line falls steadily through a fortnight where the readings look like a saw.
The rate underneath is fitted to the readings themselves rather than to the smoothed line, which matters more than it sounds. Fitting the average looks tidier and is wrong twice: the average lags, so it under-reads a real change while it catches up, and its points are no longer independent, so the error bars come out far too small and everything looks certain. The scatter of the actual weigh-ins is the thing that decides whether a change is real, so that is what is used.
Where the energy number comes from
Energy in, minus energy stored, is energy out. If you logged 2,000 kcal a day for a month and your trend fell half a kilo a week, you were burning about 2,550 — no formula, no activity multiplier, no guessing how much your job counts for. This is the single most useful calculation in weight management and almost nothing free does it.
It inherits one honest weakness: it is only as good as your food logging. Under-recording is the norm rather than the exception, commonly by 10–20%, and this arithmetic cannot tell the difference between burning less and writing down less. It will read low by whatever you leave out. What it is still very good at is change: if the same logging habits give a maintenance figure that drifts down over months, that drift is real.
Common questions
Why 7,700 kcal per kilo?
It is the conventional energy value of a kilogram of body mass, and it is an approximation — the true figure depends on how much of the change is fat and how much is lean tissue and water. Over weeks, in the middle of a steady diet, it is close enough to be useful. Over the first fortnight, when much of the change is glycogen and water, it is not, which is part of why this tool asks for a fortnight before answering.
My weight has not moved in three weeks. Is that a plateau?
The tool will tell you which, because it compares the size of the change against the size of the noise. A genuine plateau and three weeks of water masking a real loss look identical on a phone chart, and quite different once you have error bars. Three weeks of a truly flat trend with tight scatter is a plateau; three weeks of a flat trend with a kilo of scatter either side is simply not enough information yet.
Does anything I type leave my device?
No. It is kept in this browser's own storage so it is there when you come back, and nothing is sent anywhere. There is no account, and "Clear everything" removes it.
Is this safe to use if I have an eating disorder?
Tools that turn eating into arithmetic can make things worse for some people, and daily weighing is specifically discouraged in several treatment approaches. If that applies to you, please talk to a clinician rather than to a calculator. In the UK, Beat runs a helpline on 0808 801 0677.