What you burn, what to eat, and how uncertain both are
Every calculator returns one number. That number comes out of a regression fitted to a few hundred people and carries about a ten percent standard error, which at 2,300 calories is a band nearly 500 wide. This one prints the band, and it shows what happens to the timeline when maintenance falls as you do.
Where the numbers come from
Resting energy expenditure is estimated with the Mifflin-St Jeor equation, which is what clinical practice uses and which has the smallest error of the three equations in common circulation. Harris-Benedict runs about five percent high on modern populations, and Katch-McArdle needs a body-fat percentage measured properly rather than guessed from a photograph. Maintenance is that figure multiplied by an activity factor, and the activity factor is where most of the error enters — almost everybody picks one category too high, which is why the descriptions on this page are written to push the choice down.
Why a range and not a number
The equation's published standard error is about ten percent. For someone whose estimate lands at 2,300 calories, the honest statement is between roughly 2,070 and 2,530 — and the difference between the ends of that band is about half a kilogram a month. A calculator that prints the midpoint alone has not simplified the answer, it has removed the part that decides whether the plan works. The right use of the band is to start in the middle, weigh yourself over a fortnight, and let the scale tell you which end you are.
Why the timeline is longer than you were told
Dividing the weight to lose by 7,700 calories per kilogram is the arithmetic every calculator prints, and it assumes maintenance stays where it started. It does not. A lighter body costs less to run, so the same plate is a smaller deficit every week, and the loss decelerates. This page computes both figures and shows the gap between them, because that gap is the reason plans stall in the third month — and it is arithmetic rather than willpower.
What this page will not do
It will not plan below 1,200 calories for women or 1,500 for men. Sustained intake beneath those figures is where micronutrient requirements stop being met, and a calculator that prints whatever number it is asked for is the exact tool somebody in trouble reaches for. Nothing here is medical advice, none of it is a measurement of you, and if you are pregnant, under eighteen, managing a medical condition or have any history of disordered eating, these numbers are not the right tool.