Goal date

Why flat arithmetic overstates how fast you get there

The usual calculation is one line: multiply the kilograms you want to lose by 7,700 kcal, divide by your daily deficit, and that is your number of days. It is the arithmetic almost every goal-date tool stops at, and on a small goal it is close enough.

On a large one it is not, because it assumes the deficit stays the same size. It does not. A lighter body costs less to run — every kilogram you lose takes roughly 10 kcal a day of resting burn with it, and more once the activity multiplier is applied. Hold your eating steady and the gap between burn and intake narrows every week. The error compounds, so the bigger the goal, the more optimistic the flat answer.

What this page does instead

It steps through the plan a day at a time. Each day it recomputes your resting rate from your current weight with Mifflin–St Jeor, applies your activity factor, works out that day's actual deficit, and takes the corresponding fraction of a kilogram off. The result is a curve that flattens rather than a straight line, and a date that is usually later than the simple sum promises. Both numbers are shown so you can see the gap.

The weight where your intake becomes maintenance

If you plan by a fixed daily calorie figure rather than a pace, there is a weight at which that figure simply is your maintenance — and at that point you stop losing, no matter how long you keep going. Most calculators will happily print a date beyond it, which is how you end up with tools promising weights you could never reach on the plan you entered.

This page solves for that weight directly and tells you when your goal sits below it. If it does, the answer is not a later date; it is a smaller intake or a higher burn.

How close is this to the NIH model?

The reference model for this is Hall and colleagues' work, which underlies the NIH Body Weight Planner. It goes considerably further than this page: it tracks fat and lean tissue separately, accounts for the changing energy density of what is actually being lost, and models adaptive thermogenesis — the extra slowdown in metabolism beyond what the weight change alone explains.

This page models the weight term only. So it is more honest than flat arithmetic and still slightly optimistic against the full model. One figure from that work worth carrying around, because it is so often misquoted: a sustained change of about 100 kJ a day leads to roughly 1 kg of eventual weight change, with about half of it arriving within a year.

Reading the result

Treat the date as a projection with a wide band, and expect the scale to disagree with it week to week — water, glycogen and salt move body weight far faster than fat does. Judge by the multi-week trend.

A goal below a BMI of 18.5 is flagged rather than dated, and paces above about 1% of bodyweight a week are flagged too. If you are on a GLP-1 medication or have had bariatric surgery your curve will not follow this one, and your clinical team's plan replaces it. Not for under-18s, pregnancy or breastfeeding.

To set the intake side, use thedeficit calculator; to plan the climb back to maintenance afterwards, thereverse diet calculator.