Weight loss calculator

Most weight-loss calculators use a rule that is known to be wrong. This one runs the published model that replaced it, and draws the old rule alongside so you can see the error.

Tool · Rubric v2.3

Almost every weight-loss calculator on the internet uses a rule that is known to be wrong. This one runs the published model that replaced it, and draws the old rule alongside it so you can see the size of the error.

Inputs

Sex

Age

Height

Current weight

Daily calories

Activity

Hall model — what the evidence predicts 3,500-kcal rule — what most calculators show

Why there are two lines

Almost every weight-loss calculator assumes a pound of body weight is worth 3,500 kilocalories, and that a steady deficit therefore produces steady, linear loss forever. Under that rule, eating 500 kcal below maintenance predicts a pound a week — this week, next week, and in five years' time.

Bodies do not work that way, and this has been established for over a decade. Weight responds to a change in intake slowly, and the response fades out rather than continuing: a smaller body costs less to run, and metabolism adapts beyond what the size change alone accounts for. Eventually expenditure falls to meet intake and weight stops changing. The 3,500-kcal rule has no such stopping point, so its error grows every week you project it forward.

The Lancet paper that established the modern model states it directly: the 3,500-kcal rule grossly overestimates actual weight loss.¹ The dashed red line is that overestimate, drawn to scale against the same inputs.

The gap is not a rounding error. Over a year it is routinely tens of pounds — which is the difference between a plan that is working and a plan you abandon because you believe you have failed it.

What this tool does, and does not do

Does	Does not
Project body weight over time for an intake you specify, using a published rule	Recommend a calorie intake, a diet, or a target weight
Show how far the common 3,500-kcal rule departs from that projection	Account for medication, illness, pregnancy or disordered eating
State every equation and constant it uses, below	Replace a clinician who knows your history

It is arithmetic on a population average, not a forecast about you.

The method, in full

Any calculator on this subject should publish its equations. Here are ours, in the order they run.

1 · Maintenance calories

Resting metabolic rate from the Mifflin–St Jeor equation,² multiplied by a conventional activity factor.

RMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + s (s: male +5, female −161) TDEE = RMR × activity factor

2 · Where the weight settles

This is the finding that replaced the 3,500-kcal rule. A sustained change in intake produces an eventual weight change of roughly one pound for every 10 kcal per day, then stops.¹³

ΔIntake = intake − TDEE (kcal/day) Δweight_end = ΔIntake ÷ 10 (pounds)

A 500 kcal/day deficit therefore settles at about 50 lb below where you started — not 50 lb per year, but 50 lb in total, approached and then held.

3 · How long it takes to get there

The approach is gradual, with about half the eventual change reached in the first year.¹ We model it as a single exponential with a one-year half-time.

weight(t) = start + Δweight_end × (1 − e^(−t / τ)) τ = 365 / ln(2) ≈ 527 days

4 · The comparison line

naive(t) = start − (TDEE − intake) × t ÷ 3500 (pounds)

Fixed maintenance, no adaptation, no end point. That is the rule this page exists to argue with.

Two honest caveats about our own implementation.

First, the published rule also states that about 95% of the change arrives by three years. A single exponential fixed to a one-year half-time reaches roughly 87% by then, so our curve is slightly conservative in years two and three. Real dynamics are not a clean exponential and no two-parameter curve satisfies both anchors at once; we chose to match the one-year figure because it is the one that matters over the range we plot.

Second, this is the published rule of thumb, not the full simulation. The NIH Body Weight Planner tracks glycogen, fluid and protein turnover separately and was fitted to controlled feeding studies. For clinical work, use that. We publish the difference rather than implying there is none.

Limitations worth stating

It assumes you eat what you entered
Self-reported intake is unreliable, and under-reporting is the largest single source of error in real weight-loss outcomes — typically far larger than the difference between any two models.
Activity factors are coarse buckets
The multipliers are convention, not measurement. Two people who both select "moderately active" can differ by hundreds of kilocalories a day.
Individual variation is large
The rule was derived from group data. Real people scatter around it, and no calculator can tell you where in that scatter you sit.
It says nothing about health
A projected number on a scale is not a statement about whether reaching it is advisable, achievable, or good for you.

Sources

Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet. 2011;378(9793):826–837. PMID 21872751. pubmed.ncbi.nlm.nih.gov/21872751
Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition. 1990;51(2):241–247. PMID 2305711. pubmed.ncbi.nlm.nih.gov/2305711
Dynamic energy balance and obesity prevention. Journal of Obesity & Metabolic Syndrome. 2018;27(4):203–212. States the 10 kcal/day per pound rule and its time course. pmc.ncbi.nlm.nih.gov/PMC6513301
National Institute of Diabetes and Digestive and Kidney Diseases. Body Weight Planner. The full simulation this tool approximates. niddk.nih.gov