Editorial policy

How we work

Health content is easy to write and hard to trust. This page is where we show the working: where every number comes from, who checks it, what we get wrong, and how we are paid.

Last reviewed 31 August 2026·Next review February 2027·Version 1.0

Where our numbers come from

Two kinds of source, and we never mix them. Body-composition figures come from the published trials of the medications themselves. Protein targets come from clinical nutrition guidelines, not from the trials.

Wilding JPH et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med 2021;384:989–1002. Lean mass Jastreboff AM et al. Tirzepatide once weekly for the treatment of obesity. N Engl J Med 2022;387:205–216. Lean mass Nunes Heinzen E et al. Bimagrumab with semaglutide in adults with obesity (BELIEVE). Nat Med, published online 3 March 2026. Lean mass Bauer J et al. Optimal dietary protein intake in older people (PROT-AGE). J Am Med Dir Assoc 2013;14:542–559. Protein Deutz NEP et al. Protein intake and exercise for optimal muscle function with ageing (ESPEN). Clin Nutr 2014;33:929–936. Protein Cava E, Yeat NC, Mittendorfer B. Preserving healthy muscle during weight loss. Adv Nutr 2017;8:511–519. Method

Where a claim rests on a single study, we say so on the page. Where the evidence disagrees with itself, we say that too rather than picking the tidier number.

The formula the calculator uses

In full, with no rounding hidden. It starts from a baseline share of the weight lost and adjusts for the four things the literature shows change it. The result is deliberately reported as a range.

The 30% baseline was set from the earlier trials. In March 2026 the BELIEVE trial measured the same thing directly over 72 weeks and found that on semaglutide alone, 71.8% of the weight lost was fat — meaning 28.2% was not. That is a newer and more direct measurement arriving within two points of the constant we were already using, and we would have changed the number if it had not.

total_loss = start_weight − current_weight
protein_per_kg = protein_g ÷ current_weight_kg
lean_share = 30%  (baseline)
protein ≥ 1.6 g/kg  −8  · ≥ 1.2  −4  · ≥ 0.9  +1  · below 0.9  +8
training ≥ 4/wk  −11  · 3  −9  · 2  −5  · 1  −2  · none  +5
loss rate > 1.4%/wk  +11  · > 1.0%  +6  · < 0.5%  −3
age ≥ 65  +5  · ≥ 50  +3
lean_share = clamp(lean_share, 10%, 48%)
range = ±6 points, +6 if protein is not tracked, +2 if fewer than 8 weeks in
lean_loss = total_loss × lean_share  (reported as a range)

Two details worth stating. Your starting lean mass is estimated with the Boer formula from height, weight and sex at birth — that is what those questions are for. And if you tell us you do not track protein, the calculator assumes 0.75 g per kg and widens the range, rather than pretending to a precision it does not have.

This is an estimate from population data, not a measurement of you. A DXA or BIA scan measures; this infers. We show a range because the honest answer is a range — anyone quoting you a single decimal is overselling their method.

Who reviews it, and how often

01 No registered dietitian has been appointed yet, so nothing on this site makes a clinical claim. The calculator states its formula and its sources on the page and says plainly that it estimates rather than measures. When a reviewer is appointed, their name and the review date will appear at the top of every page carrying a number. 02 The whole site is re-checked every six months, and any page is re-checked immediately if a major trial or a guideline changes. 03 Reviewers are paid a flat fee for their time. They are not paid per page, and nobody who sells anything — a manufacturer, a pharmacy, an advertiser — sees a page before it is published.

Corrections

If something here is wrong, we want to know, and we would rather hear it from you than leave it up. Write to the address below — it goes to a person, not a form.

Report an error
corrections@forgefitai.com

Factual errors are fixed within two working days. Anything we change materially gets a dated correction note at the foot of that page — we do not quietly edit and move on.

How we make money

One way, and only one: we sell the tracker. It costs $49 once, for 12 weeks. That is the entire business model, and you are reading all of it.

There is no advertising on this site and no affiliate links. Nothing we mention — a food, a scale, a brand — earns us a commission if you buy it, so nothing we recommend is chosen because it pays. We take no money from manufacturers of GLP-1 medication, compounding pharmacies, or telehealth prescribers. The calculators stay free and need no account. If the free page answers your question, that is a good outcome — we would rather you use it and never pay us than pay for something you did not need.