How does the calculator work?

It does two steps. First it finds your lean body mass, which is everything in your body that is not fat: muscle, bone, organs and water. Then it sets your protein target from that lean mass instead of your total weight, so extra body fat does not push the number up.

If you enter your body fat, lean mass is simply your weight minus your fat. If you leave it empty, the calculator estimates lean mass from your height and weight with the Boer formula, a standard equation from 1984. It is an estimate: two people with the same height and weight can carry very different amounts of muscle. A body fat reading from a smart scale, calipers or a DEXA scan makes the answer closer.

Why base protein on muscle mass and not body weight?

Your muscles use the protein you eat to repair and grow. Body fat does not. The studies behind most protein advice were run on fairly lean, trained people, so their targets in grams per kg of body weight assume a body like theirs. Applied to someone carrying more fat, the same numbers overshoot.

So the calculator converts your lean mass into a lean-adjusted weight: what you would weigh at 15% body fat for men or 25% for women. It applies the research targets to that weight, and never to more than your real weight. The one exception is fat loss, where the research already gives targets per kg of lean mass.

How much protein for each goal?

Goal	Target	Where it comes from
Build muscle	1.6 to 2.2 g per kg of lean-adjusted weight a day	A review of 49 trials found no extra muscle gain above about 1.6 g per kg (Morton et al.); 2.2 g is the upper intake Schoenfeld and Aragon cite.
Lose fat, keep muscle	2.3 to 3.1 g per kg of lean body mass a day	Recommended while dieting, when muscle is easiest to lose (Helms et al.).
Stay fit	1.4 to 2.0 g per kg of lean-adjusted weight a day	Enough for most people who exercise (ISSN position stand, Jäger et al.).
Over 65	At least 1.2 g per kg of body weight a day if you are active	Older muscles respond less to protein, so they need more (PROT-AGE, Bauer et al.).

Take the low end if you eat plenty of protein already and just want a floor. Aim higher if you train hard, are dieting, or are older.

How should you split protein across the day?

Spread it out. Schoenfeld and Aragon suggest about 0.4 g per kg of body weight per meal across at least four meals, which is why the calculator shows a per-meal number for four meals. For an 80 kg person that is about 32 g a meal, roughly what one chicken breast gives you.

Is lean body mass the same as muscle mass?

No. Lean mass includes bone, organs and water as well as muscle, so your muscle alone is a part of the number shown. Lean mass is still the best simple stand-in, because it is what grows or shrinks when you build or lose muscle, and it is what the protein research measures.

Who should not use these numbers?

Talk to a doctor before raising protein if you have kidney disease: the PROT-AGE group notes that people with severe kidney disease who are not on dialysis may need to limit protein. The calculator is also not made for people under 18, during pregnancy, or for medical nutrition plans.

Sources

Educational content, not medical advice. Training and diet changes carry risk; talk to a physician before changing your diet if you have a medical condition.

How much protein do you need for your muscle mass?