Protein Per Kg Lean Mass: Resetting My Post-Bulk Cut

I ended my bulk at 84.1kg and 17.1% body fat. Then I opened the protein calculator and found it would give me the exact same number on a cut as it did on a bulk.

Hold on. Everything I had read said protein needs to go up when you start cutting. So I did what any suspicious lifter with a spreadsheet habit would do: I read the calculator's source code. What I found changed how I use the tool, and it is the single most useful thing I can tell you before you let any macro calculator set your protein target.

Short version: this protein per kg lean mass calculator fixes your protein at a constant 2.3 grams per kilogram of lean body mass. It does not know whether you are bulking or cutting. If you are cutting, that constant is on the low end. If you are bulking, it is already higher than it needs to be. The tool is not wrong, it is just phase-blind, and you are the one who has to supply the phase adjustment.

What the calculator actually does behind the button

Here is the exact formula the page runs, lifted from its own JavaScript:

var leanKg = weight * (1 - bodyfat / 100);
// ~2.3 g protein per kg lean mass
proteinG = leanKg * 2.3;

That is the whole protein decision. Two lines. There is no branch that asks "are you in a surplus or a deficit," no multiplier that shifts with your goal, and no cap that raises protein when your deficit gets aggressive. The five-option goal selector (cut fat, maintain, gain muscle, and so on) does change your calories, because it changes the fat and carbohydrate split. It does not touch the protein number at all as long as you entered a body fat percentage.

So if you weigh 84.1kg at 17.1% body fat, the calculator gives you 69.7kg of lean mass and 160g of protein. That answer is identical whether you set the goal to "cut fat" or "gain muscle." Run it on the page and you will see all five goals return 160g of protein, with only the carb and fat grams moving. If you want your own numbers, the tool is one paste away, and the math is 30 seconds of arithmetic.

Why 2.3 g per kg of lean mass is a bulking number, not a cutting number

2.3 g/kg of lean mass lands close to 1.8 g/kg of total body weight for a moderately lean lifter, which is the well-established sweet spot for a surplus. In a bulk, that is plenty, because the extra calories themselves are driving protein synthesis. You are not asking protein to do all the work.

During a cut the job changes. Now protein is the main defense against muscle breakdown, and the research on lean athletes in a deficit keeps pointing higher. The classic range from Eric Helms and colleagues sits around 2.3 to 3.1 g/kg of lean mass during caloric restriction, and practical nutrition coaching often targets 2.5 g/kg and up when the deficit is steep or body fat is already low. So the number this calculator hands you is roughly the floor for a cut, not the midpoint. Treat its output as a starting point and add 10 to 30 percent yourself.

That 10 to 30 percent is not a rounding error, it is 16 to 48 grams of protein on the example above, or roughly one to two extra servings of a high-protein food. Those grams have to come from somewhere, and it will be from fat and carbs, because the deficit total is fixed.

Recalculating your lean mass when the scale and the mirror disagree

The input this formula is most sensitive to is body fat percentage, not weight. Lean mass is doing all the work, so a small error in your body fat estimate moves your protein target more than the same error in weight.

Say you are 84.1kg. If your real body fat is 15 percent but you estimate 18, you compute 68.9kg of lean mass instead of 71.5kg. At 2.3 g/kg that is a 6 gram difference in protein. At the 2.8 g/kg a cut really wants, it becomes a 7.3 gram difference. The error compounds upward the higher you push your protein ratio, which is exactly why the tools that make you guess your body fat are the ones that quietly sabotage a cut.

Use the calculator's own layout to check for drift: enter your weight and a realistic body fat range, then run the arithmetic by hand once with the lower and upper ends. If your lean mass swings by more than 2 or 3 kilograms across that range, get a measurement you trust before you commit to a protein target. The tool can compute a number in a millisecond, it just cannot tell you whether your body fat estimate is honest.

The full workflow for resetting your protein, in order

1. Get a real lean mass number. Weight times (1 minus body fat percentage). If your body fat is a guess, bracket it: compute lean mass at the low end and the high end of your plausible range.
2. Run the calculator to get its baseline. It will give you 2.3 g/kg of lean mass. Write that down as your floor.
3. Add your cutting premium. Multiply the baseline by 1.1 for a gentle deficit and 1.3 for an aggressive one. That puts most people in the 2.5 to 3.0 g/kg band the evidence actually supports.
4. Let the calculator set your remaining macros. This is where the tool earns its keep. It already prorates the deficit based on your activity multiplier and fills the leftover calories with carbs and fat. Change the goal to "cut fat" and it does that part properly.
5. Recheck every 4 to 6 weeks. Lean mass moves as you lose weight. A target that fit at 84.1kg is 3 to 4 grams too high by the time you are 80kg, and the calculator will not remind you.

What "protein per kg of lean mass" gets right that total-body-weight math gets wrong

If you had set protein by total body weight during your bulk, 1.8 g/kg of 84.1kg is 151g. On a cut, the same 1.8 g/kg formula keeps shrinking as you lose weight, even though your muscle has not changed. That is backwards. Protein protects lean tissue, so the target should track lean tissue, and lean tissue barely moves in a well-run cut.

This is the one structural choice this calculator gets exactly right: it divides protein by lean mass, not by the number on the scale. Baseline your protein on the tissue you are trying to keep, and you will not accidentally cut your protein the same week you cut your weight.

What the tool will not do for you

Three honest limits, all visible from the source:

• It will not raise protein for a cut. 2.3 g/kg is one constant, and the phase adjustment is yours.
• It will not validate your body fat input. Feed it 25 percent when you are 18 and it will happily compute 160g of protein for a body you do not have.
• It will not tell you which foods to hit the number with. The food cards below the result are reference examples, not a plan, and they are not scaled to your grams.

None of that makes the calculator less useful. It makes the lean mass arithmetic instant, which is the part most people get wrong by a kilogram or two every time. Use it for that, apply your own cutting premium, and your protein macro stops being a guess.