Body Recomposition Calculator

By Rick Campbell · Updated 15 Sept 2026 · Sourced to primary literature · Not medical advice

The scale can sit still while fat mass falls and lean mass rises. That split is recomposition. This page takes two weights and two body-fat percentages, splits each day into fat mass and lean mass, and shows which compartment moved.

Lean mass here is everything that is not estimated fat: muscle, bone, organs and water. A tape or formula percent misses by about 3 to 5 points, so a one-point change can be noise. The same method, the same time of day, weeks apart, is the only comparison this form will treat as real.

Enter starting and later weight and body-fat percent. You get fat and lean change, a flag when the scale was almost flat while fat fell, and a 2.2 g/kg protein figure for the later weight.

In brief

Fat mass is weight times body-fat percent. Lean mass is the rest. Recomp is those two moving in different directions.
A scale change under 0.6 kg with fat mass down 0.3 kg or more is flagged as scale-flat, fat-down.
Use one method both times. Mixing Navy, RFM and calipers invents a change that was never measured.
Protein at 2.2 g per kilogram plus resistance training is what makes a small surplus or deficit more likely to land on the right tissue.

What you'll see here

Fat mass and lean mass at both time points, the change in each, a flag when the scale hid fat loss, and a protein target.

What the two compartments are

Fat mass is the kilogram figure implied by your body-fat percent. At 80 kg and 25%, fat mass is 20 kg and lean mass is 60 kg. Later, at 80 kg and 22%, fat mass is 17.6 kg and lean mass is 62.4 kg. The scale did not move. The compartments did.

Lean mass is not the same as skeletal muscle. A litre of water, a glycogen refill or a hard training week will raise lean mass on any two-compartment split. That is why this page wants weeks between readings, not two mornings.

The lean-body-mass and FFMI calculators use a single day's percent. This page exists for the difference between two days, which is the only way a flat scale becomes useful.

Why the scale is the wrong scoreboard

A new lifter in a small deficit, or a trained person eating around maintenance with high protein, can lose fat and add lean tissue in the same month. The net kilogram change can be near zero. Calling that a stall is how people cut themselves out of the lean mass they just built.

Morton and colleagues, pooling resistance-training trials, found that protein intakes around 1.6 g per kilogram, with diminishing returns above that, supported greater lean-mass gains than lower intakes. Helms and colleagues, writing for natural contest preparation, placed cutting protein in a higher band, often 2.3 to 3.1 g per kilogram of lean mass, to protect tissue in a deficit.

This result shows 2.2 g per kilogram of later bodyweight, the cutting tier used on the deficit and protein pages. It is insurance, not a hypertrophy programme, and it is the number you can actually fit next to a fat floor.

The error band you cannot ignore

Navy tape, relative fat mass and calipers each carry a typical error of about 3 to 5 percentage points against a four-compartment model. A drop from 24% to 23% can be the same person on a different Tuesday.

DEXA is better and still not a chemical analysis. Different machines, hydration and software versions move the lean figure. If you have a scan, use that scan both times. Do not mix a scan with a tape.

This form flags scale-flat fat-down only when weight moved by less than 0.6 kg and fat mass fell by at least 0.3 kg. Smaller fat changes are left without that headline so a 1-point tape wobble does not become a story.

When both compartments fall

If fat mass and lean mass both drop, the deficit is eating tissue from both sides. Raise protein toward the cutting tier, keep resistance training, and do not chase a faster scale drop. The deficit calculator will not plan faster than 1% of weight a week for this reason.

New calorie cuts after a lean-mass drop are how 'recomp' attempts become a smaller, softer version of the same person. Hold calories, lift, and re-measure in four weeks.

If fat mass did not fall, the time gap is too short or the method changed. Recheck before you change food.

How to take the two readings

Same device or same tape protocol, same time of day, after voiding, before food. For a waist-based percent, measure midway between the lowest rib and the top of the hip bone after a normal breath out. The waist-measurement guide on this site is the protocol.

Four to eight weeks is a useful gap for a tape. Two weeks is mostly water. If you only have a scale, this page cannot split the compartments. Use the check-in calculator for expected versus actual kilograms instead.

Women should compare like cycle weeks where possible. A luteal-phase reading next to a follicular reading will invent lean-mass change.

What to do with the split

If fat is down and lean is up or flat, keep protein and training and do not cut harder because the scale was boring. If both are down, raise protein and stop deepening the deficit. Carry later weight into the protein and macro calculators.

Body-fat method pages on this site exist so you can pick one method and stay on it. Switching methods mid-block is how fake recomp appears.

This is not a scan. It will not tell a clinician how much visceral fat you carry. For central fat risk, use the tape screens on the visceral-fat and waist-to-height pages.

How it's calculated

Fat mass

Fat mass (kg) = weight (kg) × body-fat % ÷ 100

Computed at both dates with the percent you enter. Same method both times.

Lean mass

Lean mass (kg) = weight (kg) − fat mass (kg)

Includes muscle, bone, organs and water. Not a muscle-mass scan.

Scale-flat, fat-down flag

|later kg − start kg| < 0.6 and fat-mass change ≤ −0.3 kg

A 1-point tape change can still be noise. Read the flag with the method error in mind.

Worked example: 82 kg at 24% to 81.5 kg at 21%

Start fat mass = 82 × 0.24 = 19.68 kg. Start lean mass = 82 − 19.68 = 62.32 kg.
Later fat mass = 81.5 × 0.21 = 17.12 kg. Later lean mass = 81.5 − 17.12 = 64.38 kg.
Fat mass change = 17.12 − 19.68 = −2.56 kg. Lean mass change = 64.38 − 62.32 = +2.06 kg.
Scale change = 81.5 − 82 = −0.5 kg, inside 0.6 kg, with fat down more than 0.3 kg, so the scale-flat fat-down flag is on.
Protein at 2.2 g/kg on the later weight is 81.5 × 2.2 = 179 g a day.

Where this number is used in the real world

Reading a flat month on the scale without calling it a stall.
Checking whether a cut is taking lean mass as well as fat.
Comparing two tape or caliper readings taken weeks apart.
Setting a 2.2 g/kg protein target while composition is changing.
Deciding whether to hold calories or stop cutting after both compartments fell.

Frequently asked questions

Is recomposition possible, or is it just a smaller deficit?

Both things can be true. New lifters, people returning from a layoff, and people who raise protein and start resistance training while eating near maintenance often show fat down and lean up on a two-compartment split. Trained people in a large deficit usually lose some lean mass. Morton's protein meta-analysis supports a high protein floor if the goal is to keep or add lean tissue. This page reports the split you measured. It does not promise that everyone can add muscle in a deep cut.

Why do you insist on the same body-fat method?

Navy tape, RFM, calipers and DEXA do not share a zero point. A 3-point gap between methods is normal and has nothing to do with fat lost. If you start on the Navy formula and finish on a gym scale, the 'recomp' is the method change. Pick one protocol, write it down, and repeat it. The body-fat methods guide on this site is there so you can choose once.

How small a fat-mass change should I ignore?

Anything under about 0.3 kg of estimated fat, or a 1-point change in percent, is inside typical tape and formula error. The scale-flat headline only fires when fat mass is down at least 0.3 kg and weight moved by less than 0.6 kg. If your two readings are twelve days apart, ignore the split and wait. Water will dominate a short gap no matter how careful the tape is.

What protein number should I actually eat?

This result shows 2.2 grams per kilogram of later bodyweight, the same cutting tier as the deficit page. Morton's pooled data found most of the lean-mass benefit near 1.6 g/kg, with a plateau above that for many trainees. Helms placed contest-prep protein higher relative to lean mass. 2.2 g/kg of total weight is a simple adult target that fits next to a fat floor on this site. The protein calculator will place it beside your calories.

Can I recomp in a calorie surplus?

A small surplus with high protein and progressive lifting is how most people add lean mass. Fat mass often rises a little as well. That is still a useful split: you want the lean change larger than the fat change. A large surplus just raises fat mass. The surplus calculator on this site caps the rate for that reason. This page will show the compartments either way.

Does a falling body-fat percent always mean fat loss?

No. If weight rose a lot and percent fell a little, fat mass can still be up. Always read the kilogram split, not the percent alone. The reverse is also true: weight down and percent flat can still be a fat-mass drop if lean mass fell in step. The table is there so you do not manage the percent as if it were a separate organ.

Keep going

A single number rarely tells the whole story. Alongside the recomp result, the body fat calculator, the lean body mass calculator, the protein calculator, the FFMI calculator and the check-in calculator each add a different angle on the same measurements. For the reasoning behind the numbers, read Recomp vs weight loss and How much protein.

Sources

Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med 2018;52:376-84. doi.org/10.1136/bjsports-2017-097608
Helms ER, Aragon AA, Fitschen PJ. Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. J Int Soc Sports Nutr 2014;11:20. doi.org/10.1186/1550-2783-11-20
Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: National Academies Press; 2005. doi.org/10.17226/10490

Cite this page

Quoting a figure from here in an article, a report or a piece of coursework? Use whichever of these your style guide asks for.

APA: Campbell, R. (2026). Body Recomposition Calculator. Body Stats. https://bodystats.co/app/body-recomposition-calculator
Plain text: “Body Recomposition Calculator”, Body Stats, last updated 15 September 2026, https://bodystats.co/app/body-recomposition-calculator

Every formula and threshold on this page is written out with its primary source on our methodology page. These results are informational and educational, not a diagnosis or a substitute for professional advice. See the medical disclaimer.

Last updated 15 September 2026. Written by Rick Campbell; not medically reviewed. See review status.