Body Recomposition Calculator

Use the Body Recomposition Calculator

Free body recomposition calculator projects fat loss and muscle gain month by month from your calories, protein, body fat and training age. Plan a real recomp.

How to Use Body Recomposition Calculator

1. Step 1: Enter your weight and body fat percentage

   Pick kg/cm or lb/ft, then fill in Current weight and Current body fat (%). Body fat is the input that matters most here — it splits your weight into the lean and fat compartments the projection moves energy between. A caliper or tape estimate within 3 points is accurate enough.

2. Step 2: Set your lifting experience and sessions per week

   Choose Lifting experience from novice to highly trained. This sets your monthly muscle-gain ceiling: 1.0% of body weight for a novice down to 0.125% for a highly trained lifter, halved for women. Then drag Lifting sessions per week — the frequency penalty disappears at four sessions.

3. Step 3: Choose a calorie strategy

   Pick maintenance for a classic recomp, a mild 10% deficit if you carry more fat, or a lean 5% surplus if you are already lean. Each setting shows how much of your growth ceiling it preserves, from 100% in a surplus to 35% in a 20% deficit.

4. Step 4: Set your protein target

   Move the Protein slider to at least 1.6 g/kg, where the growth benefit plateaus in the research. The label converts it to grams per day at your body weight, and the growth multiplier next to it updates as you drag.

5. Step 5: Read the month-by-month projection

   Choose a plan length of 3 to 12 months and read the table: weight, lean mass, fat mass, body fat percentage and the calories to eat in each month. The calorie column climbs on its own because maintenance is recalculated from your new lean mass every row.

6. Step 6: Fix your biggest limiter

   The growth factor table lists every multiplier applied to your rate, and the summary names the lowest one. Change that single setting first — raising protein or adding a fourth session usually moves the projection more than any calorie tweak.

Key Features

• Month-by-month projection of lean mass, fat mass and body fat percentage
• Katch-McArdle maintenance calories recalculated each month as lean mass changes
• Muscle gain ceilings by training age, with separate rates for men and women
• Growth multiplier breakdown that names your single biggest limiting factor
• Protein, fat and carbohydrate targets with a visual macro split
• FFMI headroom check against the drug-free ceiling of 25 (men) and 22 (women)

Understanding Results

Formula

The projection runs a two-compartment energy model, one month at a time. Your weight is first split into lean mass and fat mass using your body fat percentage, then basal rate comes from Katch-McArdle, which reads lean mass and nothing else:

BMR = 370 + (21.6 × lean mass in kg)
Maintenance = BMR × activity factor (1.2 to 1.9)
Target calories = maintenance × (1 + strategy), where strategy is −20% to +5%

Monthly muscle gain starts from a training-age ceiling — 1.0% of body weight for a novice, 0.5% intermediate, 0.25% advanced, 0.125% highly trained, halved for women — then five multipliers are applied: calorie strategy (1.00× in a surplus down to 0.35× in a 20% deficit), protein (1.00× at 1.6 g/kg or more, 0.85× at 1.2–1.6, 0.65× below), lifting frequency (1.00× at five sessions down to 0.70× at two), body fat available (1.20× to 0.70×), and genetic headroom, which tapers linearly to zero over the last 6 FFMI points before the drug-free ceiling.

Fat change (kg) = [monthly energy balance − (muscle gained × 2,500)] ÷ 7,700

The two constants are the point: a kilogram of adipose tissue stores about 7,700 kcal, while a kilogram of lean tissue costs only about 2,500 kcal to build because it is 70–75% water. New lean mass is paid for out of the energy balance first, and whatever remains moves the fat compartment. Every month the loop recalculates lean mass, fat mass, BMR, maintenance and target calories from the updated body composition.

Reference Ranges & Interpretation

Muscle gain ceilings by training yearfollow the model popularised by Alan Aragon and Lyle McDonald: about 0.9 kg (2 lb) per month in year one, 0.45 kg in year two, 0.23 kg in years three and four, and 0.11 kg from year five. Women average roughly half these figures. A recomp reaches only a fraction of the ceiling — typically 40–75% at maintenance and 25–40% in a moderate deficit.

Body fat bands that favour recomposition:men above 20% and women above 28% carry enough stored energy that the model applies a 1.1–1.2× bonus. Men at 12–18% and women at 22–28% are neutral. Below 8% for men and 18% for women a 0.7× penalty applies, because at that leanness the body defends its remaining fat. Essential body fat is roughly 5% for men and 12% for women, and the projection will not model below it.

Fat-free mass index headroom:Kouri's 1995 analysis of drug-free and steroid-using athletes found an FFMI of about 25 marks the natural ceiling for men; 22 is the commonly cited equivalent for women. An FFMI of 19 or below leaves full headroom, 22 halves the rate for a man, and 25 reduces it to zero. An outlook of “Excellent” means the model allows over 0.6% of body weight in month one; “Poor” means under 0.18%, which no home body fat measurement can resolve.

Assumptions & Limitations

This is a model, not a measurement, and it inherits every error in its inputs. Body fat percentage is the largest of these: consumer bioimpedance scales are commonly off by 5 percentage points and calipers by 3, and that error propagates straight into lean mass, basal rate and every projected row. Predictive maintenance equations carry their own ±10% spread because non-exercise activity varies enormously between individuals eating the same diet.

The model assumes consistent progressive resistance training, adequate sleep, and that you actually eat the target calories — self-reported intake is routinely under-reported by 20–30%. It also assumes you adjust calories as maintenance drifts upward with new lean mass; hold month one's number all year and the later rows will overstate your fat loss. Muscle gain is treated as smooth and monthly, whereas real hypertrophy arrives unevenly and often front-loads in the first eight weeks of a new programme.

The assumptions break entirely for several groups: anabolic steroid users (whose FFMI ceiling does not apply), people returning to training after a layoff (muscle memory makes regain far faster than the training age suggests), anyone pregnant or breastfeeding, adolescents still growing, and people with thyroid disease, PCOS, Cushing's syndrome or on corticosteroids. Speak to a physician or a registered dietitian before making significant dietary changes, particularly if you have any history of disordered eating.

Complete Guide: Body Recomposition Calculator

A body recomposition calculator exists for one specific frustration: twelve weeks of hard training, and the scale reads 82.1 kg instead of 82.0 kg. Nothing happened, apparently. Except the caliper reading went from 22% to 20.4% body fat, which means roughly 1.3 kg of fat left and 1.4 kg of lean tissue arrived in its place. The two changes cancelled almost exactly on the scale, and the only instrument capable of noticing was the one measuring composition rather than mass. That swap is what recomposition means, and it is why a recomp calculator has to track two compartments instead of one scale reading. Modelling it takes exactly that: two numbers, two compartments, and an honest accounting of where the energy went.

Six Months at 82 kg: What the Body Recomposition Calculator Projects

Take a concrete case and follow it all the way through. A 32-year-old man, 178 cm, 82 kg, 22% body fat, lifting four days a week, two years into consistent training. Split his weight into compartments first: 22% of 82 kg is 18.0 kg of fat, leaving 64.0 kg of lean mass. That lean figure is what drives everything else, so the Katch-McArdle equation is the right basal formula here — it reads lean mass and nothing else:

BMR = 370 + (21.6 × lean mass in kg) = 370 + (21.6 × 64.0) = 1,752 kcal/day
Maintenance = 1,752 × 1.55 (lifting 3–5 days) = 2,715 kcal/day

Now the muscle side. Two years of training puts him at roughly 0.5% of body weight per month as a ceiling — about 410 g. He does not get the ceiling, because a recomp is not a bulk. Eating at maintenance rather than in a surplus costs about 15% of it, four sessions a week rather than five costs another 5%, and being at 22% body fat gives back 10% because there is plenty of stored fuel to build from. One more factor bites: 64 kg of lean mass at 178 cm is an FFMI of 20.2 against a drug-free ceiling near 25, and that closing gap trims a further 20%. Multiply it all out and month one allows roughly 290 g of new lean tissue. Here is what six months of that looks like, with maintenance recalculated each month as lean mass changes:

What Six Months at Maintenance Actually Buys

Read the last row carefully, because it is the honest answer nobody wants: at true maintenance calories, six months buys 1.7 kg of lean mass and 0.6 kg of fat loss. One percentage point of body fat. Meanwhile the scale went up1.1 kg. This is what a textbook recomp actually looks like, and it is why so many people abandon one at week eight convinced it is not working. Drop the same man to a 10% deficit — 2,443 kcal instead of 2,715 — and the six-month picture changes completely: about 1.1 kg of lean gained, 6.9 kg of fat lost, and body fat falling from 22% to roughly 14.7%. A third less muscle, and more than twelve times the fat loss. The calculator above lets you flip between those settings and watch which compartment pays.

The Two Constants That Make a Recomp Possible

Simultaneous fat loss and muscle gain sounds like it violates energy balance. It does not, because fat and muscle are not priced the same. A kilogram of adipose tissue stores about 7,700 kcal; roughly 87% of it is triglyceride at 9 kcal per gram. A kilogram of skeletal muscle stores only about 2,500 kcal, because it is 70–75% water — you are really depositing around 200 g of protein plus the fluid and the metabolic cost of assembling it.

Three times more energy per kilogram in one direction than the other. That asymmetry is the entire mechanism. Burning 7,700 kcal of stored fat funds three kilograms of new muscle, so a person carrying 18 kg of fat is sitting on enough energy to build far more tissue than they will ever build. Nothing needs to arrive from outside the system; the energy just moves compartments. It is also why recomp gets harder as you get leaner — not because the physics changes, but because at 8% body fat the body defends what little is left with falling leptin, rising ghrelin and a measurable drop in testosterone.

Who Actually Recomps, and Who Is Wasting a Year

Four groups reliably gain muscle and lose fat at the same time, and the reason is the same in each case: an unusually large gap between current lean mass and trainable potential.

• Untrained beginners. A first year of lifting supports roughly 0.9 kg of muscle per month for men and half that for women, which is enough to swamp a modest deficit.
• Returning lifters. Muscle memory is structural: myonuclei acquired during previous training persist for years, so regaining 8 kg of lost mass can take a quarter of the time it took to build it.
• Overweight lifters above roughly 25% body fat (men) or 32% (women).Large fat stores mean large energy availability. Longland's 2016 trial put men in a brutal 40% deficit for four weeks and the high-protein group still gained 1.2 kg of lean mass while losing 4.8 kg of fat.
• Anyone who has never trained properly despite years in a gym. Training age counts consistent progressive overload, not gym membership months.

The group that should stop trying: a lean, advanced lifter at 12% body fat with an FFMI of 23. Their monthly ceiling is around 0.25% of body weight before any recomp penalty, and the leanness penalty cuts it further — maybe 90 g a month, which no body fat measurement you own can detect against its own 3-point error bar. That lifter should pick a direction. Check where you sit on that curve with the FFMI calculator, which compares your fat-free mass to the drug-free ceiling of about 25 for men and 22 for women.

Why 1.6 g/kg Is the Number That Keeps Appearing

Morton's 2018 meta-analysis pooled 49 studies and 1,863 participants and found the benefit of extra protein on lean mass plateaued at 1.62 g per kg of body weight per day, with the confidence interval stretching to 2.2 g/kg. For our 82 kg example that is 133 g minimum, 180 g at the upper bound. Below the plateau the loss is real and quick: a lifter at 1.0 g/kg gives up something like a third of the muscle a recomp could have produced, because in an energy deficit dietary amino acids are the difference between muscle protein being spared and being oxidised for fuel.

During a deficit the sensible target sits higher than the maintenance plateau — 2.0 to 2.4 g/kg — which is exactly what Longland used. There is also a practical reason to overshoot: protein carries a thermic effect of 20–30% against 5–10% for carbohydrate and 0–3% for fat, so 40 g of protein swapped for 40 g of carbohydrate quietly removes about 25 kcal a day from the usable total. Work out your own floor with the protein intake calculator for muscle gain, then set the slider above to match.

Recomp, Cut, or Bulk? A Decision Framework

Body fat percentage and training age between them decide this, and the honest answer is often “not a recomp”. Use the row that matches you:

One nuance the table cannot hold: time horizon. A recomp is the right call for someone with no deadline who wants to avoid ever being visibly bulky or visibly depleted. A cut-then-bulk cycle reaches the same endpoint faster but passes through phases you may not want to be in during a wedding or a beach holiday. If speed matters more than smoothness, the weight loss calorie calculator will set a straightforward deficit instead.

The Maintenance Number That Will Not Sit Still

Here is the trap almost every recomp guide skips. Maintenance calories are not a constant, and in a recomp they drift upward. Each kilogram of new lean mass adds 21.6 kcal to basal rate through Katch-McArdle, which becomes roughly 33 kcal of maintenance once a 1.55 activity factor is applied. Add 1.7 kg of muscle over six months and true maintenance has risen about 57 kcal a day.

That sounds trivial. Over the following six months it is 10,000 kcal, or 1.3 kg of fat that comes off without you touching anything — which is pleasant if you wanted fat loss and a genuine problem if you were trying to hold weight. The month-by-month table in the calculator recalculates that number every row, so the “Eat” column climbs on its own. If your own maintenance estimate is more than a few months old, rebuild it with the TDEE calculator before trusting any recomp target built on top of it.

How Do You Measure a Recomp When the Scale Is Flat?

You need instruments that separate the compartments, and you need to understand their error bars. DEXA is the practical reference standard at roughly ±1–2 percentage points, but hydration state alone can shift a reading by a full point, so scan at the same time of day in the same fed and hydrated state. Consumer bioimpedance scales are worse than their marketing suggests — commonly ±5 points, and they drift with hydration, skin temperature and time since your last meal. Seven-site calipers in trained hands land around ±3 points, and, importantly, they are consistently wrong, which makes them fine for tracking change even when the absolute number is off.

The cheapest reliable signal costs nothing: waist circumference at the navel, measured every two weeks first thing in the morning. Waist tracks fat mass closely and is far more repeatable than any home body-fat method. A waist falling 2 cm while your bench press and body weight both hold is a recomp, and no scale on earth would have told you. Pair it with the lean body mass calculator to convert each new body fat reading into the lean and fat figures this projection actually runs on.

Four Mistakes That Stall a Recomp

Quitting at week eight. The example above moved body fat 0.4 points in two months. That is inside the error bar of every home measurement, so eight weeks of a genuinely working recomp is indistinguishable from eight weeks of nothing. Sixteen weeks is the minimum before the signal clears the noise.

Replacing lifting with cardio. Resistance training is the signal that tells the body which tissue to keep when energy is short. Without it, a deficit strips lean mass alongside fat: roughly 25% of weight lost in an unsupported diet is fat-free mass, against under 5% when heavy lifting and adequate protein are in place. Cardio helps the energy side and does almost nothing for the muscle side.

Treating “maintenance” as permission to stop counting. Eating at maintenance is harder than eating in a deficit, because there is no scale feedback when you drift 300 kcal high. Most people who believe they are recomping are in a small surplus and simply gaining slowly.

Ignoring sleep.Nedeltcheva's crossover trial restricted sleep to 5.5 hours and found that while total weight loss was unchanged, the proportion coming from fat fell by 55% and the loss of fat-free mass rose sharply. Sleep is not a lifestyle footnote in a recomp — it decides which compartment the weight comes out of. Seven hours is the floor, and eight is better if training volume is high.

References

1. Longland TM, et al. “Higher compared with lower dietary protein during an energy deficit combined with intense exercise promotes greater lean mass gain and fat mass loss.” American Journal of Clinical Nutrition, 2016. PubMed 26817506
2. Morton RW, 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.” British Journal of Sports Medicine, 2018. PubMed 28698222
3. Nedeltcheva AV, et al. “Insufficient sleep undermines dietary efforts to reduce adiposity.” Annals of Internal Medicine, 2010. PubMed 20921542
4. Kouri EM, et al. “Fat-free mass index in users and nonusers of anabolic-androgenic steroids.” Clinical Journal of Sport Medicine, 1995. PubMed 7614327
5. U.S. Centers for Disease Control and Prevention. “Assessing Your Weight” and adult physical activity guidance. cdc.gov

Frequently Asked Questions