Can you lose fat and build muscle at the same time? Enter your body, calories, protein and training, and this body recomposition calculator projects your weight, fat mass and lean mass over 12 and 24 weeks, with an honest range and side-by-side scenarios.
Don’t know it? Measure it with the body fat calculator, or leave blank for a rough estimate from BMI.
Not sure? Find your number with the protein calculator.
Your projection
Weight
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Fat mass
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Lean mass
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WeightFat massLean massLikely range
Scenarios side by side
Week by week
| Week | Weight | Fat mass | Lean mass | Body fat |
|---|
Individual results vary widely. This is a simplified model, not a prediction for you. Water, glycogen and gut content can move the scale by 1–2 kg in a day, and people on the same plan gain muscle at very different rates. Track your real weekly averages and waist with the progress tracker and adjust after 3–4 weeks.
How this body recomposition calculator works
Body recomposition means losing fat and keeping or adding lean mass (muscle, plus the water and organs that go with it) at the same time. The calculator runs a simple day-by-day energy balance for 24 weeks, then splits each change in weight between fat and lean tissue. Every step is listed below so you can see exactly what drives the numbers.
- Maintenance calories. Resting burn from the Mifflin-St Jeor equation, times your day-to-day activity factor, plus the extra calories from your lifting sessions (about 3.5 METs for a typical 50-minute session).
- Your intake. The number you enter, or our estimate: the same rule as the calorie calculator, a deficit for losing about 0.5% of body weight a week, never below 1,500 calories for men or 1,200 for women.
- Your body adapts. As you lose weight, you burn less. We lower your maintenance by about 24 calories a day for every kg lost (and raise it for every kg gained), the rule of thumb from the NIH body-weight model.
- Where the weight comes from. Without lifting, the share of weight change that is lean tissue follows Forbes’ rule: the leaner you are, the bigger the lean share. Lifting during a deficit protects most of that lean tissue, and eating at least moderate protein protects a little more.
- Muscle gain from training. Lifting adds lean mass at a steady rate that is fastest for beginners, a bit higher with 4+ sessions, a bit lower with low protein, and shrinks as your deficit grows, reaching zero at about a 500-calorie daily deficit.
- The shaded range combines a ±10% error in maintenance calories, muscle gain at half to one-and-a-half times our central rate, and lifting protecting anywhere from 40% to all of your lean mass in a deficit.
Every assumption and its source
| Assumption | Value used | Source |
|---|---|---|
| Energy in 1 kg of fat mass lost or gained | 9,440 kcal (39.5 MJ) | Hall 2008. The familiar 7,700 kcal/kg (3,500 kcal/lb) rule describes mixed tissue, not pure fat. |
| Energy in 1 kg of lean mass | 1,816 kcal (7.6 MJ) | Hall 2008 |
| Share of weight change that is lean (no lifting) | 10.4 / (10.4 + fat mass in kg), Forbes’ rule | Forbes, as modelled in Hall 2008 |
| Fall in calorie burn per kg lost | 24 kcal/day per kg | Hall et al. 2011, Lancet (100 kJ/day per kg) |
| Resting burn and activity | Mifflin-St Jeor × 1.2, 1.375 or 1.55; accurate to about ±10% for most people | Mifflin 1990; Frankenfield 2005 |
| Calories burned lifting | 3.5 METs, 50 min, 2.5 or 4 sessions a week | Compendium of Physical Activities, code 02054 |
| Lean loss prevented by lifting in a deficit | 90% (40–100% in the shaded range) | Sardeli 2018 meta-analysis: lifting prevented about 93.5% of diet-induced lean loss in older adults (we round down; the study’s confidence interval was wide) |
| Extra lean kept with moderate or high protein in a deficit | Up to 0.6 kg per 12 weeks | Krieger 2006: intakes above 1.05 g/kg kept about 0.6 kg more fat-free mass |
| Beginner lean gain with lifting | About 2.5% of lean mass per 12 weeks (about 1.5 kg for an average man) | Benito 2020 meta-analysis of 111 studies: about 1.5 kg on average |
| Trained lifters | Half the beginner rate | Our assumption: gains slow with training age (ACSM 2009), but no study gives a single number |
| 4+ sessions a week | 1.25 × the gain rate | Our conservative reading of Schoenfeld 2017: more weekly sets, more growth |
| Low protein | 0.8 × the gain rate (moderate 0.9) | Morton 2018: extra protein added about 0.3 kg to typical gains |
| Deficit blunts muscle gain | Gain falls in a straight line to zero at a 500 kcal/day deficit | Murphy and Koehler 2022 meta-regression |
| Body fat if left blank | 1.2 × BMI + 0.23 × age − 10.8 (men) − 5.4 | Deurenberg 1991 (rough: error of several percentage points) |
| Lowest body fat modelled | 5% for men, 13% for women; further loss comes from lean tissue | Our safety floor, so the projection never shows extreme leanness as a goal |
| Range of individual responses | Muscle gain from 0.5× to 1.5× the central rate | Hubal 2005: muscle size gains ranged from about 2% to 59% in 585 people on the same program |
What the projection cannot tell you
- It is not a forecast for you. Sleep, stress, steps, medications, hormones and how closely you follow the plan all change the result.
- Lean mass is not the same as muscle. It also includes water and glycogen, which is why the scale drops fast in week one of a diet and why lean mass can dip on low-carb diets.
- Home scales cannot see this. Bioimpedance scales are not accurate enough to track changes this small. Use weekly average weight, waist and strength instead.
- Recomposition is slow after the first months, especially for trained lifters. Most people see the clearest results with a modest deficit, high protein and 2–4 strength sessions a week.
Put it into practice: the 12-week fat loss program combines a modest deficit with strength training. Read more: how to lose fat and keep muscle after 35, protein after 35, what happens to your metabolism after 35 and strength training after 35.
Sources
- Hall (2008), International Journal of Obesity: what is the required energy deficit per unit weight loss?
- Hall et al. (2011), The Lancet: quantification of the effect of energy imbalance on bodyweight
- Mifflin et al. (1990), American Journal of Clinical Nutrition: a new predictive equation for resting energy expenditure
- Frankenfield et al. (2005), Journal of the American Dietetic Association: accuracy of resting metabolic rate equations
- Compendium of Physical Activities: conditioning exercise MET values
- Sardeli et al. (2018), Nutrients: resistance training prevents muscle loss induced by caloric restriction in older adults
- Krieger et al. (2006), American Journal of Clinical Nutrition: protein and carbohydrate intake during energy restriction, a meta-regression
- Benito et al. (2020), International Journal of Environmental Research and Public Health: resistance training and whole-body muscle growth in men
- American College of Sports Medicine (2009): progression models in resistance training for healthy adults
- Schoenfeld, Ogborn and Krieger (2017), Journal of Sports Sciences: weekly training volume and muscle growth
- Morton et al. (2018), British Journal of Sports Medicine: protein supplementation and gains in muscle mass and strength
- Murphy and Koehler (2022), Scandinavian Journal of Medicine and Science in Sports: energy deficiency impairs lean mass gains from resistance training
- Deurenberg et al. (1991), British Journal of Nutrition: BMI as a measure of body fatness
- Hubal et al. (2005), Medicine and Science in Sports and Exercise: variability in muscle size and strength gain after resistance training
This calculator gives general education, not medical advice. Check with your doctor before a calorie deficit or new training plan if you have diabetes, heart or kidney disease, take regular medication, or have a history of eating disorders. Very low-calorie diets should only be followed with medical supervision.
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