How FitCommit Calculates Calories and Macros

TL;DR

FitCommit starts with BMR, estimates total daily energy expenditure, and then applies the goal you choose. Cutting offers a sliding deficit from 15% to 40% below TDEE, with 25% as the recommended default. Protein scales with lean mass and deficit size. Fat is 25% to 30% of target calories on cuts and 25% on bulks and maintenance, with a floor of 0.5 g per kg of body weight or 20% of calories, whichever is higher. Carbohydrates fill the remaining calories.

1. Why Katch-McArdle and not Harris-Benedict or Mifflin-St Jeor

Harris-Benedict (1919) and Mifflin-St Jeor (1990) both estimate BMR from total body weight, height, age, and sex. That works reasonably well for the general population but breaks down at the tails. Two people with identical weight, height, age, and sex can have wildly different resting metabolic rates if one is 15% body fat and the other is 30%, because fat-free mass is the dominant metabolic driver. The weight-based formulas do not know about that difference.

Katch-McArdle (1996) drops everything except lean body mass. The formula is BMR = 370 + 21.6 x (LBM in kg). It was validated across a wide range of body compositions and produces tighter estimates for lean, athletic, or over-fat populations than the weight-only equations. Our users skew toward people who train and care about body composition, so the Katch-McArdle assumption is the better fit.

When the user has completed an AI body scan, we use its lean-mass estimate. When they have not, we use sex-specific average body fat to estimate lean mass from weight: 18% body fat for men and 25% for women. These defaults are starting assumptions, not measurements.

2. Activity multipliers and TDEE

The five-tier activity model is standard in sports nutrition and maps to ACSM activity categories. FitCommit uses the following multipliers, applied to BMR to produce TDEE:

The biggest individual error source in TDEE estimates is self-reported activity. People who walk 8,000 steps and lift three times a week almost always pick "very active" when "moderate" is closer. If your weight is not moving the way the math says it should after 2 to 3 weeks, drop one tier before changing calories.

3. Goal adjustments: deficit, surplus, maintenance

FitCommit uses percentage-of-TDEE adjustments rather than flat calorie deltas. A 500-calorie deficit is aggressive for a 120-lb woman (roughly 28% of TDEE) and trivial for a 250-lb man (roughly 17%). Percentages scale properly across body sizes.

Cutting targets range from 15% below TDEE for a gentler deficit to 40% for a very aggressive deficit. The recommended default is 25%. Smaller deficits generally suit leaner starting points and longer cuts. Larger deficits require more body fat to lose, closer monitoring, and a shorter runway.

• Bulking offers 5%, 10%, and 15% surplus levels, with 10% as the default.
• Maintenance uses estimated TDEE directly.
• General-planning floors stop targets at 1,500 calories per day for men and 1,200 for women. These are planning guardrails, not individualized medical advice.

A percentage-based adjustment scales with estimated energy expenditure more usefully than treating one flat calorie number as appropriate for everyone.

4. Protein: grams per kilogram of lean body mass

Protein is the most important macro to get right during any phase. It drives satiety, preserves muscle under a deficit, and supplies the amino acid pool for muscle protein synthesis during a surplus. FitCommit sets protein based on lean body mass rather than total body weight, because fat tissue does not require protein to maintain.

• Cutting: 2.3 to 3.1 g per kg of lean mass. Protein rises as the selected deficit becomes more aggressive. The recommended 25% deficit uses 2.6 g/kg.
• Bulking: 2.3 g per kg of lean mass. Above the general strength-sport guideline of 1.6 to 2.2 g/kg total body weight and well above the 0.8 g/kg RDA for sedentary adults.
• Maintenance: 2.3 g per kg of lean mass. Same as bulking. Maintenance phases benefit from sustained high protein because lean mass retention is still an active process.

On the per-meal side, Phillips and Van Loon (2011) identified a leucine threshold near 2.5 to 3.0 g per meal that maximally stimulates muscle protein synthesis. Spread across 3 to 5 meals, our totals easily clear this threshold at every body weight and goal we publish.

A concrete example. A 180-lb male (81.6 kg) cutting at moderate activity, estimated at 18% body fat: lean mass is 0.82 x 81.6 is about 66.9 kg. Protein target: 2.6 x 66.9 is about 174 g/day.

5. Fat: percentage with a safety floor

Fat intake is set as a percentage of target calories with a minimum floor based on total body weight and calorie intake.

• Cutting: 25% to 30% of target calories, rising with the selected deficit.
• Bulking and maintenance: 25% of target calories.
• Floor: 0.5 g per kg of body weight or 20% of target calories, whichever produces more fat.

The floor prevents the fat target from collapsing when calorie intake becomes low. It is a planning constraint, not a guarantee that a target is appropriate for a particular medical situation.

FitCommit calculates both values and uses the higher result before assigning remaining calories to carbohydrates.

6. Carbs: remainder with a 50 g floor

Carbohydrates take whatever calories are left after protein and fat are allocated. Protein and fat are both essential macros (the body cannot synthesize all required fatty acids or amino acids); carbs are functionally non-essential in the narrow sense that the body can produce glucose via gluconeogenesis. But they are critical fuel for training and brain function and most people train and feel better with adequate carbs.

The 50 g floor exists so that aggressive cuts at low body weights do not accidentally default to ketogenic numbers. If the remainder drops below 50 g of carbs, we recompute: carbs are held at 50 g, fat is increased to soak up the calorie budget, and protein stays at the lean-mass target. This is rare at weights above 140 lbs.

On timing, we publish per-meal splits on every variant page: protein is divided evenly across 4 meals by default, carbs are concentrated around training when the user specifies a training schedule, and fat is distributed across non-training meals. The training timing heuristic is less evidence-supported than the daily total (Aragon and Schoenfeld, 2013, found no clear performance advantage to nutrient timing once daily totals are matched) but is included because it matches how most athletes structure their days in practice.

7. A full worked example

Let's walk through a 180-lb male, cutting, moderate activity. You can reproduce this with the macro calculator.

1. Convert weight. 180 lbs / 2.2046 is 81.65 kg.
2. Estimate lean mass. Male average body fat is 18%. Lean mass is 81.65 x (1 minus 0.18) is 66.95 kg.
3. Calculate BMR. BMR is 370 + (21.6 x 66.95) is 370 + 1446 is 1,816 cal/day.
4. Calculate TDEE. Moderate activity multiplier is 1.55. TDEE is 1,816 x 1.55 is 2,815 cal/day.
5. Apply the recommended cutting deficit. Target calories is 2,815 x 0.75, which rounds to 2,111 cal/day.
6. Protein. 2.6 g/kg lean mass x 66.95 kg is 174 g/day. Protein calories is 174 x 4 is 696 cal.
7. Fat. The recommended cut uses 27% of 2,111 calories, which is about 63 g. The floor is the higher of 0.5 g/kg body weight or 20% of target calories, and both are below the 27% target here.
8. Carbs. Remaining calories after 174 g protein and 63 g fat produce 212 g of carbohydrates.

Final target: 2,111 calories, 174g protein, 212g carbs, 63g fat. Rounding in intermediate steps can shift totals by a few calories; the production code holds full precision until the last rounding step.

8. What we do not claim

A few honest limits on these numbers, because any good methodology also says where it breaks.

• These are starting targets, not prescriptions. BMR predictions have a standard error of roughly 8 to 15% for any individual. If your weight does not track the math after 2 to 3 weeks, adjust calories up or down by 5 to 10% and keep going.
• Activity self-reporting is the largest error source. People overestimate effort. If you are uncertain, use a lower activity tier and let weight trends tell you whether to move up.
• The body fat defaults (18% male, 25% female) are population averages. If your actual body fat is materially different, the lean mass estimate and therefore every downstream target will be off. Use the AI body scan or a calipers estimate to fix this.
• We do not currently account for menstrual cycle effects, pregnancy, lactation, medical conditions, or medications that alter metabolism. Consult a physician or registered dietitian for any of those cases.
• Children and adolescents are outside scope. FitCommit is for adults 18 and older.

9. Reviewer and oversight

The macro framework described here is reviewed by Andrew Menechian, FitCommit's Head of Fitness. Methodology prose and calculator constants are reviewed together so the explanation and output stay aligned.

Andrew's areas of expertise include macro calculation, TDEE and BMR estimation, cutting and fat loss protocols, lean bulking, the Katch-McArdle formula, and body composition. He is also one of FitCommit's beta trainers, working directly with users to validate that the calculated targets produce real-world results.

Reviewed by Andrew Menechian, Head of Fitness, FitCommit

Andrew reviews FitCommit's calorie, macro, and body-composition methodology. His background includes 12+ years in fitness, PN1, PNC 1&2, Poliquin PICP 1&2, bodybuilding, powerlifting, and coaching education.

10. Primary sources

The formulas, ranges, and safety floors on this page are drawn from the following sources. Where a direct link is available we include it; otherwise the citation is complete enough to locate in PubMed or the publisher's archive.

1. Katch VL, McArdle WD, Katch FI. Essentials of Exercise Physiology, 5th ed. (2016).

   Origin of the Katch-McArdle BMR equation. Uses lean body mass rather than total body weight to remove fat-mass error from resting metabolic rate estimates.

2. Jager R, Kerksick CM, et al. ISSN position stand: protein and exercise. J Int Soc Sports Nutr (2017).

   Protein requirements for training athletes: 1.4 to 2.0 g/kg for general, up to 2.3 to 3.1 g/kg lean mass during an energy deficit to preserve lean mass. Informs our cutting protein target of 2.6 g/kg lean mass.

3. Helms ER, Aragon AA, Fitschen PJ. Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. J Int Soc Sports Nutr (2014).

   Contest-prep evidence synthesis. Supports aggressive protein (2.3 to 3.1 g/kg lean mass) during deficits, fat floors, and resistance training for lean mass preservation.

4. Aragon AA, Schoenfeld BJ, et al. ISSN position stand: diets and body composition. J Int Soc Sports Nutr (2017).

   Deficit and surplus magnitudes for cutting and bulking. Supports our 20% cutting deficit and 10% bulking surplus defaults.

5. Loucks AB, Thuma JR. Luteal phase deficiency induced by low energy availability. J Clin Endocrinol Metab (2003).

   Low energy availability threshold (<30 kcal/kg LBM/day) disrupts endocrine function in female athletes. Informs our fat safety floors.

6. American College of Sports Medicine. Quantity and Quality of Exercise position stand (2011).

   Activity multiplier categories used in TDEE estimation: sedentary, light, moderate, vigorous, very vigorous. Our five-tier activity model (1.2 / 1.375 / 1.55 / 1.725 / 1.9) maps to these categories.

7. Phillips SM, Van Loon LJC. Dietary protein for athletes: from requirements to optimum adaptation. J Sports Sci (2011).

   Leucine threshold, protein distribution, and per-meal protein dose. Guides our per-meal protein recommendations on each variant page.