How FitCommit Calculates Calories and Macros

TL;DR

• BMR uses the Katch-McArdle formula: 370 + (21.6 x lean mass in kg). We estimate lean mass from sex-specific average body fat (18% for men, 25% for women) when a user does not supply a scan.
• TDEE multiplies BMR by one of five activity factors: 1.2 sedentary, 1.375 lightly active, 1.55 moderate, 1.725 very active, 1.9 extra active.
• Cutting multiplies TDEE by 0.80 (a 20% deficit). Bulking multiplies by 1.10 (a 10% surplus). Maintenance uses TDEE directly.
• Protein is set by lean body mass: 2.6 g/kg on cuts, 2.3 g/kg on bulks and maintenance.
• Fat is 35% of target calories on cuts, 30% on bulks and maintenance, subject to a safety floor of 0.7 to 1.1 g/kg of total body weight (sex and goal dependent).
• Carbs fill whatever calories are left, with a 50 g/day floor for central nervous system glucose.

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 their measured lean mass directly. 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 are population medians that place both sexes in the "Moderate" body fat category and tend to under-claim lean mass rather than over-claim it, which is the safer direction for calorie estimates.

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: TDEE x 0.80. A 20% deficit is aggressive enough to produce roughly 1 lb/week of loss for most people but shallow enough that compliance and muscle retention stay high. Deeper deficits (25%+) cost lean mass unless paired with high protein and resistance training.
• Bulking: TDEE x 1.10. A 10% surplus is the lean-bulk default. It supplies the energy needed for muscle protein synthesis without the fat accrual that 20%+ surpluses produce. Expected rate of gain: 0.25 to 0.5 lb/week for an intermediate lifter.
• Maintenance: TDEE x 1.0. Same calorie total, same protein ratio as bulking. Useful between cut and bulk phases, during a diet break, or when the goal is recomposition (slow fat loss paired with slow muscle gain).

The ISSN position stand on diets and body composition (Aragon et al., 2017) explicitly supports percentage-based deficits in the 10 to 25% range for cutting, with protein preservation being the deciding factor for lean mass retention. Our 20% default sits in the middle of that range.

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: 2.6 g per kg of lean mass. Aggressive but not extreme. Matches the upper end of the ISSN recommendation for athletes in a deficit (2.3 to 3.1 g/kg lean mass).
• 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 safety floor expressed in g/kg of total body weight. The percentage sets the upper bound. The floor sets the lower bound.

• Cutting: 35% of target calories, with a floor of 0.9 g/kg body weight for men, 1.1 g/kg for women.
• Bulking: 30% of target calories, floor of 0.7 g/kg men, 0.9 g/kg women.
• Maintenance: 30% of target calories, floor same as bulking.

Why a floor at all? Because once calories drop far enough, 30% of a small number stops being enough fat to support hormone production. Loucks and Thuma (2003) showed that low energy availability below roughly 30 kcal/kg of lean mass per day disrupts the luteal phase in female athletes. Low dietary fat is the fastest way to trigger that kind of hormonal disturbance. The female floor is higher than the male floor because women are more sensitive to energy availability in general.

In practice, the floor almost always wins during aggressive cuts on smaller body weights, where 35% of calories is a small absolute number of fat grams. For larger individuals and for bulks and maintenance, the percentage wins.

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. This is the example that appears at /macros/180-lbs-male-cutting-moderate.

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 cutting deficit. Target calories is 2,815 x 0.80 is 2,252 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 (percentage). 35% of 2,252 is 788 cal / 9 cal/g is 87.6 g. Safety floor check: 0.9 g/kg x 81.65 kg is 73.5 g. Floor is lower than the percentage, so the percentage wins: 88 g.
8. Carbs. Remaining calories: 2,252 minus 696 (protein) minus 792 (fat) is 764 cal / 4 cal/g is 191 g.

Final target: 2,252 calories, 174g protein, 191g carbs, 88g fat. These are the exact numbers that appear on the variant page. 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

Every number in this methodology, and every automated calculation that produces the 420 macro pages and the in-app targets, is reviewed by Andrew Menechian, FitCommit's Head of Fitness. Andrew designed the macro framework described above and signs off on any change to the constants. If a formula, multiplier, or protein target changes, this page and the underlying /macros collection are updated together and the change is logged with a date stamp.

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.

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.