Methodology
The methods behind the numbers.
Find the equations, assumptions, projections, recommendation logic, and limitations behind FitCalc tools.
Estimates, not exact measurements
FitCalc calculators turn a defined set of inputs into a mechanical result using a stated formula. Inputs are converted to canonical metric units internally, calculations retain unrounded values, and results are rounded only for display. Formula and component versions are recorded so changes can be tested against prior behavior.
These estimates are useful starting points, not medical diagnoses or guarantees. Real energy expenditure and body composition vary between people and across time.
Results, evidence, and recommendations
Calculated Result
A mechanical output produced by a stated equation.
Evidence-Based Range
A range supported by research or recognized guidance when the evidence supports expressing a range.
FitCalc Recommendation
A practical interpretation or preferred target chosen by FitCalc. Recommendations are identified as such rather than presented as universal scientific facts. Not every current calculator uses all three layers.
FitCalc Plan V2
A guided nine- or ten-screen browser-local assessment connects existing energy, optional body-composition, and iterative Timeline engines. Scientific equations, editorial targets, user preferences, and model assumptions are identified separately.
Mifflin–St Jeor and existing activity factors estimate BMR and maintenance. Fat loss uses Plan-specific 5%, 10%, or 15% deficits; muscle gain uses 5% or 10% planning surpluses. A 15% muscle-gain surplus is not offered because evidence in trained lifters found that faster gain primarily increased fat gain without a clear muscle-thickness advantage. Training frequency adds context, not another energy multiplier.
FitCalc Plan uses approximately 1 gram per pound of CURRENT body weight, rounded to whole grams. This practical high-protein target is not a universal physiological minimum. Adequate intake may be lower depending on context, and more protein does not automatically produce more muscle. The standalone Protein Calculator remains unchanged.
Fat defaults to 25% of calories. The 20–40% slider trades fat against carbohydrate while protein and calories stay fixed. This is a food-preference range; the general adult fat reference range is 20–35%. The existing preferred 0.6 g/kg fat constraint can narrow the range. If the target cannot accommodate fixed protein and preferred fat, fat/carbohydrate targets are unavailable rather than negative. Whole-gram rounding leaves at most 3 kcal unallocated when feasible.
Enter a body-fat estimate, use the existing Navy circumference method, or skip it. Source labels remain visible. Existing lean/fat mass functions inherit measurement error; no future body composition is predicted.
A valid fat-loss target uses the existing calculated-maintenance Timeline engine, recalculating estimated energy needs as projected weight changes while planned intake and activity stay constant. It is informed by dynamic energy-balance research, but does not reproduce the complete NIH Body Weight Planner or full Hall physiological model. The ±10% effective-deficit scenario range is not a confidence interval. Unavailable projections do not remove the valid nutrition plan. Maintain and Build Muscle do not receive artificial deadlines.
FitCalc converts the distance from current to target weight into a broad scale-weight planning range. At a 5% surplus it assumes 0.15–0.30% of current weight per week; at 10%, 0.20–0.40%. These deliberately overlapping FitCalc bands draw on off-season rate recommendations and controlled surplus research. They are not a validated physiological equation, do not predict pure muscle gain, and can vary materially with training status, adherence, water, glycogen, fat gain, and individual response.
Review average intake and multiweek weight trends together with training performance, hunger, and recovery. Approximately 2–3 weeks is an initial checkpoint, not a guarantee or automatic calorie-adjustment prescription.
Weight Loss Timeline
FitCalc uses a transparent iterative energy-balance projection rather than extending the first-week 3,500-calorie estimate forever. It holds the selected calorie target constant and updates estimated maintenance as projected body weight changes.
FitCalc Iterative Energy Balance v1
One-day steps recompute Mifflin?St Jeor maintenance at each projected weight using the selected activity factor.
Maintenance changes by approximately 100 kJ/day per kilogram of body-weight change (about 24 kcal/day/kg), consistent with the population-level dynamic rule described by Hall and colleagues.
This is not the full NIH Body Weight Planner model. Water, glycogen, body composition, adherence, and individual metabolic adaptation can materially change the actual timeline.
TDEE and Goal Plan
FitCalc estimates resting energy expenditure with the Mifflin–St Jeor equation, then multiplies it by the activity level selected by the user. The Goal Plan applies FitCalc's established calorie-adjustment and macro rules.
Mifflin–St Jeor v1; FitCalc Activity v1; FitCalc Goal Plan v1; FitCalc Macros v1
Adults ages 18–100 within the calculator's supported ranges.
A selected population-level activity multiplier can represent typical activity, and the result is a planning starting point.
Individual expenditure varies. Weekly pace uses the established static 3,500-calorie approximation.
Macro Calculator
FitCalc calculates or accepts daily calories, allocates its practical protein target first, allocates fat next, and gives carbohydrate the remaining energy.
FitCalc Macro v1
Fat loss -15%; maintenance 0%; muscle gain +10%.
Protein 2.2 g/kg; fat is the greater of 0.6 g/kg or about 25% of calories; carbohydrate is the remainder.
No split is uniquely correct. Low-calorie conflicts are shown explicitly.
Protein Calculator
The result separates a research-informed range from FitCalc's practical high-end recommendation.
FitCalc Protein v1
Evidence ranges remain context-specific: general maintenance/building 1.2–1.6 g/kg, general fat loss 1.4–1.8, resistance maintenance/building 1.6–2.2, and resistance fat loss 1.8–2.2.
A deliberately higher editorial target: general maintenance 1.8 g/kg, general fat loss 2.0, and 2.2 g/kg for general muscle building and all resistance-training contexts. It is not a universal minimum or hard ceiling.
Not pediatric, pregnancy/lactation, kidney-disease, or clinical nutrition guidance.
BMR
Mifflin–St Jeor estimates resting energy needs from age, equation sex, height, and weight. BMR is not a daily eating target.
Maintenance Calories
The same canonical BMR and activity-multiplier engine used by TDEE estimates total daily maintenance. Calibrate it against 2–3 weeks of consistent intake and weight trends.
Calorie Deficit
FitCalc subtracts 10%, 15%, or 20% from calculated or known maintenance. The displayed weekly rate is a short-term static approximation, not a long-term prediction.
BMI
Body mass index compares weight with height squared and applies the established adult BMI categories. FitCalc uses it as a quick screening reference.
BMI Standard v1
Adults within the calculator's supported ranges.
BMI is used as a screening measure.
It does not distinguish fat mass from lean mass and is not a diagnosis.
Body Fat Percentage
FitCalc uses the sex-specific U.S. Navy circumference equations with height, neck, waist or abdomen, and hip measurements where required.
Navy Body Fat v1
Adults within the calculator's supported ranges.
Circumferences are measured consistently at the locations described.
Measurement technique affects the result. It is a field estimate, not a laboratory measurement or diagnosis.
1 Rep Max
FitCalc uses the Epley equation for sets of 1–10 repetitions: lifted weight multiplied by one plus reps divided by 30. A one-rep set returns the entered weight. Exercise selection labels the result but does not change coefficients.
Prediction error varies with exercise, technique, fatigue, training status, and proximity to failure. Sets above 10 reps are rejected.
Plate Loading
The target is total barbell weight. FitCalc subtracts the selected bar, divides the remainder equally, and decomposes each side from largest selected plate to smallest using integerized hundredths of the active unit.
If the target cannot be assembled symmetrically, the closest achievable total is reported. An equally close lower load is preferred.
Running Pace
FitCalc stores distance in metres and elapsed time in seconds. Speed equals distance divided by time; pace is time divided by distance. Results round pace and elapsed time to the nearest second and speed to one decimal place.
Distance + time returns pace and speed. Distance + pace returns finish time and speed. Even splits preserve the calculated pace and use mile or kilometre markers, capped at 50 rows.
This is not a race-performance prediction and does not account for terrain, weather, fatigue, or pacing strategy.
Heart Rate Zones
Without an entered maximum heart rate, FitCalc uses the Tanaka estimate: 208 − 0.7 × age. An entered maximum takes priority but is not assumed to be medically verified.
The five FitCalc bands are 50–60%, 60–70%, 70–80%, 80–90%, and 90–100%. Users may calculate them as percent of maximum heart rate or, with resting heart rate, as Heart Rate Reserve. Whole-bpm boundaries use the next band's rounded-up start minus one, preventing gaps and overlaps.
Age formulas have substantial individual error, training systems define zones differently, and the displayed Zone 2 is not necessarily a laboratory-defined threshold.
Lean Body Mass
FitCalc uses the two-compartment model: lean mass equals body weight multiplied by one minus body-fat proportion. It requires a body-fat estimate rather than predicting one from demographic inputs.
Waist-to-Height Ratio
Waist circumference divided by height, using the same units. The common “keep waist below half your height†rule is screening context, not a diagnosis or universal risk boundary.
Fat-Free Mass Index
Standard FFMI equals fat-free mass in kilograms divided by height in metres squared. FitCalc does not apply the sample-specific normalized adjustment or interpret a score as evidence of performance-enhancing drug use.
Review and corrections
Methodology records include review dates. A date is updated when the relevant method or review changes, not merely because unrelated site code changes. If you find a formula, citation, or explanation that appears incorrect, please contact FitCalc.