Methodology & Sources
Every formula on CalcFit, with its origin. No calculator on this site uses a made-up or proprietary formula — everything traces back to a published equation, government guideline, or a widely-cited research method.
This directory describes the principal methods used by CalcFit and distinguishes published equations from informal assumptions. It is not a claim of clinical validation. Tool-specific explanations provide additional context; use the linked primary references where available.
General note: every formula below is a population-level estimate. None of them are diagnostic, and none replace a conversation with a doctor, registered dietitian, or certified trainer for anything specific to your individual health situation.
Body Composition
| Calculator | Formula / Method | Source |
|---|---|---|
| BMI Calculator | weight(kg) / height(m)² | World Health Organization |
| Ideal Weight Calculator | Hamwi, Devine, Robinson, Miller formulas | Hamwi (1964), Devine (1974), Robinson (1983), Miller (1983) |
| Body Fat % Calculator | Circumference log-formula | U.S. Navy body composition method |
| Waist-to-Hip Ratio | waist / hip | World Health Organization risk categories |
| Waist-to-Height Ratio | waist / height | Ashwell method |
| Lean Body Mass | Boer formula | Boer, P. (1984), Journal of Applied Physiology |
| Army Body Fat Calculator | Same circumference method as Navy | Legacy circumference estimate; not a current Army eligibility assessment |
| Body Frame Size | height / wrist circumference ratio | Classic medical frame-size tables |
| Body Surface Area | Du Bois & Mosteller formulas | Du Bois (1916); Mosteller (1987), NEJM |
| Body Adiposity Index | hip / height^1.5 − 18 | Bergman et al. (2011), Obesity |
| Ponderal Index | weight / height³ | Rohrer (1921) index, standard anthropometric measure |
| Skinfold Body Fat | Jackson-Pollock 3-site + Siri equation | Jackson & Pollock (1978); Siri (1961) |
Calories & Nutrition
| Calculator | Formula / Method | Source |
|---|---|---|
| BMR / TDEE Calculator | Mifflin-St Jeor equation × activity multiplier | Mifflin et al. (1990), American Journal of Clinical Nutrition |
| Calorie Deficit Calculator | 3,500 kcal ≈ 1 lb fat | Standard energy-balance approximation |
| Macro Calculator | Percentage splits by goal/diet type | ISSN position stands on protein intake |
| Calories Burned Calculator | MET × 3.5 × weight(kg) / 200 × minutes | Compendium of Physical Activities (Ainsworth et al.) |
| Water Intake Calculator | 0.5 oz per lb bodyweight baseline | Informal fluid-planning rule of thumb; not an individualized medical recommendation |
| Protein Intake Calculator | g/kg by activity/goal | ISSN Position Stand (2017) |
| Fiber Intake Calculator | 14g per 1,000 kcal | Institute of Medicine Dietary Reference Intakes |
| Alcohol Calorie Calculator | 7 kcal/g ethanol + residual carbs | USDA nutrient values |
| Intermittent Fasting Calculator | Fixed eating/fasting window arithmetic | Standard IF protocol definitions |
| Net Carbs Calculator | Total carbs − fiber − ½ sugar alcohols | Common keto-community convention |
Exercise & Performance
| Calculator | Formula / Method | Source |
|---|---|---|
| One Rep Max Calculator | Epley & Brzycki formulas | Epley (1985); Brzycki (1993) |
| Heart Rate Zone Calculator | 220 − age | Fox et al. (1971) — a population estimate, not lab-measured |
| Target Heart Rate Calculator | Karvonen / heart rate reserve method | Karvonen et al. (1957) |
| VO2 Max Calculator | (distance − 504.9) / 44.73 | Cooper (1968), 12-minute run test |
| Race Time / Marathon Predictor | T2 = T1 × (D2/D1)^1.06 | Riegel (1977), American Scientist |
| Wilks Score Calculator | 5th-order polynomial coefficient by bodyweight | Wilks (1994) original coefficients |
| FTP Calculator | 20-min power × 0.95, Coggan zones | Coggan & Allen, cycling power training methodology |
| Rowing Pace Calculator | Watts = 2.80 / (split/500)³ | Concept2 published power formula |
Pregnancy & Growth
| Calculator | Formula / Method | Source |
|---|---|---|
| Pregnancy Weight Gain | Total gain range by pre-pregnancy BMI | Institute of Medicine (2009) guidelines |
| Due Date Calculator | LMP + 280 days | Naegele's Rule |
| Pregnancy Calorie Calculator | TDEE + trimester addition (0/340/450 kcal) | ACOG / Dietary Guidelines for Americans |
| Child BMI Percentile | weight(kg) / height(m)²; no weight-category assignment | BMI arithmetic only; use the linked official CDC calculator for percentiles |
| Child Height Predictor | Mid-parental height ± range | Mid-parental-height heuristic; not the Khamis–Roche method |
Sleep & Recovery
| Calculator | Formula / Method | Source |
|---|---|---|
| Sleep Cycle Calculator | 90-minute cycle back-calculation | Standard sleep-stage cycle research |
| Sleep Debt Calculator | Recommended hours − actual, summed | National Sleep Foundation nightly ranges |
| Nap Calculator | 20-min / 90-min nap windows | Sleep inertia research on nap length |
Unit Converters & Weight Management
All 8 unit converters (weight, height, distance, temperature, volume, energy, pace, speed) use standard, internationally-defined conversion constants (e.g. 1 kg = 2.20462 lbs, 1 mile = 1.60934 km) — these are physical unit definitions, not estimates. The Weight Management category (BMI Prime, Metabolic Age, Target Weight by Date, Muscle Gain Time, Weight Loss Percentage) combines the BMI, BMR, and calorie-deficit formulas above with straightforward arithmetic — see each tool's own page for its exact derivation.
A Note on "Metabolic Age" and Similar Popular Concepts
A few calculators on this site — most notably Metabolic Age — use concepts that are popular in consumer fitness (body composition scales, fitness trackers) but are not formal clinical measurements with a single universally agreed-upon reference dataset. We say this explicitly on those tools' own pages rather than presenting them with false precision. We'd rather be honest about a formula's limitations than borrow authority it hasn't earned.
Primary references and useful tools
- Mifflin et al.: resting energy expenditure equation (1990). Activity multipliers add a separate planning assumption.
- CDC: what BMI can and cannot tell you.
- CDC child and teen BMI calculator for age- and sex-specific percentiles.
- Compendium of Physical Activities for MET-based activity estimates.
- NIDDK Body Weight Planner for a dynamic adult weight-change model rather than a fixed 3,500-calorie rule.
- ACOG: nutrition during pregnancy.
These sources do not endorse CalcFit. The list explains relevant methods; it does not establish that every calculator is clinically validated. Sleep timing, frame size and muscle-gain forecasts include informal assumptions. The metabolic-age page does not assign an age from a made-up reference population.
Questions or Corrections
If you're a clinician, researcher, or coach and think we've mischaracterized a formula, cited the wrong source, or implemented the math incorrectly, please contact us — we want this page to be right, and we'll correct it quickly.