BrainyCalc Docs — Fitness & Health

API Reference

Fitness & Health

8 clinically grounded tools for body composition, energy expenditure, nutrition, and exercise physiology. All tools use peer-reviewed formulas and return reproducible results with the same inputs.

Verified science

All fitness tools implement peer-reviewed formulas (Mifflin-St Jeor, Harris-Benedict, US Navy body fat, Devine, Robinson, Karvonen). Sources are specified per tool.

fitness_bmi fitness_bmr fitness_tdee fitness_body_fat fitness_ideal_weight fitness_target_heart_rate fitness_water_intake fitness_macros

fitness_bmi

Calculates Body Mass Index and returns the WHO weight category (Underweight / Normal weight / Overweight / Obesity).

body composition

Parameter	Type	Required	Description
`weightKg`	number	required	Body weight in kilograms (must be > 0)
`heightCm`	number	required	Height in centimeters (must be > 0)

Request / Response

{ "weightKg": 75, "heightCm": 178 }

{
  "bmi": 23.67,
  "category": "Normal weight",
  "weightKg": 75,
  "heightCm": 178
}

WHO categories

< 18.5 Underweight · 18.5–24.9 Normal weight · 25–29.9 Overweight · ≥ 30 Obesity

fitness_bmr

Basal Metabolic Rate — the calories your body needs at complete rest. Supports Mifflin-St Jeor (default) and Harris-Benedict formulas.

energy expenditure

Parameter	Type	Required	Description
`weightKg`	number	required	Weight in kilograms
`heightCm`	number	required	Height in centimeters
`ageYears`	number	required	Age in years
`gender`	enum	required	`male` or `female`
`formula`	enum	optional	`mifflin` (default) or `harris`

Request / Response

{
  "weightKg": 80, "heightCm": 180,
  "ageYears": 30, "gender": "male"
}

{
  "bmrKcal": 1880,
  "formulaUsed": "mifflin",
  "gender": "male",
  "ageYears": 30,
  "weightKg": 80,
  "heightCm": 180
}

fitness_tdee

Total Daily Energy Expenditure — multiplies BMR by an activity multiplier to estimate actual daily calorie needs.

energy expenditure

Parameter	Type	Required	Description
`bmr`	number	required	BMR in kcal (use `fitness_bmr` first)
`activityLevel`	enum	required	See activity levels below

Activity level	Description	Multiplier
`sedentary`	Little or no exercise	× 1.2
`light`	Light exercise 1–3 days/week	× 1.375
`moderate`	Moderate exercise 3–5 days/week	× 1.55
`active`	Heavy exercise 6–7 days/week	× 1.725
`very_active`	Very intense exercise / physical job	× 1.9

Request / Response

{ "bmr": 1880, "activityLevel": "moderate" }

{ "bmr": 1880, "activityLevel": "moderate", "tdeeKcal": 2914 }

fitness_body_fat

Estimates body fat percentage using the US Navy circumference method. Hip measurement required for females.

body composition

Parameter	Type	Required	Description
`gender`	enum	required	`male` or `female`
`waistCm`	number	required	Waist circumference in cm (must exceed neck)
`neckCm`	number	required	Neck circumference in cm
`heightCm`	number	required	Height in cm
`hipCm`	number	optional*	Hip circumference in cm — required for females

Request / Response (male)

{
  "gender": "male", "waistCm": 85,
  "neckCm": 38, "heightCm": 178
}

{ "bodyFatPercentage": 16.2, "gender": "male" }

fitness_ideal_weight

Returns ideal body weight using three formulas simultaneously: Devine, Robinson, and Miller. Height must be ≥ 152 cm (5 ft).

body composition

Parameter	Type	Required	Description
`heightCm`	number	required	Height in centimeters (≥ 152)
`gender`	enum	required	`male` or `female`

Request / Response

{ "heightCm": 175, "gender": "female" }

{
  "devineKg": 66.9,
  "robinsonKg": 65.9,
  "millerKg": 71.3,
  "heightCm": 175,
  "gender": "female"
}

fitness_target_heart_rate

Calculates heart rate zones using the Karvonen formula. Returns fat burn, aerobic, anaerobic, and maximum zones.

exercise

Parameter	Type	Required	Description
`ageYears`	number	required	Age in years (1–120)
`restingHeartRate`	number	optional	Resting heart rate in bpm (default: 70)

Request / Response

{ "ageYears": 35, "restingHeartRate": 62 }

{
  "maxHeartRate": 185,
  "restingHeartRate": 62,
  "zones": {
    "fatBurn": { "min": 124, "max": 148 },
    "aerobic": { "min": 148, "max": 160 },
    "anaerobic": { "min": 160, "max": 173 },
    "maximum": { "min": 173, "max": 185 }
  }
}

fitness_water_intake

Recommended daily water intake based on body weight and exercise duration. Returns liters and fluid ounces.

nutrition

Parameter	Type	Required	Description
`weightKg`	number	required	Body weight in kg — baseline is ~33 ml per kg
`activityMinutes`	number	optional	Daily exercise minutes (adds 350 ml per 30 min)

Request / Response

{ "weightKg": 70, "activityMinutes": 45 }

{
  "recommendedLiters": 2.84,
  "recommendedOunces": 96,
  "weightKg": 70,
  "activityMinutes": 45
}

fitness_macros

Macronutrient breakdown (protein, carbs, fat in grams) from a calorie target and fitness goal.

nutrition

Parameter	Type	Required	Description
`calories`	number	required	Target daily calories (must be > 0)
`goal`	enum	optional	`maintenance` (default) · `weight_loss` · `muscle_gain` · `keto`

Goal	Protein	Carbs	Fat
`maintenance`	30%	40%	30%
`weight_loss`	40%	30%	30%
`muscle_gain`	30%	50%	20%
`keto`	20%	5%	75%

Request / Response

{ "calories": 2500, "goal": "muscle_gain" }

{
  "calories": 2500,
  "goal": "muscle_gain",
  "proteinGrams": 188,
  "carbsGrams": 313,
  "fatGrams": 56,
  "percentages": {
    "protein": "30%", "carbs": "50%", "fat": "20%"
  }
}