About Our Health & Fitness Calculators
The health calculators on AI Calculator estimate body composition, daily energy expenditure, and macronutrient targets using the same evidence-based equations clinicians and registered dietitians use in practice — the Mifflin-St Jeor equation for resting metabolic rate, the Harris-Benedict revised equation as a cross-check, the US Navy circumference method for body fat percentage, and the standard WHO classification thresholds for BMI. They are educational and informational tools; they are not a substitute for medical advice from your doctor or a consultation with a qualified dietitian or fitness professional. People with medical conditions, those who are pregnant or breastfeeding, athletes with very high muscle mass, and the elderly should treat the outputs as general guidance only. Each calculator shows the formula and the assumptions it makes, so you can judge how much weight to give the result.
Calculators in This Health Suite
- BMI Calculator — body mass index from height and weight, with WHO classification (underweight < 18.5; normal 18.5–24.9; overweight 25–29.9; obese class I/II/III above 30).
- BMR Calculator — basal metabolic rate (calories burned at rest) using both Mifflin-St Jeor (preferred for general population) and Harris-Benedict (legacy reference).
- Calorie Calculator — total daily energy expenditure (TDEE) by multiplying BMR with an activity factor (sedentary 1.2 × through extreme 1.9 ×).
- Body Fat Calculator — estimates body-fat percentage using the US Navy method (neck, waist, hip circumferences and height).
- Ideal Weight Calculator — healthy-weight range using BMI 18.5–24.9 plus the Robinson, Miller, Devine, and Hamwi formulas as cross-references.
The Equations Behind the Calculators
BMI is simply weight (kg) / height (m)². It correlates loosely with body-fat percentage at the population level but is a weak indicator for individuals — muscular athletes routinely score “overweight” on BMI while having very low body-fat percentages. The Mifflin-St Jeor BMR equation is BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + s, where s is +5 for males, −161 for females. This equation was validated against indirect calorimetry in over 500 subjects and is generally accepted as the most accurate non-clinical estimate of resting energy expenditure for adults. TDEE then multiplies BMR by an activity factor: 1.2 (sedentary, desk job, no exercise), 1.375 (light exercise 1–3 days/week), 1.55 (moderate, 3–5 days), 1.725 (heavy, 6–7 days), 1.9 (very heavy, athletes / physical labour).
A Worked Example: Setting a Weight-Loss Calorie Target
Consider a 35-year-old female, 165 cm, 75 kg, with a moderately active lifestyle (gym 3×/week, otherwise desk job). BMR (Mifflin-St Jeor) = 10 × 75 + 6.25 × 165 − 5 × 35 − 161 = 750 + 1031.25 − 175 − 161 = 1,445 kcal/day. TDEE at activity factor 1.55 = 1,445 × 1.55 = 2,240 kcal/day. For a sustainable 0.5 kg/week fat loss, a 500 kcal/day deficit is conventional (since 1 kg of body fat ~ 7,700 kcal), giving a target of about 1,740 kcal/day — with macros typically split as ~30% protein, 35% carb, 35% fat. The calorie calculator handles the BMR-then-TDEE-then-deficit chain in one step.
Frequently Asked Questions
Is BMI accurate for athletes or muscular people?
Often not. BMI doesn’t distinguish muscle from fat. A bodybuilder may have BMI 30 but body-fat < 10%. For a more individual measure, use the body-fat calculator (US Navy method) or get a DEXA scan.
Why are the calorie targets only an estimate?
BMR equations are accurate to about ±10% for the average person, and activity factors are even rougher. The calculator output is a starting point. Track actual weight change over 2–3 weeks and adjust intake by ~200 kcal/day if the trend doesn’t match your goal.
Should I follow the calculator over my doctor’s advice?
No. These calculators are educational. Anyone with a medical condition (diabetes, heart disease, eating disorder history, pregnancy) should follow their clinician’s personalised guidance.
Can children use these calculators?
The BMR/TDEE equations apply to adults (18+). Children require age-and-sex-specific reference ranges; refer to the WHO/CDC growth charts and a paediatrician.
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Important: These calculators provide informational estimates only and are not a substitute for professional medical advice, diagnosis, or treatment. Always consult your physician or qualified health provider with questions about a medical condition or before making changes to your diet, exercise, or medication.