BMI & BMR Calculator
Understand your body composition and calculate exactly how many calories you need daily to maintain, lose, or gain weight.
Daily Caloric Needs (TDEE)
🏆 BMI & BMR: Foundational Health Metrics
Everything you need to know about using this calculator and the math behind it.
How it Works
Your Body Mass Index (BMI) is a measure of body fat based on height and weight. Basal Metabolic Rate (BMR) estimates the number of calories your body needs to maintain basic life functions at rest.
The Formula
BMI = weight(kg) / height(m)². BMR uses the Mifflin-St Jeor equation: (10 × weight) + (6.25 × height) - (5 × age) + (5 for men, -161 for women).
Pro Tip
BMI doesn't distinguish between muscle and fat. Athletes may have a high BMI but low body fat. BMR is the baseline for creating calorie deficits.
Frequently Asked Questions
What do the BMI categories mean? +
The WHO defines: Underweight (<18.5), Normal (18.5–24.9), Overweight (25–29.9), Obesity Class I (30–34.9), Obesity Class II (35–39.9), Obesity Class III (40+). These categories correlate with risks for cardiovascular disease, type 2 diabetes, and other conditions — but BMI is a screening tool, not a diagnostic one. A high-BMI athlete may be healthier than a "normal" BMI sedentary person.
What's the Mifflin-St Jeor equation used for BMR? +
Mifflin-St Jeor (1990) is the most accurate BMR formula for most people: Men: (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5. Women: (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161. Compared to the older Harris-Benedict equation, Mifflin-St Jeor is about 5% more accurate on average for modern populations.
What is TDEE and how is it different from BMR? +
BMR (Basal Metabolic Rate) is the calories your body burns at complete rest — just to maintain organ function, breathing, and body temperature. TDEE (Total Daily Energy Expenditure) is BMR multiplied by an activity factor. It's the total calories you burn in a typical day. Use TDEE — not BMR — as your calorie baseline for weight management.
Is BMI accurate for children, the elderly, or athletes? +
BMI has significant limitations for special populations. For children and teens, age- and sex-specific BMI percentiles are used (not raw BMI). For the elderly, normal BMI ranges may skew higher due to reduced muscle mass. For athletes and bodybuilders, BMI often misclassifies them as overweight/obese when they have very low body fat. Body fat percentage (use our Body Fat Calculator) is more informative for these groups.
How do I use these numbers to lose weight? +
Use your TDEE as your maintenance calorie level. To lose weight, create a calorie deficit of 250–500 calories/day below TDEE — this leads to 0.25–0.5 kg (0.5–1 lb) of fat loss per week. Do not go below 1,200 calories/day (women) or 1,500 (men) without medical supervision, as very low calories slow your metabolism and cause muscle loss. Combine with strength training to preserve muscle.