If you have ever tried to lose weight, gain muscle, or simply understand your body's energy needs, you have probably encountered terms like BMR, TDEE, and BMI. These three metrics form the foundation of nutritional science and fitness planning. This guide explains what each metric means, how it is calculated, and how to use it effectively.
Body Mass Index (BMI) is a simple calculation that uses your height and weight to estimate whether you are underweight, normal weight, overweight, or obese. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and remains one of the most widely used health screening tools.
BMI is a useful screening tool but has significant limitations. It does not distinguish between muscle mass and fat mass — an athlete with high muscle mass may have a high BMI despite being very lean. It also does not account for age, gender, ethnicity, or body fat distribution (visceral vs. subcutaneous fat). For a more complete picture, BMI should be combined with other measurements like waist circumference or body fat percentage.
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest to maintain basic life functions: breathing, circulation, cell production, and organ operation. BMR accounts for roughly 60-75% of your total daily calorie expenditure.
The most accurate formula widely used today is the Mifflin-St Jeor Equation, developed in 1990:
Total Daily Energy Expenditure (TDEE) is your BMR multiplied by an activity factor that accounts for your daily physical activity level. TDEE represents the total calories you burn in a day.
A calorie deficit of 500 calories per day typically results in about 0.5 kg (1 lb) of weight loss per week. Calculate your TDEE, then subtract 500 to get your target intake. For example, if your TDEE is 2,500 calories, aim for 2,000 calories daily. Never eat below your BMR without medical supervision.
A calorie surplus of 250-500 calories supports muscle growth when combined with resistance training. Protein intake should be 1.6-2.2g per kg of body weight. For example, a 70kg person training to build muscle would need 112-154g of protein daily.
To maintain your current weight, eat at your TDEE level. Track your weight over 2-4 weeks and adjust slightly if you are trending up or down.
Sleep is often overlooked but critically impacts all three metrics. Poor sleep increases cortisol (stress hormone), which can lead to higher BMR disruption, increased appetite, and fat storage. Adults should aim for 7-9 hours of quality sleep per night. Use the Sleep Calculator to find optimal sleep and wake times based on 90-minute sleep cycles.
Freemaki offers all the calculators mentioned in this guide — free, no signup, all running locally in your browser:
Calorie Calculator → BMI Calculator → Sleep Calculator →
Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before making significant changes to your diet or exercise routine.