BMI, BMR & TDEE Calculator

What is BMI and how is it calculated?

BMI (Body Mass Index) is a numeric value calculated from your weight and height. The formula is: BMI = weight (kg) / height (m)². For Imperial units: BMI = (weight in pounds × 703) / (height in inches)². The WHO classifies BMI as: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25.0–29.9), and Obese (30.0 and above). BMI was devised by Belgian mathematician Adolphe Quetelet in the 1830s as a population-level statistical tool.

Source: WHO — Obesity and Overweight Fact Sheet

What is a healthy BMI range?

The WHO defines a healthy BMI as 18.5 to 24.9 for adults. However, BMI does not distinguish between muscle and fat mass. A bodybuilder with 10% body fat may have a BMI of 30+ (classified as obese) purely due to muscle weight. Conversely, an older adult with low muscle mass may have a normal BMI while carrying excess body fat. BMI is most useful as a population-level screening tool, not an individual diagnostic. For women, some research suggests the normal range may differ slightly due to body composition differences.

Source: WHO — Obesity and Overweight Fact Sheet

What is BMR and how is it different from TDEE?

BMR (Basal Metabolic Rate) is the number of calories your body needs at complete rest to maintain basic life functions — breathing, circulation, cell production. TDEE (Total Daily Energy Expenditure) is your BMR multiplied by an activity factor that accounts for physical movement throughout the day. A sedentary person's TDEE is roughly 1.2× their BMR, while a very active person's TDEE can be 1.9× their BMR. The Mifflin-St Jeor equation is the most widely validated formula for estimating BMR.

Source: Wikipedia — Basal Metabolic Rate

What is TDEE and how do I use it to lose weight?

TDEE (Total Daily Energy Expenditure) is the total number of calories you burn per day, including all physical activity. To lose weight, consume roughly 500 calories below your TDEE per day — this creates a calorie deficit that results in approximately 0.5 kg (1 lb) of fat loss per week. To gain weight or build muscle, consume 500 calories above your TDEE. To maintain your current weight, eat at your TDEE level. This calculator shows all three targets automatically once you enter your age, sex, and activity level.

Source: Wikipedia — Total Daily Energy Expenditure

Which activity level should I choose for TDEE?

Choose Sedentary if you have a desk job and do little to no exercise. Choose Lightly Active if you exercise 1–3 days per week. Choose Moderately Active for 3–5 days per week of exercise. Choose Very Active for hard exercise 6–7 days per week. Choose Extra Active if you have a physical job or train twice per day. Most people overestimate their activity level — when in doubt, choose one level lower than you think. Your TDEE will be more accurate if you are honest about your actual movement throughout the day, not just formal workouts.

Is BMI accurate for everyone?

No. BMI has well-documented limitations. It overestimates body fat in muscular or athletic individuals and underestimates it in elderly people who have lost muscle mass. BMI also does not account for differences in body fat distribution — abdominal fat (visceral fat) is more dangerous than fat stored elsewhere, but BMI cannot distinguish between them. Some health organizations now recommend waist circumference or waist-to-hip ratio as complementary metrics alongside BMI.

Source: WHO — Obesity and Overweight Fact Sheet

Is my health data private when using this calculator?

Yes. All BMI, BMR, and TDEE calculations are performed entirely in your browser using JavaScript. Your weight, height, age, and other inputs are never sent to any server, never stored in a database, and never shared with third parties. You can verify this by opening your browser's DevTools Network tab — no outbound requests are made. Closing the tab erases all data.