BMI, BMR, and TDEE are three of the most commonly used metrics in health and fitness. Understanding what they measure, how they are calculated, and their limitations helps you make informed decisions about nutrition and exercise.
BMI (Body Mass Index)
Formula: BMI = weight (kg) / height (m)²
BMI is a screening tool that uses height and weight to categorize body weight status:
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 - 24.9 | Normal weight |
| 25.0 - 29.9 | Overweight |
| 30.0 - 34.9 | Obese (Class I) |
| 35.0 - 39.9 | Obese (Class II) |
| 40.0+ | Obese (Class III) |
BMI Limitations
- Does not distinguish fat from muscle: Athletes with high muscle mass may have "overweight" BMI despite low body fat.
- Age not considered: Older adults naturally have more body fat at the same BMI as younger adults.
- Ethnicity differences: Health risks associated with BMI vary across populations. Asian populations may face higher risks at lower BMI values.
- No fat distribution information: Abdominal fat (visceral) is more dangerous than subcutaneous fat, but BMI cannot distinguish between them. Waist circumference is a better indicator of visceral fat.
BMR (Basal Metabolic Rate)
BMR is the number of calories your body burns at complete rest — the energy needed to maintain basic functions like breathing, circulation, and cell production.
BMR Formula Comparison
| Formula | Year | Best For | Accuracy |
|---|---|---|---|
| Mifflin-St Jeor | 1990 | General population | Most accurate for most people |
| Harris-Benedict (revised) | 1984 | General population | Slightly less accurate, widely used |
| Katch-McArdle | 1996 | Lean/athletic individuals | Most accurate when body fat % is known |
Mifflin-St Jeor (recommended):
- Men: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age) + 5
- Women: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age) - 161
TDEE (Total Daily Energy Expenditure)
TDEE = BMR x Activity Multiplier. It represents the total calories you burn in a day including all physical activity.
| Activity Level | Multiplier | Description |
|---|---|---|
| Sedentary | 1.2 | Desk job, little exercise |
| Lightly active | 1.375 | Light exercise 1-3 days/week |
| Moderately active | 1.55 | Moderate exercise 3-5 days/week |
| Very active | 1.725 | Hard exercise 6-7 days/week |
| Extra active | 1.9 | Very hard exercise + physical job |
Practical Applications
- Weight loss: Consume 300-500 calories below TDEE for sustainable fat loss (~0.5-1 lb per week)
- Weight gain: Consume 300-500 calories above TDEE for muscle building
- Maintenance: Consume approximately TDEE calories to maintain current weight
Calculate your BMI with the WizlyTools BMI Calculator.