BMI / BMR / TDEE Calculator

What BMI tells you (and doesn't)

Body Mass Index = weight in kg ÷ (height in m)². It's a quick screening tool, NOT a diagnosis. BMI is useful for tracking populations and trends but has well-known limitations for individuals:

• Overestimates body fat in muscular people (most athletes register as "overweight" or "obese").
• Underestimates body fat in older adults who've lost muscle mass.
• Doesn't capture fat distribution — visceral (belly) fat is far more dangerous than subcutaneous fat at the same BMI.
• Misses ethnicity differences — Asian populations have higher metabolic risk at lower BMIs; Black populations often have lower body fat at the same BMI.

The standard BMI categories

• Under 18.5: Underweight. Check for unintentional weight loss with a doctor.
• 18.5 - 24.9: Normal range.
• 25.0 - 29.9: Overweight. Increased risk of metabolic disease.
• 30.0 - 34.9: Class I obesity.
• 35.0 - 39.9: Class II obesity.
• 40+: Class III obesity. Significantly elevated health risks.

BMR — calories you burn just existing

Basal Metabolic Rate is the energy your body uses at complete rest — heart, brain, organs, basic cell function. For a 30-year-old, 5'10", 170 lb male, BMR is around 1,700 calories/day. For the same demographics in female, it's around 1,400.

This calculator uses the Mifflin-St Jeor equation (1990), which is the current gold standard — more accurate than the older Harris-Benedict for most populations.

TDEE — total daily calorie needs

Multiply BMR by an activity factor:

• 1.2 (sedentary): Desk job, no exercise. Most office workers default here.
• 1.375 (light): Light exercise 1-3 days/week.
• 1.55 (moderate): Moderate exercise 3-5 days/week. Most "active" people.
• 1.725 (very active): Hard exercise 6-7 days/week, or physically demanding job.
• 1.9 (extreme): Twice-daily training, or extreme physical labor. Few people genuinely qualify.

Most people overestimate their activity by one level. A 4-day-a-week gym session at moderate intensity is "moderate" — not "very active."

For weight loss or gain

• To lose 1 lb/week: Eat 500 calories below your TDEE daily (3,500 cal deficit per week).
• To gain 1 lb/week (lean gain): Eat 250-500 calories above TDEE plus resistance train.
• Never go below 1,200 (women) or 1,500 (men) calories/day without medical supervision — body shuts down metabolism and you lose muscle along with fat.

Better metrics than BMI alone

• Waist-to-height ratio: Waist circumference should be less than half your height in inches. More predictive of disease risk than BMI.
• Body fat percentage: Use the Navy method or DEXA scan. 14-24% healthy male, 21-31% female.
• Resting heart rate: Under 70 bpm correlates with cardiovascular fitness more than weight does.
• How clothes fit: Better signal than the scale for visible body comp changes.