Health Metrics Explained
Understanding what health numbers mean and how to interpret them. This guide covers the most common health metrics and their implications.
Disclaimer: This guide is for educational purposes only. Always consult healthcare professionals for personalized medical advice. These metrics are screening tools, not diagnostic instruments.
Body Mass Index (BMI)
Formula
Metric: BMI = weight (kg) / height (m)²
Imperial: BMI = [weight (lb) / height (in)²] × 703
BMI Categories (Adults)
| BMI Range | Category | Health Risk |
|---|---|---|
| < 18.5 | Underweight | Increased |
| 18.5 - 24.9 | Normal weight | Lowest |
| 25.0 - 29.9 | Overweight | Increased |
| 30.0 - 34.9 | Obesity Class I | High |
| 35.0 - 39.9 | Obesity Class II | Very High |
| ≥ 40.0 | Obesity Class III | Extremely High |
Limitations of BMI
- • Does not distinguish between muscle and fat mass
- • May overestimate body fat in athletes
- • May underestimate body fat in elderly or sedentary people
- • Does not account for fat distribution
- • Cutoffs may vary by ethnicity
Body Fat Percentage
Body fat percentage measures the proportion of your total body weight that is fat tissue. It's a more accurate indicator of body composition than BMI alone.
Body Fat Categories
Men
| Essential fat | 2-5% |
| Athletes | 6-13% |
| Fitness | 14-17% |
| Average | 18-24% |
| Obese | 25%+ |
Women
| Essential fat | 10-13% |
| Athletes | 14-20% |
| Fitness | 21-24% |
| Average | 25-31% |
| Obese | 32%+ |
Measurement Methods
- Skinfold calipers: Pinches skin at multiple sites; 3-4% error margin
- Bioelectrical impedance: Sends current through body; affected by hydration
- DEXA scan: X-ray technology; most accurate but expensive
- Hydrostatic weighing: Underwater; very accurate but inconvenient
BMR & TDEE (Calorie Needs)
Basal Metabolic Rate (BMR)
BMR is the number of calories your body burns at rest to maintain basic functions like breathing, circulation, and cell production.
Mifflin-St Jeor Equation (most accurate):
Men: BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age) - 161
Total Daily Energy Expenditure (TDEE)
TDEE is your total daily calorie needs, including physical activity.
| Activity Level | Multiplier | Description |
|---|---|---|
| Sedentary | 1.2 | Little or no 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 |
Weight Management
- Maintain weight: Eat at TDEE
- Lose weight: Eat 500-750 calories below TDEE (1-1.5 lbs/week loss)
- Gain weight: Eat 300-500 calories above TDEE
Heart Rate Zones
Maximum Heart Rate
Simple formula: Max HR = 220 - age
(More accurate: 208 - 0.7 × age)
Training Zones (% of Max HR)
| Zone | % Max HR | Benefit |
|---|---|---|
| Zone 1 (Recovery) | 50-60% | Active recovery, warm-up |
| Zone 2 (Fat Burn) | 60-70% | Fat burning, endurance base |
| Zone 3 (Aerobic) | 70-80% | Cardiovascular fitness |
| Zone 4 (Threshold) | 80-90% | Lactate threshold, speed |
| Zone 5 (Maximum) | 90-100% | Maximum effort, anaerobic |
Resting Heart Rate
- Normal: 60-100 bpm
- Athletes: 40-60 bpm
- Lower is generally better (indicates cardiovascular fitness)
Blood Pressure
Blood pressure is measured in mmHg with two numbers: systolic (pressure during heartbeat) over diastolic (pressure between beats).
| Category | Systolic | Diastolic | |
|---|---|---|---|
| Normal | < 120 | and | < 80 |
| Elevated | 120-129 | and | < 80 |
| High BP Stage 1 | 130-139 | or | 80-89 |
| High BP Stage 2 | ≥ 140 | or | ≥ 90 |
| Hypertensive Crisis | > 180 | and/or | > 120 |
Waist Measurements
Waist Circumference
Waist circumference is a simple measure of abdominal fat, which is particularly associated with health risks.
Men - Increased Risk
> 40 inches (102 cm)
Women - Increased Risk
> 35 inches (88 cm)
Waist-to-Hip Ratio
Formula: WHR = Waist circumference / Hip circumference
Men
- < 0.90: Low risk
- 0.90 - 0.99: Moderate risk
- ≥ 1.00: High risk
Women
- < 0.80: Low risk
- 0.80 - 0.85: Moderate risk
- ≥ 0.86: High risk
Key Takeaways
- • No single metric tells the complete health story—use multiple measurements
- • Trends over time are more meaningful than single measurements
- • Context matters—age, ethnicity, fitness level all affect interpretation
- • These are screening tools—abnormal results warrant professional evaluation
- • Focus on what you can control: diet, exercise, sleep, stress management