Body Metrics Calculators

A weight scale is a common way to start measuring body metrics. In health and fitness, several key measures – like BMI, BMR, body fat percentage, and more – help assess your body’s composition and energy needs. Below we explain seven common metrics, giving a brief description, the calculation formula (with units), an example, and what the result means.

Body Mass Index (BMI)

BMI is a simple measure of body fat based on height and weight. It does not distinguish muscle from fat but is useful for general weight categories.

• Formula: BMI = weight (kg) ÷ [height (m)]²
• Example: For someone weighing 68 kg and 1.70 m tall: BMI = 68 ÷ (1.70²) ≈ 23.5 kg/m².
• Interpretation: A BMI of 23.5 falls in the normal range (18.5–24.9). Below 18.5 is underweight, 25–29.9 is overweight, and 30+ is obese.

Basal Metabolic Rate (BMR)

BMR is the number of calories your body needs per day at rest (for breathing, circulation, etc.). It depends on weight, height, age, and sex. A common formula (Mifflin–St Jeor) is:

• Formula (Mifflin–St Jeor):

  • Men: BMR = 10×weight (kg) + 6.25×height (cm) – 5×age (yrs) + 5
  • Women: BMR = 10×weight (kg) + 6.25×height (cm) – 5×age (yrs) – 161
• Example: A 30-year-old man who is 175 cm tall and weighs 70 kg: BMR = 10×70 + 6.25×175 – 5×30 + 5 = 700 + 1093.75 – 150 + 5 ≈ 1,649 kcal/day.
• Interpretation: This man needs about 1,650 calories per day just to maintain basic bodily functions. His actual daily calorie needs would be higher once activity is included.

Body Fat Percentage (Navy Method)

Body Fat % estimates how much of your body weight is fat. The “Navy method” uses simple measurements. (All lengths in cm; logarithms are base 10.)

• Formula (men): Body fat % = 495 ÷ [1.0324 – 0.19077×log₁₀(waist – neck) + 0.15456×log₁₀(height)] – 450.
• Formula (women): Body fat % = 495 ÷ [1.29579 – 0.35004×log₁₀(waist + hip – neck) + 0.22100×log₁₀(height)] – 450.
• Example: For a man with waist = 80 cm, neck = 40 cm, height = 175 cm: Body fat ≈ 11–12%. (For these values the formula gives about 11%.)
• Interpretation: Around 11–12% body fat is lean (common for fit men). Healthy ranges vary: roughly 6–24% for men and 16–30% for women. Higher percentages indicate more body fat.

Lean Body Mass (LBM)

LBM is the weight of everything in your body except fat (muscles, bones, organs, etc.). It shows how much of your weight is “lean” tissue.

• Formula: Lean Body Mass (kg) = weight (kg) × [1 – body fat fraction]
• Example: If someone weighs 68 kg with 20% body fat, LBM = 68 × (1 – 0.20) = 54.4 kg.
• Interpretation: 54.4 kg is the weight of non-fat parts of the body. Tracking LBM can help monitor muscle mass or weight changes not due to fat.

Waist-to-Hip Ratio (WHR)

WHR compares the circumference of your waist to that of your hips. It indicates fat distribution and related health risk.

• Formula: WHR = waist circumference ÷ hip circumference (use same units, e.g., cm).
• Example: A woman with waist = 70 cm and hips = 95 cm has WHR = 70 ÷ 95 = 0.74.
• Interpretation: A WHR of 0.74 is quite low (healthy) for a woman. Generally, a WHR above about 0.90 for men or 0.85 for women is linked with higher health risk. Lower ratios (around 0.8 or less) are usually better.

Ideal Body Weight (Devine Formula)

The Devine formula estimates a “healthy” weight based on height and sex (originally for dosing medications). It assumes 5 feet (60 inches) as a baseline.

• Formula (Men): IBW (kg) = 50 + 2.3 × (height (in) – 60)
• Formula (Women): IBW (kg) = 45.5 + 2.3 × (height (in) – 60)

• Example:

  • A man who is 5 ft 10 in (70 in): IBW = 50 + 2.3×(70–60) = 50 + 23 = 73 kg.
  • A woman who is 5 ft 4 in (64 in): IBW = 45.5 + 2.3×(64–60) = 45.5 + 9.2 = 54.7 kg.
• Interpretation: These values (73 kg for the man, 54.7 kg for the woman) are rough targets for ideal weight. They are guidelines – actual healthy weight can vary with muscle mass, bone density, and body frame.

Body Surface Area (BSA, Du Bois Formula)

BSA estimates the total surface area of the body, which is used for dosing some medical treatments.

• Formula: BSA (m²) = 0.007184 × [weight (kg)^0.425] × [height (cm)^0.725]
• Example: For weight = 68 kg and height = 170 cm: BSA ≈ 0.007184 × (68^0.425) × (170^0.725) ≈ 1.79 m².
• Interpretation: A BSA of about 1.7–1.9 m² is typical for an average adult. A larger or smaller person has a higher or lower BSA, respectively. BSA is mainly used in clinical settings (e.g., drug dosing) but illustrates overall body size.

Using these calculations can help you gauge various aspects of your health and fitness. Remember, they are general guidelines. Individual factors (like muscle vs. fat or bone structure) can affect the results. Always use them as a starting point and consult a health professional for personal advice.