BMI & Body Fat Percentage Calculator
Calculate your BMI and estimate body fat percentage using your weight, height, age, and gender. Useful for tracking healthy weight status and understanding body composition beyond the scale.
About this calculator
Body Mass Index (BMI) is calculated as BMI = weight (kg) / (height (m))², or equivalently weight / (height_cm / 100)². It provides a simple population-level screening tool for underweight, normal weight, overweight, and obesity categories (below 18.5, 18.5–24.9, 25–29.9, and 30+ respectively). However, BMI does not distinguish fat from muscle, so body fat percentage estimation adds important context. A common adjustment uses the Deurenberg formula: Body Fat % = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex = 1 for males and 0 for females. This accounts for the fact that women naturally carry more fat at a given BMI, and fat percentage increases with age even when BMI stays constant. Together, BMI and estimated body fat give a more complete picture of body composition than either metric alone.
How to use
Consider a 35-year-old woman weighing 68 kg and 165 cm tall. Step 1: Convert height: 165 / 100 = 1.65 m. Step 2: BMI = 68 / (1.65)² = 68 / 2.7225 ≈ 24.98 — just within the normal range. Step 3: Estimate body fat using the Deurenberg formula: BF% = 1.20 × 24.98 + 0.23 × 35 − 10.8 × 0 − 5.4 = 29.98 + 8.05 − 0 − 5.4 = 32.6%. This places her at the upper edge of the acceptable range for women. Despite a normal BMI, her estimated body fat percentage suggests monitoring diet and physical activity.
Frequently asked questions
Why can two people have the same BMI but very different body fat percentages?
BMI is a ratio of weight to height squared, and it cannot distinguish between lean muscle mass and adipose (fat) tissue. A muscular athlete and a sedentary person of the same height and weight will have identical BMIs, yet the athlete may carry 12% body fat while the sedentary person carries 30%. This phenomenon, sometimes called 'normal weight obesity,' means a person can be metabolically unhealthy despite a normal BMI. That's why pairing BMI with a body fat estimate — using formulas like Deurenberg or circumference-based methods — provides a more clinically meaningful assessment.
How does age affect body fat percentage at a given BMI?
As people age, muscle mass naturally declines through a process called sarcopenia, and fat tissue tends to increase even when body weight and BMI remain stable. The Deurenberg formula explicitly includes age as a variable, adding approximately 0.23 percentage points of estimated body fat for each additional year of life. A 50-year-old and a 25-year-old with the same BMI of 24 could differ by about 5–6 percentage points in body fat. This makes age-adjusted body fat estimates particularly important for older adults, whose metabolic risk may be underestimated by BMI alone.
What BMI range is considered healthy and when should I be concerned?
The WHO defines a healthy BMI as 18.5–24.9 for adults. A BMI below 18.5 is classified as underweight and may indicate nutritional deficiency or underlying illness. BMI 25–29.9 is overweight, and 30 or above is obese, with obesity further divided into Class I (30–34.9), Class II (35–39.9), and Class III (≥40). Health risks including type 2 diabetes, hypertension, and cardiovascular disease increase significantly above a BMI of 25. However, these cutoffs were developed primarily from studies of European populations — the WHO recommends lower thresholds (23 for overweight, 27.5 for obesity) for Asian populations due to higher metabolic risk at lower BMIs.