BMI & Body Fat Percentage Calculator
Calculate your BMI, estimate body fat percentage, and see healthy weight ranges for your height in seconds. Use it as a starting point for tracking body composition during weight loss or fitness programmes.
About this calculator
Body Mass Index (BMI) is the most widely used screening metric for body weight classification. The formula is: BMI = weight (kg) / height (m)². In this calculator, height is entered in centimetres, so it is divided by 100 before squaring: BMI = weight / (height / 100)². The result is rounded to one decimal place. Standard WHO classifications are: Underweight < 18.5, Normal weight 18.5–24.9, Overweight 25.0–29.9, Obese ≥ 30.0. Body fat percentage is estimated from BMI using the Deurenberg formula, which factors in age and gender: Body Fat % = (1.20 × BMI) + (0.23 × age) − (10.8 × sex) − 5.4, where sex = 1 for male and 0 for female. BMI is a useful population-level tool but does not distinguish muscle from fat — a muscular athlete may register as overweight despite low body fat. Use it alongside waist circumference and other metrics for a fuller picture.
How to use
Example: a 35-year-old woman, 68 kg, 165 cm tall. Step 1 — convert height: 165 / 100 = 1.65 m. Step 2 — BMI: 68 / (1.65)² = 68 / 2.7225 = 24.98, rounded to 25.0. Classification: upper end of Normal weight. Step 3 — estimate body fat using Deurenberg: (1.20 × 25.0) + (0.23 × 35) − (10.8 × 0) − 5.4 = 30.0 + 8.05 − 0 − 5.4 = 32.65%. For women aged 35, a body fat of ~33% falls in the acceptable-to-high range; a fitness goal of 25–30% is a reasonable target.
Frequently asked questions
What are the limitations of BMI as a measure of body fat and health?
BMI does not directly measure body fat — it is a ratio of weight to height that correlates with fatness at a population level but is unreliable for individuals. Highly muscular people (athletes, bodybuilders) are frequently classified as overweight or obese despite very low body fat. Conversely, older adults with low muscle mass can have a normal BMI while carrying excess fat, a condition called 'skinny fat' or normal-weight obesity. Ethnicity also affects the relationship between BMI and health risk — South Asian and East Asian populations face elevated metabolic risk at lower BMI thresholds. Always interpret BMI alongside other measures such as waist-to-hip ratio, body fat percentage, and bloodwork.
How is body fat percentage estimated from BMI using the Deurenberg formula?
The Deurenberg equation was derived from a large population study correlating BMI with body fat measured by densitometry. The formula is: Body Fat % = (1.20 × BMI) + (0.23 × age) − (10.8 × sex) − 5.4, where sex equals 1 for males and 0 for females. Age is included because fat mass tends to increase and muscle mass tends to decrease with age even if BMI stays constant. The sex correction reflects that women naturally carry more essential body fat than men for hormonal and reproductive reasons. The estimate has a standard error of about 3–5%, making it suitable for general guidance but not clinical precision.
What is a healthy body fat percentage for men and women by age?
Healthy body fat ranges vary by sex and age. For women: 21–33% is generally considered healthy for ages 20–39, rising to 24–36% for ages 40–59. For men: 8–20% is healthy for ages 20–39, and 11–22% for ages 40–59. Athletes typically maintain lower levels — 14–20% for female athletes and 6–13% for male athletes. Essential body fat (the minimum needed for basic physiological function) is approximately 10–13% for women and 2–5% for men. Falling below essential fat levels carries serious health risks including hormonal disruption and organ damage.