BMI and Body Fat Calculator
Computes your BMI and estimates body fat percentage from weight, height, age, and sex. Useful for health screenings, fitness goal-setting, and understanding whether your weight is proportionate for your frame.
About this calculator
Body Mass Index is defined as BMI = weight (kg) / height (m)². It classifies weight status into underweight (<18.5), normal (18.5–24.9), overweight (25–29.9), and obese (≥30). However, BMI cannot distinguish fat from muscle mass, so this calculator also estimates body fat percentage using the Deurenberg formula: Body Fat % = (1.20 × BMI) + (0.23 × age) − 16.2 (men) or − 5.4 (women). The sex constant accounts for the fact that women naturally carry more essential fat than men of the same BMI and age. Activity level is then used to contextualise results — an athlete with high muscle mass may have a high BMI but a low body fat percentage, illustrating why using both metrics together is more informative than BMI alone.
How to use
Consider a 35-year-old man weighing 85 kg and standing 178 cm tall. First, BMI = 85 / (1.78)² = 85 / 3.168 = 26.8 — classified as overweight. Next, body fat % = (1.20 × 26.8) + (0.23 × 35) − 16.2 = 32.16 + 8.05 − 16.2 = 24.0%. A body fat of 24% for a 35-year-old man sits in the 'acceptable' range (21–25%), suggesting the slight BMI overweight reading is not cause for immediate concern, especially if he has an active lifestyle.
Frequently asked questions
What is a healthy body fat percentage for men and women?
Healthy body fat ranges differ substantially by sex and age. For men, 10–20% is generally considered fit to acceptable, with athletes often below 10% and obesity typically defined above 25%. For women, essential fat alone is about 10–13%, and a healthy range is 18–28%, with obesity above 32%. These thresholds shift upward slightly with age. The Deurenberg formula used here was validated against hydrostatic weighing in large population studies, making it more informative than BMI alone for assessing adiposity.
Why is BMI not enough to assess health on its own?
BMI is a simple ratio of weight to height squared and cannot differentiate between fat, muscle, and bone mass. A heavily muscled athlete may register as 'overweight' or even 'obese' on BMI while carrying very little fat, whereas a sedentary person with low muscle mass (sometimes called 'skinny fat') may have a normal BMI but dangerously high visceral fat. Combining BMI with an estimated body fat percentage and waist circumference provides a far more accurate picture of metabolic health risk. Clinicians use BMI as a population-level screening tool, not a diagnostic measure.
How does age affect body fat percentage estimates in the Deurenberg formula?
The Deurenberg formula includes age as a variable because body composition naturally shifts with age — muscle mass tends to decline (sarcopenia) while fat mass tends to increase, even if total body weight remains stable. This means two people with identical BMI but different ages will have different estimated body fat percentages; the older individual will receive a higher estimate. The coefficient of 0.23 per year reflects average population-level changes. For very muscular or very lean individuals, this formula may overestimate or underestimate true fat percentage, and DEXA scanning remains the gold standard.