Body Mass Index (BMI) is a quick screening number that puts your weight-to-height ratio into a category: underweight, normal, overweight, or obese. It's a starting point — not a verdict. Muscular lifters and athletes routinely score 'overweight' by BMI and are perfectly healthy.
How BMI is calculated
BMI = weight (kg) ÷ height² (m). Categories: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30+ obese. It's the same formula used by the WHO and most public-health agencies worldwide.
Why BMI fails for lifters and athletes
BMI doesn't know the difference between fat and muscle. A 90 kg lifter at 175 cm with 12% body fat has a BMI of 29.4 ('overweight') but is leaner and healthier than a 75 kg office worker with 28% body fat and a BMI of 24.5 ('normal'). For anyone who trains, use Body Fat % instead.
When BMI is actually useful
For sedentary adults with average muscle mass, BMI correlates well with health risk at the population level. It's cheap, fast, and requires no tools. Doctors use it as a first screen before ordering more accurate tests.
Worked example
A 68 kg person at 165 cm: BMI = 68 / (1.65 × 1.65) = 68 / 2.7225 ≈ 25.0. That's the edge of 'overweight' — a small amount of fat loss or muscle gain would move them anywhere on the map.
- 1Combine BMI with waist circumference — waist > 102 cm (men) or 88 cm (women) is a bigger red flag than BMI alone.
- 2If you lift, ignore BMI. Use body-fat % and mirror progress instead.
Frequently asked questions
Q1.Is BMI accurate for me?
If you're sedentary with average muscle, yes. If you lift, are an athlete, elderly, pregnant, or very short/tall — probably not.
Q2.My BMI says overweight but I look lean. What now?
Measure body fat %. If it's healthy (10–20% men / 18–28% women), ignore BMI.
Q3.What BMI should I aim for?
There's no single 'best' number. Health outcomes are strongest in the 20–25 range for sedentary people; trained athletes often live comfortably at 25–28.
BMI is a screening tool, not a diagnosis. It doesn't measure body composition, fitness, or health. Talk to a doctor for a full picture.