BMI vs Body Fat Percentage: Which Number Should You Track?
BMI is quick and useful for populations, but it cannot tell muscle from fat. Here is what each measure tells you and how to combine them with your waist size.
Body mass index and body fat percentage are the two most common ways to judge whether your weight is healthy. They measure different things, and each has blind spots. Knowing how they differ helps you pick the right number for your goals.
What BMI tells you
BMI is your weight in kilograms divided by your height in meters squared. It was devised in the 1830s by Belgian statistician Adolphe Quetelet to describe populations, and was adopted for health screening in the 1970s.
Strengths: it needs only a scale and a tape measure, is free and repeatable, and for most people it correlates reasonably with body fat and health risk. Large studies link BMI above 30 with higher risks of type 2 diabetes, heart disease and some cancers.
Weaknesses: BMI cannot distinguish muscle from fat or show where fat is stored. A muscular rugby player and a sedentary office worker of the same height and weight have identical BMIs. It can also miss "normal-weight obesity": people with a healthy BMI but high body fat and low muscle, who still carry elevated metabolic risk.
What body fat percentage tells you
Body fat percentage is the proportion of your weight that is fat. It directly answers the question BMI only approximates. The American Council on Exercise considers about 14–24% healthy for men and 21–31% for women, with athletes lower.
Ways to measure it, from most to least accurate:
- DEXA scan: a low-dose X-ray that also shows fat distribution. Accurate to within 1–2%.
- Hydrostatic weighing and Bod Pod: accurate, but need specialist equipment.
- Skinfold calipers: good in trained hands, inconsistent when self-measured.
- US Navy tape method: uses neck, waist and hip measurements. Typically within 3–4%, and free.
- Smart scales (bioelectrical impedance): convenient but strongly affected by hydration, so readings can swing day to day.
Waist size: the third number
Fat stored around the abdomen, especially visceral fat around the organs, is most strongly linked to health problems. A simple check: keep your waist circumference under half your height. For someone 175 cm tall, that means a waist below 87.5 cm. UK health guidance adopted this waist-to-height ratio in 2022 because it predicts risk better than BMI alone.
Which should you track?
- If you are mainly losing weight and are not very muscular, BMI and waist size are enough. Both will fall together.
- If you lift weights or play sport, track body fat and waist size. Your BMI may barely change as you lose fat and gain muscle.
- If you are older, body fat and strength matter more, since muscle loss with age can hide behind a stable BMI.
Tracking well
Whatever you measure, consistency matters more than precision. Measure at the same time of day, under the same conditions, and focus on the trend over four to eight weeks rather than any single reading. Use the BMI calculator and the body fat calculator together, and note your waist measurement each time.
Finally, these numbers are screening tools, not diagnoses. Blood pressure, blood sugar, cholesterol and fitness are equally important. A doctor can put all of them in context.