Same BMI, Different Body Fat
BMI is your weight divided by the square of your height — one number, with no room for age, sex, or muscle. Run a single BMI of 25 through the standard body-fat formula and it fans out into a 22.3-point range: about 18.4% body fat for a young man, near 40.7% for an older woman. Same BMI. Same “overweight” label. Very different bodies.
One BMI, a Whole Ribbon of Body Fat
The shaded band is the range of estimated body fat at each BMI, from the leanest read (a 20-year-old man, lower edge) to the highest (a 70-year-old woman, upper edge). It stays a constant 22.3 points wide because age and sex shift body fat by fixed amounts the BMI never sees. The dashed line marks BMI 25 — the “overweight” threshold — where the four people below sit.
Highest read (woman, 70) Leanest read (man, 20) BMI 25
Four People, One BMI of 25
Every row below is a BMI of exactly 25 — the line where “normal” becomes “overweight.” The body-fat estimate and its category come straight from the formula. Two of them read as “Average,” two as “Obese.”
| Person (all BMI 25) | Est. body fat | Category |
|---|---|---|
| Man, age 20 | 18.4% | Average |
| Woman, age 20 | 29.2% | Average |
| Man, age 70 | 29.9% | Obese |
| Woman, age 70 | 40.7% | Obese |
Want your own numbers? The Body Fat Calculator runs this same formula (and the more accurate Navy tape method), and the BMI Calculator gives the BMI it starts from.
Why the Gap Never Closes
The estimate comes from the Deurenberg equation, the standard way to turn a BMI into a body-fat percentage:
Two of those terms have nothing to do with BMI. Being female instead of male adds a flat 10.8 points. Each year of age adds 0.23 points, so 50 years adds 11.5. Stack the two extremes — a 20-year-old man against a 70-year-old woman — and you get a fixed 22.3-point gap that rides along at every BMI. BMI can tell you where you sit on the weight-for-height scale; it structurally cannot tell you which end of that 22.3-point band you land in.
This is why a single BMI reading is a starting point, not a verdict. It is a good population-level screen and a poor individual one — which is exactly what the 22.3-point band shows.
Common Questions
Can two people with the same BMI have very different body fat?
Yes. Using the standard Deurenberg equation, a BMI of 25 works out to about 18.4% body fat for a 20-year-old man but roughly 40.7% for a 70-year-old woman — a 22.3-point gap at one identical BMI. BMI is weight over height squared; it has no term for age, sex, or how much of that weight is muscle versus fat.
Why does BMI overestimate body fat for some people and underestimate it for others?
Because BMI ignores body composition. Two of the same-BMI examples above land in the "Average" body-fat category and two land in "Obese" — the same BMI (25) spans 2 categories. A muscular person can read as a high BMI while carrying little fat, and an older or less-muscular person can read as a "normal" BMI while carrying more fat than the number suggests.
How much higher is body-fat percentage for women than men at the same BMI?
By the Deurenberg formula, a woman's estimated body fat is exactly 10.8 percentage points higher than a man's at the same BMI and age — that constant is built into the equation. Age adds 0.23 points per year, so going from 20 to 70 at a fixed BMI adds about 11.5 points on its own.
Is body-fat percentage a better measure than BMI?
For body composition, yes — body fat estimates what fraction of your weight is fat versus lean tissue, which BMI cannot do. The most reliable of the everyday estimates is the U.S. Navy tape method, which uses actual neck and waist girths. The BMI-based Deurenberg estimate shown here is a rough fallback; it is useful for showing how much a single BMI can hide, not for a precise personal reading.
- Formula: body fat % = 1.20 × BMI + 0.23 × age − 10.8 × (male ? 1 : 0) − 5.4 — the Deurenberg equation the Body Fat Calculator ships (BMI method).
- At BMI 25: plug in age 20/70 and each sex. Expected: 18.4%, 29.2%, 29.9%, 40.7% — a 22.3-point spread from 18.4% to 40.7%.
- Constants: woman − man = 10.8 points; age 20 → 70 = +11.5 points. Both independent of BMI, so the spread is 22.3 at every BMI.
- Categories: ACE body-fat ranges, sex-specific — the same thresholds the calculator uses. At BMI 25 the four people fall into 2 of them (Average and Obese).
Every number here is arithmetic on that one formula. The Deurenberg estimate is a rough BMI-based approximation, not a clinical body-fat measurement; the point is how much a single BMI can hide, not a precise personal reading.