What body fat does the BMI method estimate for a 170-pound, 70-inch, 30-year-old man?

20.0%, from a BMI of 24.4. The Deurenberg equation is 1.20 × 24.4 + 0.23 × 30 − 10.8 − 5.4. The Navy tape method puts a typical man of the same build around 17.5% — gaps of a few points between methods are normal and inside both error bands.

Why does age change the estimate when my weight hasn't changed?

The equation adds 0.23 percentage points per year of age to reflect the population-average shift from muscle to fat over time at constant BMI. It is a statistical adjustment, not a measurement of your body — a 70-inch, 170-pound man screens at 20.0% at age 30 and 26.9% at age 60 with the same 24.4 BMI.

Should I trust this if I lift weights?

No — treat it as an upper bound at best. The equation reads all extra weight as fat, so a muscular 230-pound, 74-inch athlete screens at 25.7% from a BMI of 29.5 even when his actual body fat is far lower. Use the Navy method tape view, or a measured method like DEXA, for anything that depends on the number.

Body Fat Calculator

Body Fat Calculator — BMI-Based Body Fat Estimate

The no-tape view: the Deurenberg equation converts BMI, age, and sex into a body fat estimate — body fat % = 1.20 × BMI + 0.23 × age − 10.8 (men only) − 5.4 — published from a 1991 Dutch adult study. A 70-inch, 170-pound, 30-year-old man has a BMI of 24.4 and screens at 20.0%; the same man at age 60 screens at 26.9%, because the equation assumes body fat rises with age at constant BMI. It is the roughest method on this site — convenient when no tape is at hand, and explicitly unreliable for athletes and very muscular people.

Body fat estimate

MethodSexHeight (in)Weight (lb)Age (years)

Estimated body fat

20.0%

Average (ACE range)

Behind the estimate

BMI (703 × lb / in²)24.4

Deurenberg (1991) equation: 1.20 × BMI + 0.23 × age, adjusted for sex.

Screening estimate, not a medical measurement — the Navy method typically runs within ±3–4 points of a DEXA scan.

Both methods are screening estimates computed in your browser from the numbers you enter. They do not measure body composition directly and are not a diagnosis or medical advice — consult a healthcare professional before acting on the result.

Where the Deurenberg equation comes from

Paul Deurenberg and colleagues published the formula in the British Journal of Nutrition in 1991, after regressing densitometry-measured body fat against BMI, age, and sex in a sample of more than a thousand Dutch adults and children. The adult form used here is body fat % = 1.20 × BMI + 0.23 × age − 10.8 × (1 for men, 0 for women) − 5.4, with BMI computed as 703 × weight in pounds divided by height in inches squared.

The coefficients encode two population-level facts: at the same BMI, women average about 10 percentage points more body fat than men, and body fat tends to rise with age even when BMI stays flat — the equation adds 0.23 points per year. That is why a 70-inch, 170-pound man screens at 20.0% at age 30 but 26.9% at age 60 with an identical 24.4 BMI, and why a 65-inch, 140-pound, 35-year-old woman screens at 30.6% from a BMI of just 23.3.

Why it misreads athletes and older adults

The equation only sees weight, height, age, and sex — it cannot tell muscle from fat. A muscular 74-inch, 230-pound, 28-year-old athlete carries a BMI of 29.5 and screens at 25.7%, at the edge of the ACE obese band for men, even if a tape or DEXA measurement would place him far lower. Deurenberg's own paper reported a standard error around 4 percentage points, and the miss is systematically high for strength-trained people.

The age term cuts the other way for older adults. It models the average loss of muscle with age, so an unusually fit 60-year-old is over-estimated, while someone with pronounced muscle loss can be under-estimated. If you can take three tape measurements, the Navy method view of this calculator is the better default; use the BMI-based estimate as the convenient rough cut, and read every result as a screening estimate rather than a measurement of your body.

Questions

What body fat does the BMI method estimate for a 170-pound, 70-inch, 30-year-old man?: 20.0%, from a BMI of 24.4. The Deurenberg equation is 1.20 × 24.4 + 0.23 × 30 − 10.8 − 5.4. The Navy tape method puts a typical man of the same build around 17.5% — gaps of a few points between methods are normal and inside both error bands.
Why does age change the estimate when my weight hasn't changed?: The equation adds 0.23 percentage points per year of age to reflect the population-average shift from muscle to fat over time at constant BMI. It is a statistical adjustment, not a measurement of your body — a 70-inch, 170-pound man screens at 20.0% at age 30 and 26.9% at age 60 with the same 24.4 BMI.
Should I trust this if I lift weights?: No — treat it as an upper bound at best. The equation reads all extra weight as fat, so a muscular 230-pound, 74-inch athlete screens at 25.7% from a BMI of 29.5 even when his actual body fat is far lower. Use the Navy method tape view, or a measured method like DEXA, for anything that depends on the number.