BMI and Navy body fat
BMI is one number from two measurements, and it cannot tell muscle from fat. The Navy tape method can, roughly, from three. This runs both, says plainly where each one misleads, and hands the result to the TDEE calculator — which needs a body-fat figure and currently makes you guess.
What BMI is good for, and what it is not
BMI is your weight divided by the square of your height. That is the whole formula. It was built in the nineteenth century to describe populations, not people, and it works well for exactly that: across a large group it tracks health outcomes usefully. For one person it cannot see the difference between muscle and fat, so a lifter reads as overweight and someone sedentary at a light weight reads as healthy. It also ignores where the fat sits, and around the middle is the placement that matters most.
The Navy method, and why inches
The circumference method estimates body fat from the ratio between the parts of you that hold fat and the parts that mostly do not. For men that is waist against neck; for women, waist and hip against neck. Height scales it. The equations were fitted in imperial units, so metric measurements here are converted to inches first rather than run through a re-derived metric variant — the same numbers, without a rounding step that does not belong.
Why the answer is a whole number
The method is typically within three or four percentage points of a DEXA scan, and it moves by a percent or more if you hold the tape differently. A figure like 18.4% claims a precision the tape does not have, so this rounds to whole percentages. Measure at the same time of day, before eating, and treat the direction of travel across weeks as the real signal.
The handoff to TDEE
Most calorie calculators default to Mifflin-St Jeor, which works from total weight. Katch-McArdle works from lean mass instead, which is the better basis if you are very lean or carrying a lot of weight — but it needs a body-fat percentage, and people guess. The button above carries this one across so the TDEE page starts with a measured figure rather than a hunch.
FAQ
Is the Navy method accurate?
Accurate enough to track change, not accurate enough to argue about. Expect it to sit within three to four percentage points of a DEXA scan for most people, and further out at the extremes of very lean or very heavy.
My BMI says overweight but I train. Which number do I trust?
The body-fat figure. That mismatch is the textbook case of BMI failing on an individual: it sees mass, not what the mass is made of.
Why does the women's formula need a hip measurement?
Because fat distribution differs, and waist against neck alone predicts it poorly. The extra term is what makes the estimate hold up.
Is anything I type here sent anywhere?
No. Every calculation runs in this page, and nothing is uploaded or stored.
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