How to measure body fat you can trust, and why two methods disagree
No consumer method measures body fat directly. A BIA scale sends a small current through the body and infers fat from resistance, so it moves with hydration, food and time of day. The US Navy formula estimates from circumferences. Both carry an error of several percentage points, so the number's absolute value is far less trustworthy than its direction over weeks.
Two devices, the same body, five minutes apart, and two different answers. This is not a fault in either of them — it is what estimating something you cannot see looks like.
What is a BIA scale actually measuring?
Electrical resistance. It passes a small, harmless current through the body and measures how hard it is for that current to travel. Lean tissue holds a lot of water and conducts well; fat holds little and resists. From that resistance, plus your height, weight, age and sex, an equation estimates the fat percentage.
Notice what that means: it is measuring water and inferring fat. Anything that changes your hydration changes the reading. A dehydrated body reads fatter than it is. A body full of a recent meal reads differently again. Foot-to-foot scales pass the current mainly through the lower body and extrapolate the rest.
This is why the same person can move two percentage points in a day without a single gram of fat changing.
Is the US Navy formula more accurate?
It is not more accurate so much as differently wrong, and it is wrong in a more stable way. It uses neck and waist circumference for men, plus hip for women, along with height. It does not care about hydration at all, which makes it more repeatable day to day.
What it cannot do is tell two people apart who carry the same circumferences with different amounts of muscle underneath. Its error against a laboratory method is commonly several percentage points, and it drifts most at the extremes — very lean and very heavy people are where it disagrees most.
Repeatable and biased beats accurate and noisy when what you want is a trend, which is most of the time.
Do men and women measure the waist at the same place?
No, and this is the single most common way people get an unusable number. The Navy method asks men to measure at the navel, and women at the narrowest point of the waist — which for most women is above the navel.
Measure the wrong landmark and the formula still returns a confident number. It just answers a different question.
Two other rules matter as much: measure at the end of a normal breath out, without pulling the tape tight, and measure at the same time of day. A tape pulled harder is a smaller waist that means nothing.
So how should you use the number?
As a trend from one method, never as a fact, and never mixed between methods. Pick whichever you will actually do consistently, and then compare it only to itself. A BIA reading in June should be compared to a BIA reading in July, measured under the same conditions — not to a Navy figure, and not to a gym caliper reading from last year.
If two methods disagree by four points, neither is lying to you. They are answering slightly different questions with slightly different errors, and the useful information is in which direction each one is moving.
Reading something you can act on beats reading something you cannot.