Physiology

How to measure body fat percentage: methods compared

DEXA, smart scales, calipers, tape measurements and photos all estimate body fat with different errors. Here is how accurate each one is and how to track your trend.

No method measures body fat perfectly. DEXA scans are among the most precise options available outside a lab, skinfold calipers in skilled hands are reasonably good, tape-based formulas give a rough estimate, and bathroom smart scales are the least reliable for a single reading. For most people, the best approach is to pick one affordable method, measure under the same conditions every time, and focus on the trend over weeks rather than any single number.

DEXA scan

A DEXA (dual-energy X-ray absorptiometry) scan estimates fat, lean tissue and bone, and even shows how they are distributed across the arms, legs and trunk. It is often used as a reference in research. Still, results can shift by a percentage point or more between scans depending on hydration, recent meals and the machine used. Use the same clinic and scan in the morning, fasted, if you repeat it.

Bioelectrical impedance (smart scales and handheld devices)

These devices send a weak electrical current through the body and estimate body fat from how easily it passes. Water conducts well and fat poorly, so hydration has a big effect: the same person can read several percentage points differently before and after drinking, training or a salty meal. Home scales that use only the feet are the least accurate. They can still show a useful trend if you measure daily at the same time and look at weekly averages.

Skinfold calipers

Calipers measure the thickness of skin and fat folds at several sites, such as the abdomen, thigh and chest, and plug them into an equation. Done by an experienced person, they are reasonably accurate and very good at tracking change. You can even skip the equation and simply track the sum of the millimetres at each site.

Tape measurements

The US Navy method estimates body fat from height and the circumference of the neck and waist (plus hips for women). It is free and repeatable, but it can be off by several percentage points for muscular people. On its own, the waist measurement is one of the most useful numbers you can track: if your waist shrinks while your lifts hold steady, you are losing fat.

Photos and the mirror

Photos in the same light, pose and time of day every two to four weeks are surprisingly useful. They will not give you a number, but they show changes in shape that the scale misses. BMI, by contrast, uses only height and weight and cannot tell muscle from fat, so it says little about body composition for people who train.

How to track body fat well

• Choose one method and stay with it.


• Measure at the same time of day, ideally in the morning before eating or training.


• Compare averages or repeated measurements over four to eight weeks.


• Combine at least two signals, such as waist size plus photos, or calipers plus scale weight.

FAQ

What is a healthy body fat percentage?
Rough healthy ranges are about 10–20% for men and 18–30% for women, with variation by age. Athletes often sit lower, but very low levels are hard to maintain and are not necessary for health.

Why does my smart scale jump around so much?
Mostly because of changes in hydration and glycogen. Look at the weekly average instead of daily readings.

Is DEXA worth paying for?
It is useful for a detailed snapshot or comparing two points months apart. For week-to-week tracking, a tape measure and photos give you most of the information for free.

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