Measuring Body Fat: Four Methods, Their Errors and When to Use Them

• Home bioimpedance scale: cheap and convenient, ±3–8 points, heavily affected by hydration.
• Skinfold calipers: inexpensive, ±3–5 points, highly operator-dependent.
• Tape-measure formulas: just a tape, ±3–5 points, good for repeated self-tracking.
• DEXA: ±1–2 points plus regional distribution, but low radiation, booking and cost.

Bioimpedance passes a weak current through the body: fat conducts poorly, hydrated muscle conducts well. But water intake, sodium, carbs, post-exercise dehydration and even a shower change conductivity.

Use it under fixed conditions — morning, after the bathroom, fasted, same time — and read the trend. A single reading means nothing.

Calipers must pinch skin and subcutaneous fat, not muscle; take three measurements per site and average. Common sites: triceps, subscapular, abdomen, suprailiac.

Tape formulas (e.g. US Navy) use waist and neck (plus hip for women); error comes mainly from tape placement and timing of exhalation. Fixing "end of normal exhale, same landmark" keeps error tolerable.

DEXA separates fat, lean mass and bone density and reports regional distribution. It is worth it when a fat-loss plateau needs a direction check, during a muscle-building phase, or for suspected sarcopenic obesity in older adults.

But it is a snapshot, not a dashboard. Once or twice a year is enough for most people; day to day, rely on waist and weight trends.

• Stick to one method; do not alternate scale, calipers and DEXA week to week.
• Fix the conditions: morning, fasted, after the bathroom, before exercise.
• Measure once a week at the same time and read a four-week trend, not daily numbers.
• Track waist circumference too — it is steadier and reflects visceral fat better.
• Any weight plan, especially with medication or extreme dieting, should be discussed with a clinician.