You can spend $5 on a pair of plastic calipers or $200 on a DEXA scan, and they'll give you wildly different body fat percentages. The home bathroom scale that says you're 22% will probably disagree with the gym's hand-held device that says 18%, and a clinical Bod Pod might tell you the truth is 20%. None of them is wrong, exactly — they just measure body fat through different physical principles, each with its own error sources.

Here's what each method actually measures, how accurate it really is, and which one is worth your time and money.

The methods, ranked by accuracy

You can run a free estimate using the body fat percentage calculator, which implements both the US Navy and skinfold methods.

DEXA: the practical gold standard

DEXA (Dual-Energy X-ray Absorptiometry) was originally developed for measuring bone density. It works by passing two low-dose X-ray beams through the body — one absorbed mostly by bone, the other by soft tissue. The difference lets the machine calculate fat mass, lean mass, and bone mass separately for each region of the body.

Why it's the practical reference standard: It directly measures the three major body compartments rather than estimating them, and it's the most accurate method that's actually available to consumers. A DEXA scan also tells you where your fat is — abdominal vs. peripheral — which BMI and most other methods can't.

Limitations: The radiation dose is tiny (roughly the same as a long-haul flight), but pregnant women shouldn't have one. Hydration affects results — go in moderately fasted and not after a heavy water-loading session. DEXA also tends to slightly underestimate body fat in very lean athletes and overestimate in very obese subjects, though the error is small.

Worth it if: You want one accurate baseline to compare other methods against, you're tracking long-term changes (DEXA is highly repeatable), or you suspect your BMI is misleading because of muscle mass.

Bod Pod: the gold standard's quieter sibling

The Bod Pod uses air displacement plethysmography. You sit inside a sealed chamber that measures how much air your body displaces, which gives your body density, which combined with body weight yields fat percentage.

Pros: No radiation, accurate (within 2–3% of DEXA in most populations), takes about 5 minutes.

Cons: Less widely available than DEXA. Hair, clothing, and breathing patterns affect the reading — you'll be asked to wear minimal tight-fitting clothing and a swim cap.

Hydrostatic weighing: accurate but inconvenient

Underwater weighing uses Archimedes' principle: weigh you on land, weigh you submerged, calculate body density from the difference. Highly accurate when done correctly. Largely supplanted by Bod Pod and DEXA because almost nobody wants to be fully submerged while exhaling completely. Still common in university sport science labs.

Skinfold calipers: more useful than they look

Calipers measure subcutaneous fat thickness at specific anatomical sites (commonly 3 or 7 sites). Equations like Jackson-Pollock convert the sum of those measurements into estimated body fat percentage.

Why they're underrated: A skilled tester can achieve ±3–4% accuracy, which is comparable to a Bod Pod, at a fraction of the cost. They're also excellent for tracking change over time, because the absolute reading is less important than whether the sum of skinfolds is going up or down.

The catch: Operator skill is the biggest variable. An experienced trainer's measurements will be 2–3% more accurate than a beginner's on the same body. Self-measurement is unreliable for back and triceps sites — you simply can't get consistent pinches on yourself.

Best for: Anyone working with a personal trainer who can do consistent measurements every 4–6 weeks. The trend matters more than the absolute number.

BIA (bioelectrical impedance): convenient, often wrong

BIA devices send a small electrical current through your body and measure the resistance. Fat resists current more than muscle, so the resistance reading is converted into estimated body fat. This is the technology in smart scales, hand-held gym devices, and wearable trackers.

The accuracy problem: BIA is heavily affected by hydration. A glass of water before measurement can swing your reading by 1–2%. A hard workout the day before, alcohol, salt intake, menstrual cycle phase, and even time of day all change the number. Two measurements on the same scale 30 minutes apart can disagree by 2–3%.

The good BIA devices — multi-frequency clinical BIA used in research — are reasonably accurate. The smart scale at the home goods store is closer to a guess. Hand-held BIA (the kind in some gyms) is also limited because it only measures the upper body and extrapolates everything below the waist.

Best for: Tracking trends over weeks or months, if you measure under identical conditions each time (same time of day, same hydration state, same scale, same posture). The absolute number is unreliable; the trend can still be useful.

US Navy method: free and surprisingly decent

The US Navy method uses three or four tape-measure circumferences (neck, waist, hips for women) and your height, plugged into a regression formula developed for Navy fitness assessments.

Pros: Free. Repeatable (a tape measure doesn't drift like BIA). No equipment beyond a $3 tape.

Cons: The formula was calibrated on Navy personnel, who tend to be younger and fitter than average. It loses some accuracy at very high or very low body fat. Women's measurements (which include hips) are slightly more accurate than men's.

Best for: Anyone wanting a free, repeatable home method that doesn't depend on hydration or operator skill. Pair it with the body fat calculator's US Navy mode and you get a cheap, surprisingly consistent baseline.

What about smart watches and rings?

Most consumer wearables that report "body fat percentage" use either crude BIA (Apple/Samsung's older approaches) or pure estimation from heart rate, age, and activity. The accuracy is generally worse than a tape measure. Treat the numbers as a rough trend at best, and don't make decisions based on day-to-day fluctuations.

Which one should you actually use?

The most important thing nobody mentions

Body fat percentage as an absolute number matters less than people think. Whether you're 17% or 19% has almost no health implications — both are healthy. What matters is:

1. Is your body fat percentage in a healthy range for your sex and age?
2. Is it changing in the direction you want it to?
3. Is it changing because you're losing fat, or because you're losing muscle?

Almost any method can answer those questions if you use it consistently. Obsessing over which method is "most accurate" usually misses the point: the point is to make decisions, and a slightly imprecise number used consistently beats a precise number used once.

Calculate your starting point with the body fat calculator, pick a method you'll actually use every month, and let the trend tell the story.