How to Measure Body Fat — Every Method, Ranked by Accuracy
Body fat percentage is a useful number — and one of the most mis-measured numbers in fitness. The method you use matters more than the number it gives: a DEXA scan, a smart scale, a tape measure and a pair of calipers can disagree by ten percentage points on the same person on the same day, and all of them can be right by their own method's standards.
This guide ranks every common method by accuracy, explains what each one actually measures, and shows how to use the cheap methods well — because for tracking change over time, a consistent cheap method beats an inconsistent expensive one.
Use the calculator
Start with the numbers: try our body fat calculator and the BMI calculator to work out your own figures before applying anything on this page. The worked examples below use round numbers so you can follow the maths; the calculators use your real weight, height, age and goal.
Related reading: body fat calculator, BMI calculator, Fitness After 60 — Strength, Balance and Staying Independent, Overtraining Signs — 10 Warning Signals to Watch For. You can also browse the full guides section for the rest of the series.
The key points
DEXA: the practical gold standard. Dual-energy X-ray absorptiometry (DEXA) passes two low-dose X-ray beams through the body and estimates fat, lean tissue and bone separately, including where the fat is (arms, legs, trunk). It is the most accurate widely available method, with an error of roughly 2–3 percentage points against multi-compartment models, and it also gives bone density — useful for older adults. Downsides: cost, availability (clinics and universities, not gyms), and the need to lie still for ten minutes. It is the best choice for a baseline and a yearly check, not for weekly tracking.
Hydrostatic weighing and Bod Pod. Hydrostatic (underwater) weighing estimates body density from weight on land versus weight underwater, then converts density to body fat with the Siri equation. The Bod Pod does the same thing with air displacement instead of water. Both are accurate (error around 2–3 points) and both are awkward: underwater weighing means exhaling fully underwater, and both need specialist facilities. They are excellent for research and for a one-off accurate reading, and impractical for routine use.
Skinfold calipers: cheap and good in trained hands. Calipers pinch the skin at three to seven sites and convert the thicknesses to body fat with an equation (Jackson-Pollock is the commonest). In trained hands the error is around 3–4 percentage points; in untrained hands it is much worse, because the pinch technique and site selection matter enormously. Calipers are cheap and portable, which makes them the best budget option — provided the same trained person measures the same sites every time, and you treat the trend as the truth rather than the absolute number.
Bioelectrical impedance (BIA): convenient and easily fooled. BIA scales and handheld devices pass a small current through the body and estimate fat from resistance. They are cheap and instant, which is why every smart scale has one — and they are the least reliable method for an absolute number, with errors of 3–8 percentage points depending on hydration, recent food, exercise and skin temperature. A glass of water or a workout can move the reading by several points. BIA is usable for trends if you measure under identical conditions (morning, fasted, after using the toilet), but never trust a single reading.
The Navy method: tape measure and maths. The U.S. Navy method (Hodgdon & Beckett, 1984) estimates body fat from tape measurements — waist, neck and height for men; waist, hip, neck and height for women — using a density formula. Its error is around ±3–4 percentage points, similar to calipers, and it costs nothing beyond a tape measure. Its weaknesses are measurement error (waist measured in the wrong place or after a meal) and the fact that it estimates from circumferences, so very muscular people read higher than they are. It is the best free method for most people, and it is the method our body-fat calculator uses.
Photos, waist and the mirror: underrated. For tracking change, the simplest tools are underrated: a waist measurement, a waist-to-height ratio, progress photos in the same light and pose, and how clothes fit. Waist circumference tracks visceral fat better than body-fat percentage does, and a photo comparison over eight weeks shows changes no single number captures. The best system is a combination: one quantitative method (Navy or calipers) measured monthly, plus waist and photos, all recorded the same way each time.
At a glance
| Method | Accuracy (error) | Cost | Best for |
|---|---|---|---|
| DEXA scan | ±2–3 points | High | Baseline, yearly check, bone density |
| Hydrostatic / Bod Pod | ±2–3 points | High | One-off accurate reading |
| Skinfold calipers | ±3–4 points (trained) | Low | Budget tracking, same measurer |
| Navy method (tape) | ±3–4 points | Free | Free tracking, most people |
| BIA scale | ±3–8 points | Low | Trends only, identical conditions |
| Waist + photos | No % figure | Free | Tracking change, visceral fat |
Your plan, step by step
- Pick one method and stick with it — consistency beats accuracy for tracking change.
- Measure under identical conditions: morning, fasted, same tape position, same measurer.
- Record waist circumference and take progress photos alongside any body-fat number.
- Re-measure monthly, not daily; judge the trend over 8–12 weeks.
- Use the body-fat calculator (Navy method) for a free estimate, and a DEXA scan for a baseline.
The formula behind it
The Navy method (Hodgdon & Beckett, 1984) uses the density form of the equation, with all measurements in centimetres:
Men: body fat % = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
Women: body fat % = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
Worked example — a man with a waist of 84 cm, neck of 39 cm and height of 178 cm: waist − neck = 45 cm; log10(45) = 1.653; log10(178) = 2.250. Density = 1.0324 − 0.19077 × 1.653 + 0.15456 × 2.250 = 1.0324 − 0.3154 + 0.3478 = 1.0648. Body fat = 495 ÷ 1.0648 − 450 = 464.9 − 450 ≈ 14.9%. The typical error is ±3–4 percentage points, so the honest reading is 'about 12–19%', not '14.9%'. Use the body fat calculator to run your own numbers.
How to use this guide
The pattern that works is the same for every topic on this page: understand the principle, apply it for two to four weeks, and judge it by what changes — not by how it feels on day one. Most fitness and nutrition advice fails not because it is wrong but because it is applied for three days and then abandoned. Pick the one or two changes from this guide that fit your life, do them consistently for a month, and only then decide whether they are working. If you try to change everything at once, you will not know what helped, and you will not sustain any of it.
Related pages on this site go deeper on the numbers behind this guide: the calculators apply the same formulas shown here to your own weight, height, age and goal, and the protein, macro and body-composition articles work through the maths for specific cases. Use this guide for the understanding and the calculators for your own figures — neither replaces the other.
What this guide does NOT tell you
No guide can replace individual advice. The numbers and ranges on this page are population estimates from published research, and they describe what works on average — your body, your health conditions, your medicines and your training history all shift the answer. This guide does not diagnose anything, prescribe anything, or predict your results. If you have a medical condition (especially heart disease, diabetes, kidney or liver disease, an eating disorder, or a hormone condition), are pregnant, or take prescription medicine, the right next step is a clinician or a registered dietitian, not a more detailed article.
The other limit is honesty about evidence. Some claims in fitness are supported by randomised trials; others are supported by tradition, by a single small study, or by nothing at all. This guide says which is which where it matters, and where the evidence is thin it says so rather than dressing a guess up as a fact. A guide that admits uncertainty is more useful than one that pretends to know everything — and a reader who knows the limits of the advice is safer than one who follows it blindly.
Frequently asked questions
What is the most accurate way to measure body fat?
A multi-compartment laboratory model is the most accurate, but of the methods available to the public, a DEXA scan is the practical gold standard (error around ±2–3 percentage points), followed by hydrostatic weighing and the Bod Pod. Calipers and the Navy method are close behind at ±3–4 points when done well. Smart scales (BIA) are the least accurate for an absolute number, at ±3–8 points depending on hydration.
Why does my smart scale give a different number every day?
Because bioelectrical impedance measures water, not fat, and your hydration changes daily — a glass of water, a salty meal, a workout, or a hot day can move the reading by several percentage points. The scale is measuring something real (your body's electrical resistance), but it is not measuring body fat directly. Measure at the same time, in the same conditions, and look at the monthly trend rather than any single day.
How often should I measure body fat?
Monthly is enough. Body fat changes slowly — a good rate of fat loss is 0.5–1% of body weight a week — and weekly measurements are mostly noise from water and measurement error. Measure monthly under identical conditions, and judge the trend over eight to twelve weeks. Weighing yourself weekly (for weight) and measuring body fat monthly (for composition) is the right split.
Can the Navy method be wrong for muscular people?
Yes. The Navy method estimates body fat from circumferences, and a muscular person with a large neck and a thick waist from muscle (not fat) will read higher than they really are — the formula cannot tell muscle from fat around the waist. The same limitation applies to BMI. For very muscular people, calipers or a DEXA scan are better, because they measure tissue rather than circumferences.
Is waist-to-height ratio better than body fat percentage?
For health risk, it is arguably more useful: waist-to-height ratio tracks visceral fat — the fat around the organs that is most linked to metabolic and cardiovascular risk — and the target is simple (keep your waist under half your height, a ratio below 0.5). Body-fat percentage tells you composition; waist-to-height tells you risk. Track both: composition for the goal, waist for the health signal.
Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.