Body Fat Percentage Explained — Ranges, Methods and What Matters
Body fat percentage is the number most people actually want when they step on the scales. Weight alone cannot tell you whether you lost fat or muscle, whether you are getting leaner or just lighter, or whether a 'healthy' weight is hiding too much fat. Body fat percentage answers those questions — but only if you understand what it measures, how it is measured, and how much error each method carries.
This guide explains the number from the ground up: the healthy ranges by age and sex, the six common measurement methods ranked by accuracy, the formulas behind the cheap methods, and what to track instead of a single reading. The headline is reassuring: you do not need an expensive scan to track your body composition well — you need one consistent method, measured the same way, over months.
Use the calculator
Start with the numbers: try our body fat 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 calculator uses your real weight, height, age and goal.
Related reading: body fat calculator, BMI calculator, How to Measure Body Fat, Healthy Body Fat Percentage, full guides section.
The formula behind body fat percentage
Body fat percentage is fat mass divided by total body mass, times 100 — a simple ratio that hides a difficult measurement problem, because fat mass cannot be weighed directly without a laboratory. The cheap methods estimate it from measurements: the U.S. Navy method (Hodgdon & Beckett, 1984) uses the density form with all measurements in centimetres — for men, body fat % = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450, and for women, 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450 — with a typical error of ±3–4 percentage points. The BMI-based method uses the Deurenberg formula (Deurenberg et al., 1991): body fat % = 1.20 × BMI + 0.23 × age − 10.8 × sex (1 for men, 0 for women) − 5.4. BMI itself is weight (kg) divided by height (m) squared, with WHO categories of underweight below 18.5, healthy 18.5–24.9, overweight 25–29.9 and obese at 30 or above. And the waist-to-height ratio — waist divided by height, with 0.5 as the widely used threshold (Ashwell et al.) — needs no formula at all and tracks visceral fat better than body fat percentage does. Each method answers a slightly different question, and the honest reading of any of them is a range, not a single number.
The key points
Body fat percentage is fat mass over total mass. A person who weighs 80 kg with 16 kg of fat is at 20% body fat; the same person at 75 kg with 12 kg of fat is at 16%. The ratio is what matters, which is why two people at the same weight can have very different body fat percentages — one is muscular, the other is not. This is also why body fat percentage is a better goal than weight alone: losing 5 kg of fat while gaining 1 kg of muscle improves the percentage by more than the scales suggest.
Healthy ranges depend on age and sex. Women carry more essential fat than men (about 10–13% versus 2–5%), and healthy ranges rise with age as muscle is lost. For men, a commonly used healthy range is roughly 8–19% at age 20–39, rising to 11–22% at 40–59 and 13–25% at 60–79 (Gallagher et al., 2000). For women, the equivalent ranges are roughly 21–33% at 20–39, 23–34% at 40–59 and 24–36% at 60–79. These are population ranges from the American Council on Exercise and Gallagher's data — your healthy range depends on your health, activity and medical history, not just the table.
DEXA is 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. Its error is roughly ±2–3 percentage points against multi-compartment models, and it also gives bone density. Downsides: cost, availability 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.
The Navy method is the best free method. 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 — with an error of 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 method our body-fat calculator uses, and the best free option for most people.
Smart scales are the least reliable for an absolute number. Bioelectrical impedance (BIA) scales pass a small current through the body and estimate fat from resistance. They are cheap and instant — and 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.
Track the trend, not the number. Every method has an error of at least ±2–3 points, so a single reading is almost meaningless. What matters is the trend over 8–12 weeks, measured the same way, under the same conditions. Use one method, measure monthly, and record waist circumference and progress photos alongside it — the waist tracks visceral fat better than any percentage, and photos show changes no number captures.
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 (trained) | ±3–4 points | Low | Budget tracking, same measurer |
| Navy method (tape) | ±3–4 points | Free | Free tracking, most people |
| BMI-based formula | ±4–5 points | Free | Rough estimate, large groups |
| BIA scale | ±3–8 points | Low | Trends only, identical conditions |
| Waist + photos | No % figure | Free | Tracking change, visceral fat |
A worked example: the same man, three methods
Take a 35-year-old man, 80 kg, 178 cm, waist 86 cm, neck 40 cm. His BMI is 80 ÷ 1.78² = 25.2, which the WHO classifies as overweight — but BMI cannot tell muscle from fat, so we need more. The Navy method: waist − neck = 46 cm, log10(46) = 1.663, log10(178) = 2.250; density = 1.0324 − 0.19077 × 1.663 + 0.15456 × 2.250 = 1.0324 − 0.3173 + 0.3478 = 1.0629; body fat = 495 ÷ 1.0629 − 450 = 465.7 − 450 ≈ 15.7%. The BMI-based Deurenberg formula: 1.20 × 25.2 + 0.23 × 35 − 10.8 × 1 − 5.4 = 30.2 + 8.1 − 10.8 − 5.4 = 22.1%. The two methods disagree by more than six points — which is normal, because the BMI-based formula cannot see that this man is muscular, and the Navy method's ±3–4 point error applies to both. His waist-to-height ratio is 86 ÷ 178 = 0.48, below the 0.5 threshold, which suggests his visceral fat is in the healthy range. The honest conclusion is a range, not a number: probably 13–20% body fat, with a healthy waist-to-height ratio, and a BMI that overstates his risk because it cannot see his muscle. That is how body fat percentage should be read — as a range, cross-checked against the waist, tracked over time.
How to use this guide
Pick one method and stick with it: for most people, the Navy method (tape measure) plus a waist measurement and monthly photos is the right combination. Measure under identical conditions — morning, fasted, after using the toilet, tape at the same place (waist at the navel, relaxed, not sucked in) — and record the numbers. Re-measure monthly, not daily, and judge the trend over 8–12 weeks. If you want a baseline, get a DEXA scan once a year and use the cheap method for the months in between. And use the body-fat calculator on this site to run your own numbers with the Navy formula, then compare the result with your waist-to-height ratio: if the two disagree, trust the waist-to-height ratio for health risk and the Navy trend for change over time.
What this guide does NOT tell you
No body-fat method is exact, and this guide does not pretend otherwise: the errors quoted here (±2–3 points for DEXA, ±3–4 for Navy and calipers, ±3–8 for BIA) are typical values from the published literature, and your individual error may be larger. Body fat percentage also does not diagnose anything — it is a screening and tracking number, not a medical test, and it should be used alongside waist measurements, blood pressure, blood sugar and the other markers your clinician follows. The healthy ranges on this page are population ranges (Gallagher et al., 2000; American Council on Exercise) and they do not account for your medical history, ethnicity, or body type — athletes, very muscular people and older adults all read differently. This guide does not give calorie targets or diet plans; if you want to change your body composition, the calorie and protein numbers belong in a plan discussed with a clinician or registered dietitian, respecting the safety floors of 1,200 kcal/day for women and 1,500 kcal/day for men.
Frequently asked questions
What is a healthy body fat percentage?
It depends on age and sex. For men, roughly 8–19% at age 20–39, rising to 11–22% at 40–59 and 13–25% at 60–79; for women, roughly 21–33% at 20–39, 23–34% at 40–59 and 24–36% at 60–79 (Gallagher et al., 2000). These are population ranges — your healthy range depends on your health, activity and medical history, not just the table.
What is the most accurate way to measure body fat?
DEXA is the practical gold standard, with an error of roughly ±2–3 percentage points, and it also gives bone density. Hydrostatic weighing and the Bod Pod are similarly accurate. For a free method, the Navy tape-measure method (±3–4 points) is the best option for most people, and it is the method our calculator uses.
Why do different methods give different numbers?
Because each method measures something slightly different and each has its own error. The BMI-based formula cannot see muscle, so it overestimates body fat in muscular people; the Navy method estimates from circumferences, so it also reads high in very muscular people; BIA scales are thrown off by hydration. A disagreement of 5–6 points between methods is normal — read the range, not a single number.
Are smart scales accurate for body fat?
Not for an absolute number. BIA scales have errors of 3–8 percentage points depending on hydration, food, exercise and skin temperature — a glass of water or a workout can move the reading by several points. They are usable for trends if you measure under identical conditions every time, but never trust a single reading.
Is BMI or body fat percentage more useful?
Body fat percentage is more useful for tracking change, because it separates fat from muscle; BMI is more useful as a quick population screening tool. The best combination is body fat percentage (or the Navy estimate) plus waist-to-height ratio, which tracks visceral fat better than either. BMI alone cannot tell muscle from fat.
How often should I measure body fat?
Monthly, not daily. Every method has an error of at least ±2–3 points, so daily readings are mostly noise — water, food and measurement error. Measure monthly under identical conditions, judge the trend over 8–12 weeks, and record waist circumference and photos alongside the number.
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.