Step on the scales and you'll get one number. But that number — your weight — tells you almost nothing about whether you're healthy. Two people can weigh exactly the same and have completely different body compositions: one might be lean and muscular, the other might have high body fat and low muscle. Body fat percentage is the number that actually tells the story.
What Is Body Fat Percentage?
Body fat percentage is simply the proportion of your total body weight that is fat. The rest — bones, muscles, organs, water — makes up lean mass. A person who is 30% body fat at 70kg has 21kg of fat and 49kg of lean mass.
Healthy ranges vary by sex and age. As a general guide:
- Men: essential fat 2-5%, athletes 6-13%, fitness 14-17%, acceptable 18-24%, obese 25%+
- Women: essential fat 10-13%, athletes 14-20%, fitness 21-24%, acceptable 25-31%, obese 32%+
Women naturally carry more essential body fat due to hormonal and reproductive functions. Our body fat calculator estimates your body fat percentage using body measurements.
Method 1: Skinfold Calipers
A trained technician pinches your skin at multiple sites (tricep, abdomen, thigh, etc.) with calipers and uses the measurements in a formula to estimate total body fat. Reasonably accurate (±3-5%) when done properly, affordable, and widely available. The downside: accuracy depends heavily on the technician's technique, and results can be inconsistent between different practitioners.
Method 2: Bioelectrical Impedance Analysis (BIA)
Those bathroom scales that give you a body fat reading — and the handheld devices at gyms — work using BIA. A small electrical current passes through your body and the resistance is measured. Fat conducts electricity poorly; muscle (which contains water) conducts it well. The device uses this to estimate fat percentage.
BIA is convenient and cheap but variable in accuracy (±3-8%). Readings are significantly affected by hydration levels, recent eating, and time of day. Always measure under consistent conditions for meaningful tracking. Use our BMI calculator alongside BIA results for a fuller picture of your body composition trends.
Method 3: DEXA Scan
A DEXA scan (Dual-Energy X-ray Absorptiometry) is the gold standard for body composition measurement. It produces a detailed full-body image showing fat, lean mass, and bone density, broken down by body region. Accuracy is within about 1-2%.
The downsides: it costs £50-£150 per scan and requires an appointment at a clinic or hospital. It's worth it for serious athletes or those with specific health concerns, but overkill for general fitness tracking.
Method 4: Hydrostatic Weighing
You're weighed on land, then weighed while fully submerged in water. Since fat is less dense than lean tissue, the difference reveals your body composition with excellent accuracy. It's accurate (±1-3%) but practically inconvenient — you need access to a specialist tank. Once common in research settings, it's been largely superseded by DEXA.
Method 5: Tape Measure (Navy Method)
The US Navy developed a body fat estimation formula using circumference measurements: neck, waist, and hips. It's what our body fat calculator uses. Accuracy is roughly ±3-5%, comparable to calipers. The advantage is that you need nothing except a tape measure, making it genuinely practical for regular tracking at home.
Which Method Should You Use?
For most people, the tape measure (Navy) method or a BIA scale used consistently is perfectly adequate for tracking progress. The key is consistency: always measure at the same time of day, under the same conditions, with the same method. Trends over weeks and months tell you far more than any individual measurement.
If you want a precise one-time benchmark — before a transformation programme, for instance — a DEXA scan is worth the investment.
Why Not Just Use Weight?
Because weight fluctuates with water, food in your digestive system, and muscle gain. Someone who loses 2kg of fat while gaining 1kg of muscle will only show 1kg of weight loss — but their body composition has improved dramatically. If you're only tracking weight, you'd never see this win.
Further reading: NHS resources on body composition and healthy weight provide evidence-based guidance for managing your health. Explore the NHS healthy weight hub.