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Guide to Body Fat
Why fat matters?
Body fat is essential for health - protects internal organs, stores vitamins A, D, E, K, produces hormones (leptin, estrogens), and regulates body temperature. Too little fat is as dangerous as too much. Women need more fat for reproduction.
Healthy ranges
- Men - 6-17% (essential), 18-24% (athletic), 25-31% (average), 32%+ (obesity)
- Women - 14-20% (essential), 21-24% (athletic), 25-31% (average), 32%+ (obesity)
Measurement methods
- Calipers - cheapest and popular
- Scale with BF function - convenient but less accurate
- DEXA - most accurate but expensive
- Hydrostatic weighing - accurate for athletes
Fat distribution
Visceral fat (around organs) is more dangerous than subcutaneous - increases risk of heart disease and diabetes. Pear-shaped fat distribution typical for women. Diet and exercise reduce both types.
Three people with a BMI of 24.7 can be 12% or 28% body fat
BMI knows your mass and your height and nothing else. Three men at 80 kg and 180 cm all score 24.7, comfortably mid-normal — yet measuring their waists gives 12.1%, 21.2% and 27.6% body fat. The waist is doing all the work, and BMI cannot see it.
How it works
- Estimates body fat percentage from circumference measurements using the US Navy method.
- Needs only a tape measure, which is why it survives alongside far more expensive techniques.
- Places the result against recognised ranges rather than implying a single target.
men: 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450 women: 495 ÷ (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450 all measurements in centimetres the waist term dominates — it is what BMI omits entirely
Worked example
Three men, all 80 kg and 180 cm, differing only in waist measurement.
- waist 82 cm → 12.1% body fat
- waist 94 cm → 21.2%
- waist 104 cm → 27.6%
- all three have a BMI of exactly 24.7
- the spread is 15.6 percentage points
Identical BMI, radically different body composition. This is the specific failure BMI is known for, and a tape measure round the waist resolves it for the cost of nothing.
Reading the result
- The Navy method is an estimate with real error — typically within about 3 to 4 percentage points of a laboratory measurement such as DEXA, and worse at the extremes. Treat the output as a band, not a figure, and pay attention to the direction it moves over time rather than the decimal.
- Measurement technique dominates the error. Measure the waist at the navel, relaxed and at the end of a normal exhale, with the tape snug but not compressing. A centimetre of inconsistency moves the result by roughly a percentage point, so measure the same way every time.
- Waist-to-height ratio is simpler and predicts health outcomes about as well. Keeping the waist under half your height is a usable target that needs no formula at all, and it sidesteps the estimation error entirely.
- Very low body fat is not a health goal. Essential fat is around 3% for men and 12% for women, and sustained levels near those figures carry hormonal and immune consequences. Athletic ranges are not targets for the general population.
Common questions
- Why does BMI say I am normal when I know I am not lean?
- Because BMI cannot distinguish muscle from fat or see where mass sits. All three men in the example score 24.7 while ranging from 12.1% to 27.6% body fat. Adding one waist measurement recovers the information BMI throws away.
- How accurate is a tape-measure estimate?
- Within roughly 3 to 4 percentage points of a laboratory method for most people, and less reliable at very high or very low body fat. It is far better than BMI alone and far worse than DEXA, which is the right place to put your confidence.