If you'd like to burn some fat, it's useful to know how much fat you actually have to track your progress. However, body fat percentage is tricky to measure: and no matter what commercials claim, most methods are inaccurate.
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BMI vs body fat percentage
Body Mass Index (BMI) is often calculated by doctors for obese people, but it is very inaccurate for fit people. BMI equals your weight in kilograms divided by the square of your height in meters; therefore, athletes with lots of muscle are identified as overweight by their BMI.
Body fat percentage is something completely different: it’s the mass of all the fat in your body divided by your total mass (including bones, muscle, water and food in your body, etc.). Part of this fat is essential to normal functioning of the body, so you cannot go below a certain level without causing yourself huge health trouble.
How much fat is good?
Women naturally have more fat than men, stored in their breasts, thighs, and buttocks, so the “fit” and “athletic” fat percentages are very different for the sexes. Female bodybuilders can get down to 11-12% for a competition (and men can achieve 4-5%), but only for a short while. Athletic women have up to 20% fat, while male athletes are below 13-14%. A woman looks fit at up to 24% of fat, while a man will still look good at 16-17%. In fact, getting too lean is bad for strength and performance, and women should not go below 15% due to possible hormonal and fertility problems. As for obesity, it begins at 30% for women and 25% for men.
There are many methods available, some expensive and heavily promoted, but their accuracy varies wildly.
- Tried and true – pictures and calipers. The best and cheapest method – all you need are calipers (a pincer-like tool that costs a few dollars). Measure your weight often and use calipers to pinch the skin and measure its thickness (always measure the same point, at the same time of day and on an empty stomach). Results can be inaccurate, but you will see the overall trend clearly (learn how to lose belly fat).
- Body fat analyzers – these devices are based on the method of bioelectrical impedance analysis (BIA): it uses the fact that muscle is mostly water and conducts well, while fat is a much worse conductor). An electric current is sent through the body, then the device uses an equation to calculate the fat percentage. The problem is that handheld analyzers are very inaccurate: the current actually tries to avoid fat, the readings depend on how much water you drink, and the equations themselves can be off (more info here).
- DEXA – this is a full-body X-ray test that is based on the fact that different tissues respond differently to X-rays. It is considered the “gold standard” of fat calculation (and costs around $100 per session), but studies show that it can till be very inaccurate. Doing it once is pointless, and doing it regularly just costs too much.
- Bod Pod – this is a sealed egg-shaped chamber, in which the amount of air displaced by your body is measured together with your weight and density. One session costs around $74, and Bod Pods are very popular due to clever advertising, but they can be up to 7% off.
So should you try and measure your body fat rate? Definitely. Should you spend hundreds of dollars on devices, though? Not really. Just get a caliper, a mirror, and a scale, take frequent measures, and record them. But remember: your body fat will not reduce until you work out often and eat a healthy diet!
Body Composition Methods: Comparisons and Interpretation – Ncbi.nlm.nih.gov
Measuring body composition – Ncbi.nlm.nih.gov
Understanding your Measurements – Tanita.eu