Body Fat
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Within three to four percentage points of a DEXA scan for most people, as long as the tape work is careful. Tape placement is the main source of error. Measure at the same spots, under the same conditions, and the method tracks change well even when the absolute number sits a little high or low.

Men measure the waist horizontally at the navel and the neck just below the larynx. Women measure the waist at the narrowest point of the torso, the neck the same way, and the hips at the widest point. Keep the tape level and snug without digging in, and breathe out normally before reading it.

The formula behind it, published by Deurenberg and colleagues, found that people carry more fat at the same BMI as they get older. Age corrects for that drift. The estimate is still rough, because BMI alone cannot see muscle. A muscular lifter will read high on BMI and get a body fat estimate that overshoots.

The ACE chart puts the average band at 18 to 24 percent for men and 25 to 31 percent for women, with the fitness band a step below that. Essential fat, 2 to 5 percent for men and 10 to 13 for women, is the floor the body needs to function. Sitting at the essential floor year round comes with real hormonal costs, so treat it as a competition phase rather than a target.

The Navy method, if you own a tape measure. It tracks your actual shape instead of inferring fat from height and weight. The quick estimate works for a rough figure when you have nothing to measure with. Whichever you pick, stay with it. Switching methods midway makes the trend impossible to read.

Every two to four weeks. Fat loss for most people runs slow enough that weekly readings mostly show measurement noise. Measure in the morning before eating, at the same spots each time, and take each measurement twice to catch tape slips.