Body Fat Calculator — US Navy, BMI-based, YMCA, Jackson-Pollock

Real-world use cases

• Cutting check before a powerlifting meet

  You weigh 84 kg and want to hit the 83 kg class without losing strength. The scale alone can't tell you what to cut. Run the Navy and Jackson-Pollock numbers weekly: if your waist drops 2 cm but body fat only moves from 16% to 15%, you're losing water and glycogen, not fat. Hold the diet steady and recheck in 7 days instead of panic-cutting calories.

• Postpartum recomposition tracking for a new mum

  Four months after birth you're 68 kg, up from a pre-pregnancy 60 kg. BMI calls you "overweight" and it stings. Enter waist, neck and hip and the Navy formula reads 29% body fat with WHR 0.79, squarely in the healthy female fitness band. Track the same three tape numbers monthly so you watch composition recover rather than chasing the scale weight back to a pre-baby figure that ignored muscle.

• Online coach screening a remote client

  A client 1,200 km away sends you a 95 kg / 1.82 m intake form. BMI flags 28.7, "overweight." You ask for waist 88 cm and neck 41 cm; the Navy estimate comes back 18%, which means he's carrying real muscle, not fat. You set a maintenance-plus-training plan instead of an aggressive deficit. Four tape measurements over a phone beat a single bathroom-scale number every time.

• Settling a smart-scale argument with yourself

  Your impedance scale said 17% on Monday and 21% on Thursday and you can't both be right. Type Thursday's measurements into the Navy and YMCA formulas: both land near 18%, with no 4-point swing. That gap on the scale was hydration, not fat gained over a long weekend. You stop trusting the day-to-day scale reading and switch to a weekly tape measurement you can actually reproduce.

Common pitfalls

• Measuring waist at the belt line instead of the narrowest point. A belt-line reading on a man with a slight belly can read 2-3 cm smaller, dropping the Navy estimate from 20% to 16% — measure about 2 cm above the navel after a normal exhale.

• Trusting the BMI-based number when you lift. Deurenberg only sees height and weight, so a 90 kg lifter and a 90 kg sedentary worker get the same body fat. If the four formulas disagree by over 5%, weight the Navy and Jackson-Pollock results, not the BMI one.

• Comparing today's result against a smart-scale reading from a different day. BIA swings 1-2% with hydration alone, so a tape number of 18% can look "wrong" next to a scale that said 21% after a salty dinner. Re-measure with the tape on the same day to compare like with like.

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