What is “body fat percentage” and why estimate it?

Body fat percentage is the portion of your total body weight that comes from fat mass. If you weigh 180 lb and your body fat is 20%, that implies roughly 36 lb of fat mass and 144 lb of everything else (muscle, water, bone, organs, glycogen). People track body fat because it helps answer a question BMI can’t: “How is my weight distributed?”

This matters in real life. Two people can have the same scale weight and even the same BMI, but very different body composition. One might be athletic with more lean mass, the other might have less lean mass and more fat mass. A body fat estimate can also be useful during recomposition: your scale weight might stay flat while waist size shrinks and strength increases — which is a win, even if the scale isn’t exciting.

With that said, no tape-measure method is perfect. The goal here isn’t to produce a “medical grade” truth. The goal is to give you a fast, repeatable estimate that’s good enough to track trends. If you measure the same way each time, you can see whether you’re moving in the direction you want.

This calculator uses the US Navy Method. It estimates body fat based on relationships between your height and a few circumferences. It’s popular because it requires no special equipment beyond a tape measure, it works for both men and women, and it tends to be reasonably consistent when measured the same way.

The method uses base-10 logarithms (log10). In Imperial units (inches), the formulas are:

• Men: Body Fat % = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76
• Women: Body Fat % = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387

If you enter metric values (cm/kg), the calculator converts centimeters to inches internally and then applies the same formula. That way you can measure in whatever system is easiest.

• Estimated body fat % (primary output)
• Category label (Athletic / Fitness / Average / High) based on common fitness reference ranges
• Lean mass estimate (optional) if you enter your weight
• Shareable summary (great for accountability or progress screenshots)

Real examples (so you can sanity-check your inputs)

Suppose you enter: Height 70 in, Neck 15 in, Waist 34 in, Weight 180 lb. The calculator uses (waist − neck) = 19. It computes body fat % via the formula and returns an estimate around the “fitness/average” border for many men. Then it estimates lean mass:

• Lean Mass ≈ Weight × (1 − bodyFat%)
• Fat Mass ≈ Weight × (bodyFat%)

Suppose you enter: Height 165 cm, Neck 34 cm, Waist 74 cm, Hip 96 cm, Weight 62 kg. The calculator converts cm → inches and applies the female formula (waist + hip − neck). You’ll get a % estimate plus optional lean mass and fat mass.

Small changes in the tape can shift the outcome. A 1–2 cm difference (especially on waist or hip) can move the estimate noticeably. That’s why the best practice is consistency, not obsession. If the number jitters, zoom out and compare month-to-month trends.

How to get the most accurate estimate

If you want this calculator to be genuinely useful (Omni-style useful), the biggest lever is your measurement process. Here’s a simple protocol:

• Measure at the same time of day (many people choose morning).
• Relax your stomach — don’t “suck in”.
• Keep the tape level around your body (not angled).
• Take two measurements and average them if they differ.
• Track a trend: weekly for motivation, monthly for calmer accuracy.

The category is a friendly label to help interpret the number. It’s not a moral judgment and not a medical diagnosis. Different sports, ages, and goals change what “good” looks like. Use it as a quick signal, then decide your next action based on how you feel, your labs/health markers, and your goals.

• Men: Athletic ~6–13%, Fitness ~14–17%, Average ~18–24%, High ~25%+
• Women: Athletic ~14–20%, Fitness ~21–24%, Average ~25–31%, High ~32%+

Again: these are rough and overlap across sources. What matters most is “where am I now” and “am I moving toward my goal in a sustainable way?”

Frequently Asked Questions

• Is the Navy Method accurate?

  It’s a practical estimate. It can be “close enough” for many people, especially for tracking trends. But it’s not as precise as a lab method like DEXA. The win is simplicity + repeatability.

• Why does the calculator ask for neck?

  The Navy Method uses neck circumference as a proxy signal related to overall body composition. It helps the formula adjust for individual body frame differences.

• Do I need weight?

  No. Weight is optional. If you enter weight, we also estimate lean mass and fat mass. If you skip weight, you still get body fat %.

• What if my result seems “wrong”?

  First, re-check measuring technique and units. Tape placement (especially waist and hip) is the #1 cause of weird results. If you still suspect it’s off, treat it as a trend tool rather than an absolute truth.

• Should I use BMI instead?

  BMI is easy, but it doesn’t measure fat directly. Many athletic people get flagged as “overweight” by BMI. Body fat estimate is often more useful when you care about composition.