Body Fat Calculator

Why measure body fat instead of just weight

The bathroom scale can't tell muscle from fat, and BMI — which only uses height and weight — labels many muscular people "overweight" while missing soft, low-muscle individuals at a healthy weight. Body fat percentage gets closer to what actually matters for health: how much of you is fat versus everything else. This calculator uses the U.S. Navy circumference method, which estimates body fat from a few tape-measure readings, no special equipment required. Everything is computed in your browser; your measurements are never uploaded.

The formula, and why it looks the way it does

The Navy method fits a logarithmic equation to the relationship between body circumferences and fat. The two formulas differ by sex because fat distributes differently:

Men: %BF = 495 ÷ (1.0324 − 0.19077·log₁₀(waist − neck) + 0.15456·log₁₀(height)) − 450

Women: %BF = 495 ÷ (1.29579 − 0.35004·log₁₀(waist + hip − neck) + 0.22100·log₁₀(height)) − 450

The key driver is waist − neck (for men) or waist + hip − neck (for women): subtracting neck circumference removes the part of your girth that is mostly muscle and bone, isolating the abdominal region where fat accumulates. Height appears as a normalizing term. Women's hip measurement is included because they store proportionally more fat there, and leaving it out would understate the estimate.

A worked example

Take the tool's male defaults: height 175 cm, neck 38 cm, waist 85 cm. First, waist − neck = 47, so log₁₀(47) ≈ 1.672 and log₁₀(175) ≈ 2.243. The denominator is 1.0324 − 0.19077 × 1.672 + 0.15456 × 2.243 ≈ 1.0324 − 0.3190 + 0.3467 ≈ 1.0601. Then 495 ÷ 1.0601 − 450 ≈ 466.9 − 450 ≈ 16.9%. The tool places that in the "Fit" category and renders it on a color-coded gauge. The math is sensitive: an inch off on your waist can shift the result by more than a full percentage point, which is why consistent measuring technique matters so much.

Measuring correctly is half the accuracy

The estimate is only as good as the tape work. The tool reminds you to measure the waist at navel level, the neck just below the larynx (the Adam's apple), and, for women, the hips at their widest point. Keep the tape snug but not compressing the skin, breathe normally, and measure on bare skin first thing in the morning for consistency. Take each reading two or three times and use the average. You can enter measurements in centimeters or inches — the unit toggle converts your existing values automatically — but the formula runs internally in centimeters either way.

Reading your category

The tool sorts your result into ACE (American Council on Exercise) bands, with different thresholds for men and women because women carry more essential fat:

"Essential fat" is the minimum your body needs for hormones, nerves, and organ protection — dropping below it is not a goal but a health risk. The horizontal scale beneath the gauge shows where you fall across the full range.

Accuracy, limits, and good practice

• Expect roughly ±3–4%. The Navy method correlates reasonably with DEXA scans for average builds, but it relies on circumferences alone, so it can mislead for very lean, very muscular, or very obese bodies.
• Track the trend, not one number. Because measurement noise is real, the most useful signal is the direction over weeks, measured the same way each time.
• Don't chase the lowest possible figure. Driving below essential-fat levels harms health; the "Athletic" and "Fit" bands are the sustainable targets for most people.
• It's an estimate, not a diagnosis. For medical decisions, pair it with a professional assessment rather than relying on a tape measure alone.