FITTA tool

How to estimate body-fat percentage from circumferences

Estimate body fat from neck, waist, and hip with the U.S. Navy formula

These calculators provide estimates based on population-level formulas and do not replace assessment by a dietitian, physician or exercise professional.

Turn a one-off body-fat reading into a weekly trend in FITTA

FITTA logs girths next to weight, meals, and training so you can see whether fat is falling while lifts hold.

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Practical guide

How to use FITTA better.

Body-fat percentage is the share of mass that is not muscle, bone, organs, or water. This calculator runs the U.S. Navy circumference method published by Hodgdon and Beckett in 1984: height, neck, and waist for men; women also enter hip. The output is a field estimate, typically within 3 to 4 percentage points of hydrostatic weighing. Use it to track training and diet trends, not to diagnose. FITTA stores the same girths next to weight, meals, and sessions.

What the U.S. Navy method is and how the formula works

The Navy circumference method estimates body-fat percentage from predicted body density using girths and height. Hodgdon and Beckett (1984), at the Naval Health Research Center, fit the equations against hydrostatic weighing: about r = 0.90 in men and r = 0.85 in women, with a standard error near 3.5 points. Density becomes percent fat with the Siri equation: percent fat equals 495 divided by density, minus 450.

This page uses centimeters and log base 10, never the natural log. Male density is 1.0324 minus 0.19077 times log10 of waist minus neck, plus 0.15456 times log10 of height. The male waist site in the original protocol is the abdomen at the navel. Female density is 1.29579 minus 0.35004 times log10 of waist plus hip minus neck, plus 0.22100 times log10 of height. The published inch forms are equivalent: men, 86.010 × log10(abdomen − neck) − 70.041 × log10(height) + 36.76; women, 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387. Do not mix unit systems.

Male example, 178 cm, waist 85 cm, neck 38 cm: log10(47) = 1.6721 and log10(178) = 2.2504. Density = 1.0324 − 0.19077 × 1.6721 + 0.15456 × 2.2504 = 1.0612. Percent fat = 495 / 1.0612 − 450 = 16.4 percent. Female example, 165 cm, waist 72 cm, hip 98 cm, neck 32 cm: log10(138) = 2.1399 and log10(165) = 2.2175. Density = 1.29579 − 0.35004 × 2.1399 + 0.22100 × 2.2175 = 1.0368. Percent fat = 495 / 1.0368 − 450 = 27.4 percent. Those are the coefficients this calculator applies.

How to measure neck, waist, hip, and height

The equation only holds if the landmarks match the protocol. Measure in the morning, fasted, after voiding, standing, with a non-stretch tape. Height is barefoot, heels and back to a wall. Neck sits just below the larynx, tape nearly perpendicular to the neck axis, not compressed; a slight downward slope at the front is acceptable. Take two or three readings and average to the nearest millimeter.

For men, waist is the abdomen at the navel, tape parallel to the floor, arms at the sides, belly relaxed, at the end of a normal expiration, no sucking in. The male equation does not use hip. For women, waist is the narrowest point on the torso, usually between the navel and the xiphoid. Hip is the largest protrusion of the buttocks, tape level. One extra centimeter at the waist typically raises the result by 0.5 to 1 point. A neck read 2 cm larger, from a looser tape or a lower site, drops percent fat by a similar amount. In inches, the same sites apply: convert after measuring, or convert inputs first and stay in one system.

How to read the body-fat result

The number describes composition. It is not a diagnosis and it is not an aesthetic score. Adult fitness bands used by ACE and similar charts split essential, athletic, fitness, average, and high. Men: 2 to 5 essential, 6 to 13 athletic, 14 to 17 fitness, 18 to 24 average, 25 and above high. Women: 10 to 13 essential, 14 to 20 athletic, 21 to 24 fitness, 25 to 31 average, 32 and above high. The Navy uses the method as a service ceiling, roughly 22 to 26 percent for men and 33 to 36 percent for women by age, not as a gym target.

On the same man at 175 cm with a 38 to 39 cm neck, an 80 cm waist lands near 12.9 percent and a 92 cm waist near 21.4 percent: athletic versus average, possibly at the same scale weight if lean mass differs. On a woman at 165 cm, 68 / 94 / 33 cm is about 22.8 percent (fitness); 72 / 98 / 32 cm about 27.4 (average); 80 / 104 / 32 cm about 34.3 (high). Treat cutoffs as zones. A 1-point swing between two mornings is inside tape and equation noise.

Men 14–17% and women 21–24%: typical fitness band
Men 18–24% and women 25–31%: population-average band
Men ≥25% and women ≥32%: high fat on fitness charts
Navy service ceilings are occupational limits, not physique goals
Compare the same protocol every 3–4 weeks, not a single reading

Common mistakes when you measure or interpret

The most common male error is measuring the narrowest waist instead of the navel. That shrinks (waist − neck) and underestimates fat. The reverse, measuring above the navel after a meal, inflates it. For women, swapping hip for the iliac-crest girth, or taking waist at the navel instead of the narrowest point, moves the result 2 to 4 points. Using the natural log, or feeding centimeter girths into the inch equation, produces a clinically meaningless number.

A thick lifting neck pulls the estimate down: at 178 cm and an 85 cm waist, a 38 cm neck yields 16.4 percent and a 42 cm neck about 13.2 percent. The fit was on Navy personnel, not contest bodybuilders. Flexing the traps, holding a breath, measuring after training, edema, and pregnancy also bias the reading. Do not compare one result from this page with a same-day DEXA and call the gap a scale error. The methods do not measure the same thing at the same precision.

Men: waist at the navel, not the narrowest point
Women: hip at the largest protrusion, not the iliac crest
Log base 10; keep units consistent (this page is metric)
Do not suck in the abdomen or cinch the neck tape
Do not run the male equation on the female input set

Using the percentage in training and diet

If the result sits in the average or high band and composition confirms extra fat, the usual path is a 300 to 500 kcal deficit below daily expenditure, protein at 1.6 to 2.2 g per kg, and strength work two to four days a week. Losing 0.4 to 0.7 kg per week protects lean mass better than a crash cut. Remeasure girths every 3 to 4 weeks, same time of day, same tape. A 2 to 3 cm drop at the waist with a stable neck often shows up in percent fat before the scale moves 3 kg.

If you already sit in the athletic band and lifts are falling, the deficit is too large. Essential fat below about 5 percent in men and 12 percent in women raises hormonal and performance risk; do not chase those floors unsupervised. If weight rises and percent fat falls, the gain is leaner. If weight falls, percent fat stalls, and loads drop, you are losing lean mass. FITTA keeps girths, weight, meals, and sessions in one stream so the reading becomes a trend instead of a one-off calculation.

When to pair it with BMI, skinfolds, and lab methods

BMI answers whether the weight-to-height ratio sits in a population-risk zone. The Navy method answers whether extra mass looks like trunk fat. Two adults at BMI 26.8 can carry 12 or 28 percent fat; only the girths split those cases. Skinfolds (Jackson-Pollock and similar) read subcutaneous fat at fixed sites and drift when the tester changes. DEXA, hydrostatic weighing, and air-displacement plethysmography are lab references, with different cost and access.

A working rule: BMI and waist rise together, the gain is fat. BMI rises, waist and Navy percent hold, and loads go up, the gain is leaner. A high Navy percent with a normal BMI points to little muscle and central fat. Recalculate after 3 to 4 kg or at the end of a training block. This page is a field estimate. Clinical calls, especially at essential-fat lows, in obesity, or with hormonal change, belong to a physician or dietitian.

Frequently asked questions

How do you calculate body fat percentage

The U.S. Navy method estimates density, then converts with Siri. In centimeters, male density = 1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height). Female density = 1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height). Percent fat is 495 divided by density, minus 450. This page applies those coefficients.

How can I measure body fat at home

Use a non-stretch tape. Measure height barefoot. Neck sits just below the larynx, tape not cinched. For men, waist is at the navel at the end of a normal breath out, no sucking in. For women, waist is the narrowest point on the torso and hip is the largest protrusion of the buttocks. Repeat twice, average, and let the calculator run the formula.

What is a healthy body fat percentage for men and women

There is no single ideal number. On adult fitness charts, conditioned men often sit between 14 and 17 percent and women between 21 and 24. Below that is athletic territory. Above 25 in men and 32 in women, those charts label fat as high. The best point still depends on performance, menstrual cycle, age, and medical history.

Is 20 percent body fat high

It depends on sex. In men, 20 percent sits in the average band, about 18 to 24, and still under typical Navy service ceilings. In women, 20 percent is athletic, about 14 to 20. A single reading is not a diagnosis. Cross-check waist, training performance, and a four-week trend before you change diet or volume.

How accurate is the U.S. Navy body fat method

Hodgdon and Beckett reported correlations near 0.90 in men and 0.85 in women versus hydrostatic weighing, with a standard error of about 3 to 4 points. It is useful for field trends, not as a lab test. A thick neck, edema, pregnancy, and extreme obesity bias the result. It does not replace DEXA, Bod Pod, or a clinician.

What is the difference between BMI and body fat percentage

BMI is weight divided by height squared and does not split muscle from fat. Two adults at BMI 26 can carry 12 or 28 percent fat. The Navy method estimates the fat fraction from trunk girths. Use BMI for population weight-to-height risk and percent fat to see whether a changing scale weight is fat or lean mass.

How do I lower body fat without losing muscle

Hold a 300 to 500 kcal deficit, keep protein at 1.6 to 2.2 g per kg, and lift two to four days a week. Losing 0.4 to 0.7 kg per week usually protects lean mass better than a steep cut. Remeasure girths every 3 to 4 weeks under the same conditions. FITTA logs weight, meals, and sessions so you can see whether the decline is holding.

These calculators provide estimates based on population-level formulas and do not replace assessment by a dietitian, physician or exercise professional.

Your data

Turn a one-off body-fat reading into a weekly trend in FITTA

How to use FITTA better.

What the U.S. Navy method is and how the formula works

How to measure neck, waist, hip, and height

How to read the body-fat result

Common mistakes when you measure or interpret

Using the percentage in training and diet

When to pair it with BMI, skinfolds, and lab methods

Frequently asked questions

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