Body Fat Calculator

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The US Navy Body Fat Formula

The US Navy formula runs the circumference measurements (in centimetres) through a logarithmic body-density equation, then converts density to body fat percentage. The equations differ for men and women:

Sex	Formula	Measurements
Men	495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450	Height, neck, waist
Women	495 / (1.29579 - 0.35004 x log10(waist + hip - neck) + 0.22100 x log10(height)) - 450	Height, neck, waist, hip

Worked example (male): Height 180 cm, neck 38 cm, waist 85 cm

waist - neck = 47 cm

log10(47) = 1.6721, log10(180) = 2.2553

density = 1.0324 - (0.19077 x 1.6721) + (0.15456 x 2.2553)

= 1.0324 - 0.31898 + 0.34857 = 1.06199

BF% = 495 / 1.06199 - 450 = 466.1 - 450 = 16.1%

That places this lifter in the "Fitness" category on the US Navy chart. If the same man had a waist of 95 cm instead, the estimate jumps to about 22.1% - "Average" - which shows how sensitive the formula is to abdominal circumference. A 10 cm waist change corresponds to roughly 6 percentage points of body fat in this height range.

Where to Measure

Accurate circumference readings make or break the formula. Use a flexible cloth or fibreglass tape, pull it snug against the skin without compressing tissue, and take each reading twice - use the average. Measure first thing in the morning before eating or drinking, when waist circumference is most stable.

Site	Location	Tips
Neck	Just below the larynx (Adam's apple)	Tape slopes slightly downward to the front. Do not flex neck muscles or tilt the head up.
Waist	At the navel (belly-button level)	Stand relaxed, do not suck in. Measure at the end of a normal breath out.
Hip (women only)	Widest point of the buttocks	Feet together, tape level all the way round, snug but not compressing.
Height	Standing straight, no shoes	Against a wall, feet flat, heels together, looking straight ahead.

Common mistakes: measuring the waist at the narrowest point (that is the ACSM site, not the Navy site), wearing bulky clothing over the tape, or tensing the neck muscles. Any of these skews results by 1-3 percentage points.

What Is a Healthy Body Fat Percentage?

The American Council on Exercise (ACE) body fat ranges below are the most widely cited guidance for non-athletes. Women carry more essential fat than men because of reproductive and hormonal function - about 12% in women versus 3% in men per ACSM's position stand.

Category	Men	Women	Description
Essential fat	2-5%	10-13%	Minimum for organ function and hormone health
Athlete	6-13%	14-20%	Competitive endurance or strength athletes, visible muscle definition
Fitness	14-17%	21-24%	Active adults with a regular training habit
Average	18-24%	25-31%	Typical sedentary adult
Obese	25%+	32%+	Elevated cardiometabolic risk

ACSM also publishes age-adjusted ranges because body fat rises slightly with age even in healthy people. For men aged 20-29 the "average" midpoint sits around 17.5%, rising to about 25% for men over 60. For women the equivalent values rise from about 25% to 31%. The Navy's own enlistment standards, last updated in 2025, cap body fat at 22-26% for male sailors (by age band) and 33-36% for female sailors.

How Accurate Is the US Navy Method?

DEXA (Dual-Energy X-Ray Absorptiometry) is the gold-standard reference for body composition, with whole-body precision around 1-2% under controlled conditions according to the Obesity Medicine Association. Against DEXA, the US Navy formula has a typical standard error of around 3-4% - good enough to track trends over months, but not precise enough to declare "I am exactly 18.4% fat".

Method	Accuracy (vs DEXA)	Cost	Availability
DEXA scan	Gold standard (+/- 1-2%)	$50 - $150	Clinical facilities
Hydrostatic weighing	Very good (+/- 2-3%)	$50 - $100	University labs
Bod Pod (air displacement)	Good (+/- 2-3%)	$50 - $75	Sports science labs
US Navy method (this tool)	Moderate (+/- 3-4%)	Free (tape measure)	Anywhere
Skinfold calipers	Moderate (+/- 3-5%)	$10 - $30	Needs a trained tester
Bioelectrical impedance (home scales)	Variable (+/- 3-8%)	$20 - $200	Home use
BMI-based estimate	Poor (+/- 5-10%)	Free	Anywhere

The Navy formula tends to overestimate body fat in people with thick necks (weightlifters, rugby forwards) and underestimate in those with a lot of visceral fat but a small abdominal circumference. Consistency matters more than absolute accuracy - if you measure the same way every time, a drop from 22% to 19% is meaningful even if the true DEXA numbers were 24% and 21%.

Why Track Lean Body Mass?

Lean body mass (LBM) is your weight minus fat mass. It includes muscle, bone, organs, connective tissue, and body water. Tracking LBM alongside body fat is more informative than the scale alone because it tells you whether weight changes came from fat or from muscle.

Example: A person weighing 80 kg at 16.1% body fat has a fat mass of 80 x 0.161 = 12.9 kg and an LBM of 67.1 kg. Three months of strength training later they weigh 79 kg at 13% body fat. Fat mass is now 10.3 kg, LBM is 68.7 kg. They only lost 1 kg on the scale but they lost 2.6 kg of fat and gained 1.6 kg of lean tissue - a body-recomposition win that a bathroom scale would have made invisible.

For a quick height-and-weight screening, the BMI Calculator gives a complementary view (though it cannot separate muscle from fat). Once you know your LBM, the Lean Body Mass Calculator shows you alternative LBM formulas, and the TDEE Calculator estimates daily calorie needs from your lean mass and activity level.

Body Fat and Health Risk

Body fat at either extreme raises health risk. The New England Journal of Medicine and WHO both identify excess visceral (abdominal) fat as a stronger predictor of cardiovascular disease and type 2 diabetes than total body fat. Waist circumference on its own is a useful screening tool: over 102 cm (40 in) in men or 88 cm (35 in) in women flags elevated metabolic risk per NHS guidance.

Body Fat Level	Associated Health Considerations
Below essential (< 5% men, < 12% women)	Hormonal disruption, amenorrhea, weakened immunity, bone density loss, RED-S syndrome risk
Athlete range	Optimal performance window. Sustained very low levels in non-athletes may be unhealthy.
Fitness range	Low cardiometabolic risk, good energy levels, sustainable long-term
Average range	Generally healthy; upper end brings slightly elevated risk, especially with high waist circumference
Obese range (> 25% men, > 32% women)	Increased risk of type 2 diabetes, cardiovascular disease, obstructive sleep apnea, joint problems

If you are looking to change body composition, a small calorie deficit paired with resistance training preserves muscle while reducing fat - faster deficits tend to sacrifice more lean mass. Recheck measurements every 4-6 weeks rather than weekly; daily variation from hydration and gut contents can swing results by 1-2 percentage points and obscure real progress.

Common Questions About the Navy Formula

Why does the men's formula not use hip circumference? The original Hodgdon and Beckett Navy research (1984) found that for men, the waist-minus-neck measurement alone captured enough of the variance in fat distribution to predict body density accurately. Adding hip data did not improve the fit for male subjects but did for female subjects, who typically carry more fat in the gluteofemoral region.

Does the formula work for teenagers? The Navy formula was calibrated on adult active-duty service members aged 18 and older. It tends to overestimate body fat in adolescents and is not reliable for children under 18. For young people, clinical growth charts from the CDC are more appropriate.

Does it work for bodybuilders or very muscular men? Not well. Heavily muscled lifters often have unusually thick necks, which the formula treats as "less waist-neck difference" and therefore as lower body fat - but their hypertrophied trunk musculature may also inflate the waist circumference, sometimes cancelling out. DEXA is worth the money if you have an unusually muscular build and want a real number.

What about pregnancy? Body fat estimates during pregnancy are not meaningful and the formula has not been validated for pregnant women. Wait until postpartum recovery before re-baselining.

Tracking Progress Over Time

Body composition changes slowly. A well-structured fat-loss phase typically drops body fat by 0.5-1.0 percentage points per week early on, tapering to about 0.3-0.5 points per week as the deficit becomes harder to sustain. Strength-training beginners can sometimes add 0.5-1.0 kg of lean mass per month while simultaneously losing fat ("recomposition") - a phenomenon documented in a 2020 meta-analysis in the Journal of Sport and Health Science by Barakat and colleagues.

Phase	Typical BF% Change	Typical LBM Change
Aggressive cut (500-750 kcal deficit)	-0.5 to -1.0 pp/week	-0.1 to -0.3 kg/week
Moderate cut (250-500 kcal deficit)	-0.25 to -0.5 pp/week	Maintained with training
Recomp (maintenance kcal)	-0.1 to -0.25 pp/week	+0.1 to +0.3 kg/week (beginners)
Lean bulk (250-500 kcal surplus)	+0.1 to +0.3 pp/week	+0.2 to +0.5 kg/week

Weigh and measure in the same conditions each time: morning, after the toilet, before food or drink, minimal clothing. Consistency beats precision - the Navy formula's 3-4 point error is a fixed bias for your body, so relative change over time is what the number is good for.

All calculations run entirely in your browser. Your measurements are never sent to any server.

Sources

• US Navy - Body Composition Assessment Instruction
• ACSM Complete Guide to Fitness and Health (Body Composition)
• WHO - Obesity and Overweight Fact Sheet
• NHS - Healthy Weight and Waist Size Guidance
• Obesity Medicine Association - DEXA as the Gold Standard
• ACE Fitness - Percent Body Fat Categories