Body Fat Calculator

The U.S. Navy Method Formula

The Navy method (developed for the U.S. Naval Health Research Center) estimates body fat from body circumferences and height. It relies on the fact that fat tends to accumulate around the waist and hips, so those measurements correlate strongly with total body fat.

The formulas differ by sex. Using inches for all measurements:

Men:
%BF = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76

Women:
%BF = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387

Note that men do not use the hip measurement, while women do. The log10 is a base-10 logarithm.

Worked example 1 — Male

A man is 70 in tall, with a 15 in neck and a 34 in waist.

waist − neck = 34 − 15 = 19
86.010 × log10(19)  = 86.010 × 1.2788 = 110.00
70.041 × log10(70)  = 70.041 × 1.8451 = 129.24
%BF = 110.00 − 129.24 + 36.76 = 17.5%

Worked example 2 — Female

A woman is 65 in tall, with a 13 in neck, 30 in waist, and 40 in hips.

waist + hip − neck = 30 + 40 − 13 = 57
163.205 × log10(57) = 163.205 × 1.7559 = 286.55
97.684 × log10(65)  = 97.684 × 1.8129 = 177.10
%BF = 286.55 − 177.10 − 78.387 = 31.1%

Worked example 3 — Leaner male

A man is 72 in tall, 16 in neck, 32 in waist.

waist − neck = 16
86.010 × log10(16) = 86.010 × 1.2041 = 103.57
70.041 × log10(72) = 70.041 × 1.8573 = 130.10
%BF = 103.57 − 130.10 + 36.76 = 10.2%

The metric versions of these formulas use centimeters and slightly different constants, but the calculator handles both unit systems automatically.

From Body Fat to Lean Body Mass and Fat Mass

Once you know your body fat percentage, two more useful numbers fall out immediately, provided you also know your body weight.

Fat mass is the weight of fat you carry:

Fat mass = total weight × (body fat % ÷ 100)

Lean body mass (LBM) is everything else, muscle, bone, water, and organs:

Lean body mass = total weight − fat mass

Example

Our first example gave a man a body fat percentage of 17.5%. Suppose he weighs 175 lb.

Fat mass = 175 × 0.175 = 30.6 lb
Lean body mass = 175 − 30.6 = 144.4 lb

Why does lean body mass matter? Because it is the metabolically active tissue that burns calories, drives strength, and largely determines how you look. When people 'lose weight,' the goal is almost always to lose fat mass while preserving lean mass. Tracking LBM over a diet tells you whether you're losing the right kind of weight, dropping fat, or burning away hard-earned muscle.

A useful rule of thumb: if your scale weight drops but your lean body mass stays roughly constant, your fat loss is on track.

Body Fat Percentage Categories

Raw numbers only mean something in context. The American Council on Exercise (ACE) publishes widely used body fat percentage ranges by sex. Because women carry more essential fat, healthy thresholds are shifted about 8-10 points higher than for men.

A few things worth noting:

• •Essential fat is the floor. Dropping below it is dangerous and generally only touched briefly by competitive physique athletes on stage.
• •The fitness range is a realistic, sustainable target for most active people who want visible muscle definition.
• •The average category is not 'unhealthy' by itself, but the upper end trends toward elevated health risk.
• •Body fat naturally creeps up with age. A 45-year-old at 22% is doing better relative to peers than a 22-year-old at the same percentage.

Use these bands as a compass, not a verdict. Where you feel strong, energetic, and healthy matters more than hitting a specific decimal.

Navy Method vs. Other Body Fat Measurement Methods

The Navy method is popular because it's free, fast, and repeatable at home. But it's one of several approaches, each with trade-offs in accuracy, cost, and convenience.

The key insight: for most people, the precise number matters less than consistency. The Navy method may read a percentage point or two off from a DEXA scan, but if you measure the same way each time, the change it shows is reliable, and change is what a fitness journey is really about.

The Navy method's main weakness is that it assumes a typical fat distribution. It can misjudge people with unusually muscular midsections, very tall or short frames, or fat that sits atypically. In those cases the estimate drifts, but the trend line usually still holds.

How to Take Accurate Measurements

The Navy formula is only as good as your tape work. A half-inch error on the waist can shift your result by more than a full percentage point, so measure carefully and consistently.

Use a flexible cloth or vinyl tape measure, keep it snug but not compressing the skin, and measure on bare skin or over thin clothing.

1. •Neck — Measure just below the larynx (Adam's apple), with the tape sloping slightly downward toward the front. Don't flare your neck out.
2. •Waist — For men, measure horizontally at the navel. For women, measure at the narrowest point of the natural waist. Relax your stomach; don't suck in.
3. •Hips (women only) — Measure the largest horizontal circumference around the buttocks, feet together.
4. •Height — Stand straight against a wall without shoes.

Consistency tips: Measure first thing in the morning, before eating or drinking, after using the bathroom. Take each measurement two or three times and average them. Round to the nearest half inch or centimeter.

Because a bloated stomach, a heavy meal, or even posture can nudge the waist reading, always measure under the same conditions when comparing over time.

How to Use This Body Fat Calculator

This body fat calculator turns four or five simple measurements into a full body-composition snapshot. Here's what each field does.

Inputs

• •Sex — Selects the correct Navy formula. Men and women use different equations because of differences in essential fat and fat distribution.
• •Height — Your standing height, in inches or centimeters.
• •Neck circumference — Measured just below the Adam's apple.
• •Waist circumference — At the navel (men) or narrowest point (women).
• •Hip circumference — Required for women only; the calculator hides or ignores it for men.
• •Weight (optional) — Add it to unlock fat mass and lean body mass outputs.

Outputs

• •Body fat percentage — Your primary result, from the U.S. Navy method.
• •Body fat category — Where you land: athlete, fitness, average, and so on.
• •Fat mass and lean body mass — Shown when you enter weight.

The tool works in both imperial and metric units, switch and the fields relabel automatically. Enter your numbers once, read your result instantly, and re-enter every few weeks to watch the trend.

Interpreting Your Result and Changing Your Body Fat

Got your number? Here's how to act on it.

First, locate yourself on the category table and decide whether you actually want to change anything. Plenty of people are perfectly healthy in the 'average' range and simply want to maintain.

If your goal is to lower body fat percentage, remember the equation has two levers: reduce fat mass, or increase lean mass, ideally both.

• •Sustain a modest calorie deficit. Roughly 300-500 calories below maintenance yields about 0.5-1 lb of fat loss per week without crashing your metabolism.
• •Prioritize protein. Aim for 0.7-1 g per pound of body weight to preserve lean body mass while dieting.
• •Lift weights. Resistance training is what tells your body to keep (or build) muscle while you lose fat.
• •Be patient with the last few points. Going from 25% to 18% is far easier than 12% to 9%.

If your goal is to raise lean mass, a slight calorie surplus plus progressive strength training is the recipe.

Track body fat percentage and lean body mass together. If your body fat drops while lean mass holds steady, you're recomposing successfully, even if the scale barely moves. This is exactly the scenario the scale alone would make you panic over.

Common Mistakes and Myths to Avoid

The Navy method is forgiving, but a few habits quietly wreck accuracy or lead people to misread their results.

Mistake 1 — Measuring inconsistently. Taking your waist at the navel one week and above it the next makes your trend meaningless. Pick a spot and stick to it.

Mistake 2 — Sucking in your stomach. It feels good but corrupts the waist reading and understates your body fat. Stand relaxed and breathe normally.

Mistake 3 — Chasing a single-day number. Water retention, sodium, and meals can swing your waist by an inch. Look at the multi-week average, not one reading.

Mistake 4 — Treating the estimate as clinical fact. The Navy method carries a ±3-4% margin. If it says 18%, your true value is probably somewhere between 14% and 22%.

Myths

Myth: 'Spot reduction works.' You cannot burn fat from a specific area by exercising it. Doing crunches builds abdominal muscle but does not preferentially melt belly fat; fat loss happens body-wide.

Myth: 'Lower is always better.' Below essential-fat levels, health suffers, hormones, immunity, and energy all decline. There is a healthy floor.

Myth: 'Muscle weighs more than fat.' A pound is a pound. Muscle is simply denser, so it takes up less space, which is why body composition, not scale weight, is the number that matters.