Body Fat Calculator

What is body fat percentage — and why does it matter?

Body fat percentage is the fraction of your total body weight made up of adipose (fat) tissue. Everything else — muscle, bone, organs, connective tissue, and water — is your lean mass. A 180-pound person at 20% body fat carries 36 pounds of fat and 144 pounds of lean tissue.

Scale weight alone tells you almost nothing useful. Two people can weigh exactly the same yet look and perform very differently because one has significantly more muscle and less fat than the other. Body fat percentage fills that gap by separating fat mass from lean mass.

BMI (body mass index) is a step in the right direction — it at least accounts for height — but it still cannot distinguish muscle from fat. A muscular athlete and a sedentary person of the same height and weight will have identical BMI values yet very different health profiles. Body fat percentage is more informative because it directly measures what you actually care about: how much of you is fat. If you want a fuller picture, you can also calculate your BMI here alongside your body fat estimate.

Disclaimer: all estimates on this page are approximations; accuracy varies by method and individual. Consult a qualified healthcare professional before making decisions based on any body composition estimate.

How body fat is estimated

Several methods exist, and they vary substantially in cost, accuracy, and convenience.

U.S. Navy circumference method

Developed by the U.S. Naval Health Research Center, this approach uses a flexible tape measure and a logarithmic formula. For men, you measure height, waist circumference (at the navel), and neck circumference (below the larynx). For women, hip circumference (at the widest point) is added to the equation because women naturally carry more fat in the hips and thighs. The formula then applies log₁₀ arithmetic to produce an estimate. This is one of the most practical field methods — it requires no specialized equipment and typically falls within ±3 to 4 percentage points of laboratory results for most adults.

BMI-based formula (Deurenberg)

When circumference measurements are unavailable, a BMI-based approach (such as the Deurenberg formula used here) can provide a rough estimate using only height, weight, age, and sex. Because BMI conflates muscle and fat, this method introduces larger errors for muscular or very lean individuals — typical error is ±5% or more. Think of it as a ballpark figure rather than a precise reading.

Other measurement methods

• Skinfold calipers — A trained technician pinches and measures fat folds at standardized sites (e.g., tricep, suprailiac, thigh). Accuracy depends heavily on the technician's skill; typical error is ±3–5%.
• Bioelectrical impedance (BIA) — Consumer scales and handheld devices send a small electrical current through the body and estimate fat from resistance. Convenient but sensitive to hydration; readings can swing 3–5% based on when you last ate or drank.
• DEXA scan — Dual-energy X-ray absorptiometry is a clinical imaging tool that maps bone, fat, and lean tissue with ±1–2% accuracy. It is the closest thing to a gold standard accessible outside a research lab, though it requires a medical facility and involves low-level radiation exposure.
• Hydrostatic (underwater) weighing — Based on Archimedes' principle, this method compares weight in air to weight submerged in water. Accuracy is ±1–3% but the setup is rarely available outside university or sports-science labs.

A worked example and body fat reference ranges

Example calculation (Navy method, male)

Suppose a man is 70 inches tall (5'10"), measures his waist at 34 inches (at the navel), and his neck at 15 inches. Plugging into the Navy formula: 86.01 × log₁₀(34 − 15) − 70.041 × log₁₀(70) + 36.76. Working through the arithmetic: log₁₀(19) ≈ 1.279, log₁₀(70) ≈ 1.845, so the result is roughly 86.01 × 1.279 − 70.041 × 1.845 + 36.76 ≈ 110.0 − 129.2 + 36.76 ≈ 17.6%. That lands squarely in the "Fitness" category for men.

Body fat category reference table

Reference ranges based on American Council on Exercise (ACE) guidelines. "Essential fat" is the minimum required for normal physiological function. Going below it is associated with serious health risks.

How to interpret and use your number

A single body fat reading is a snapshot, not a verdict. One measurement taken on one day tells you relatively little. What matters far more is the trend over weeks and months. Measure consistently — same method, same time of day, same hydration state — and track the direction of change rather than fixating on an absolute number.

Keep in mind that different methods will give you different numbers, sometimes by several percentage points. Switching from a bioimpedance scale to the Navy method mid-journey will appear to show a large change even if your body composition has barely shifted. Stick with one method for tracking purposes and treat readings from other methods as a separate data point.

Your body fat percentage interacts directly with your calorie needs. As lean mass increases, so does your basal metabolic rate (BMR) — the calories your body burns at rest. If you are trying to fine-tune your nutrition, pair a body fat estimate with a TDEE (total daily energy expenditure) calculation for a more personalized calorie target.

Body fat percentage also complements an ideal weight estimate, which tells you a goal weight range but not how much of that weight should be fat versus lean tissue. Together, the three metrics — body fat %, TDEE, and ideal weight — give you a much richer picture of where you are and where you are headed.

Tips and common mistakes

• •Measure the same spots every time. Waist circumference can read 1–2 inches differently depending on whether you measure at the navel versus one inch above it. Nail down your exact measuring spots and use them every time.
• •Do not compare readings from different methods. A bioimpedance scale and a DEXA scan can differ by 4–6% for the same person on the same day. Pick one method and stick with it for longitudinal tracking.
• •Avoid chasing very low body fat. Below approximately 5% for men and 13% for women, the body enters a zone of essential fat. Dropping below these levels impairs hormonal function, immune response, and bone density. Extremely lean physiques visible in physique competitions are typically transient peak conditions, not year-round targets.
• •Measure in a rested, fasted state. Eating, drinking, and exercise all temporarily shift circumference measurements and hydration levels. Early morning, before eating, is typically the most consistent time for measurements.
• •Do not measure too frequently. Body composition changes slowly. Weekly or biweekly measurements are enough to see meaningful trends without noise from day-to-day fluctuations.
• •Account for the tape tension. Pulling the tape too tight compresses soft tissue; too loose gives an inflated reading. The tape should be snug — lying flat against the skin — but not indenting it.

❓Body fat calculator FAQ