Body Fat Calculator

What This Calculator Really Does

Body fat percentage — the share of your mass that is fat rather than lean tissue — describes composition better than weight or BMI alone. This calculator estimates it two ways: the U.S. Navy tape method, which uses circumference measurements, and the BMI-based formula, which needs nothing but height, weight, age and sex.

Both are estimates, not measurements. The tape method is typically accurate within a few points when done carefully; the BMI formula trades some precision for convenience. Tracking the trend over months matters more than any single reading.

Everything runs locally in your browser: no measurements uploaded, nothing tracked, and the page works offline after the first visit.

The Formulas, Explained Plainly

U.S. Navy method (men): BF% = 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450, measurements in inches.

Navy method (women): the same structure with waist, neck and hip terms — the calculator applies the correct variant automatically.

BMI-based estimate (Deurenberg): BF% = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex is 1 for men and 0 for women.

• Circumference inputs — the tape method leans on where fat is distributed, so it responds to actual composition changes.
• BMI inputs — a population-level regression; muscular individuals will read high because BMI cannot see muscle from fat.

How to Use It — In Order

1. Choose the method — tape measurements if you have them; BMI inputs otherwise.
2. Measure the tape points — waist at the navel, neck below the larynx (and hips for women), tape level not tight.
3. Enter height, weight, age and sex — the BMI path needs all four.
4. Read the percentage — with the healthy-range band for your sex shown alongside.
5. Repeat under the same conditions — morning, pre-meal readings make trends meaningful.
6. Track the direction — a monthly trend line beats any single measurement for judging progress.

Worked Example, Verified by Hand

BMI method: a 40-year-old man, 1.80 m, 81 kg → BMI = 81 ÷ 1.80² = 25.0:

1. 1.20 × 25.0 = 30.0.
2. 0.23 × 40 = 9.2.
3. Sum, then subtract male constants: 30.0 + 9.2 − 10.8 − 5.4 = 23.0% estimated body fat.

Navy method cross-check: a man 70 in tall, 15 in neck, 36 in waist: log₁₀(21) = 1.322 and log₁₀(70) = 1.845, giving 495 ÷ (1.0324 − 0.2522 + 0.2851) − 450 = 495 ÷ 1.0653 − 450 ≈ 14.7% — a fit, lean estimate. The two methods can legitimately disagree by several points; that spread is itself information about how muscular the person is.

Common Mistakes to Avoid

• Treating estimates as lab results — DEXA scans carry their own error; tape and BMI formulas carry more.
• Pulling the tape tight or measuring over clothing — both bias the Navy method systematically.
• Comparing raw numbers across methods — the tape method and BMI regression answer with different biases; track each against itself.
• Aiming below healthy ranges — essential fat is genuinely essential; too low impairs hormones and immunity.
• Using BMI-based estimates for athletes — muscular builds inflate BMI, so the regression overshoots badly.

Limitations

• Population formulas — individual error of several percentage points is normal for both methods.
• Not valid for children or pregnancy; age-term assumptions break outside typical adult ranges.
• Hydration, recent meals and measurement technique move tape results measurably.

Expected Accuracy

Arithmetic is exact at full floating-point precision. Agreement with a four-compartment lab measurement is typically within a few points for the tape method and wider for the BMI regression — estimates, not diagnostics.

Privacy — Your Data Never Leaves This Device

Measurements stay in your browser: no account, no upload, no tracking — the tool works offline after the first visit.

Related Tools & Guides

Natural next steps from Body Fat:

• All Health & Fitness Calculators
• BMI Calculator — the input the BMI method starts from
• Ideal Body Weight — target ranges by formula
• BMR Calculator — lean mass drives metabolism
• Lean Body Mass — the non-fat remainder

Sources & Standards

The Navy circumference equations and the Deurenberg BMI regression are published, peer-reviewed estimation methods.

• CDC National Center for Health Statistics — anthropometry and body-composition context
• NIH — body composition assessment background

Bottom Line

Body fat percentage says what the scale cannot — but both formulas here are estimates. Measure consistently, track the trend, and judge against healthy ranges, not against other people.

Frequently Asked Questions