Important Note : BMI-based body-fat equations are estimates and may use different formulas for adults and children.

Use this Body Fat Calculator to estimate body fat percentage from simple body measurements rather than weight alone. It compares a U.S. Navy circumference-based estimate with a BMI-based estimate, making it useful for fitness tracking, body-composition awareness, and general progress monitoring.

Reviewed by: AjaxCalculators Editorial Team
Last updated: April 23, 2026
Method source: U.S. Navy circumference-based body fat estimate and BMI-based body fat prediction formula
Editorial standards: AjaxCalculators Editorial Policy

What This Body Fat Calculator Calculates

This calculator estimates:

• Body fat percentage (U.S. Navy method)
• Body fat percentage (BMI-based method)
• BMI

It uses the following inputs:

• Sex
• Age
• Weight
• Height
• Neck circumference
• Waist circumference
• Hip circumference for females in the Navy method

How the Body Fat Calculator Works

1) U.S. Navy Method

The U.S. Navy method estimates body fat percentage from body circumferences and height.

It is based on body-size relationships rather than body weight alone, which is why it uses tape measurements.

In practical terms:

• Men: the estimate is based mainly on height, neck, and abdomen/waist relationship
• Women: the estimate is based mainly on height, neck, waist, and hip relationship

This method is widely used as a field estimate because it is simple and does not require specialized equipment.

2) BMI-Based Body Fat Method

The BMI-based method first calculates body mass index:

BMI = weight (kg) / height² (m²)

A commonly used adult BMI-based body fat estimate is:

Body fat % = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4

Where:

• sex = 1 for male
• sex = 0 for female

This method is easy to use, but it is still only an estimate because BMI is not a direct measurement of body fat.

Assumptions and Important Notes

• This calculator gives estimates, not a direct body-composition measurement.
• The U.S. Navy method depends heavily on correct tape-measure placement.
• The BMI-based method is convenient, but BMI is not a direct measure of body fat and does not reflect fat distribution.
• Different BMI-based body fat equations may be used for adults and children, so age range matters.
• Muscular people, older adults, and people with unusual body-fat distribution may get less accurate BMI-based estimates.
• For more precise body-fat assessment, methods like DXA, hydrostatic weighing, or validated body-composition testing are more accurate than simple field formulas.

Worked Example

Suppose an adult male enters:

• Age: 30
• Weight: 80 kg
• Height: 180 cm
• Neck: 38 cm
• Waist: 90 cm

Example A: BMI-Based Estimate

Step 1: Calculate BMI
BMI = 80 / (1.80 × 1.80) ≈ 24.69

Step 2: Apply the adult BMI-based body fat formula
Body fat % = 1.20 × 24.69 + 0.23 × 30 − 10.8 × 1 − 5.4

Step 3: Calculate
Body fat % ≈ 29.63 + 6.9 − 10.8 − 5.4 = 20.33%

So the BMI-based estimate is about 20.3%.

Example B: Navy Estimate

The Navy estimate uses the neck, waist, and height relationship instead of BMI and age. Because the two methods are based on different assumptions, the two body-fat results may be close or noticeably different.

How to Use This Body Fat Calculator

1. Select your sex.
2. Enter your age.
3. Enter your weight and height in the chosen unit system.
4. Measure and enter your neck circumference.
5. Measure and enter your waist circumference.
6. If female, also enter hip circumference for the Navy method.
7. Review the body fat estimates from both methods and the BMI result.

How to Interpret the Result

U.S. Navy body fat is a circumference-based estimate.

BMI body fat is a formula-based estimate derived from BMI, age, and sex.

BMI is a size-based screening number, not a direct fat measurement.

If the two body-fat estimates differ, that does not automatically mean one is broken. It usually means the methods emphasize different body-shape signals.

Practical Uses of a Body Fat Calculator

• track body-composition trends over time
• compare circumference-based and BMI-based estimates
• monitor progress during fat-loss or fitness plans
• add body-fat context to ordinary body weight tracking
• understand why weight alone does not fully describe body composition

References

1. U.S. Navy body composition assessment guide
2. Deurenberg et al. body-fat prediction formulas from BMI, age, and sex
3. NCBI Bookshelf: strengths and limitations of BMI as a screening tool

Related Calculators

• ABSI Calculator
• Waist to Hip and Height Ratio Calculator
• Calorie Calculator
• Maintenance Calorie Calculator

Disclaimer: This calculator is for educational and fitness-tracking use only. It does not diagnose obesity, health risk, or disease. If you need medical assessment of body composition or obesity-related risk, discuss the result with a qualified clinician.