Estimate adult body fat percentage from tape measurements, compare it with a BMI-based estimate, and see fat mass, lean mass, and a simple category.
How to use our Body Fat Calculator
- Choose Sex, Units, and enter Age (years).
- Enter your height and weight using the visible fields for your unit system: Height (feet) and Height (inches) with Weight (lb), or Height (cm) with Weight (kg).
- Measure and enter Neck circumference and Waist circumference. If Sex is female, also enter Hip circumference. Keep the tape level and snug, not tight.
- Leave Comparison method on if you want the BMI-based estimate shown beside the U.S. Navy estimate, then click Calculate.
- Sanity-check the result: if the Difference between methods is large or the Estimate note says to recheck measurements, measure again before relying on the Body fat estimate.
Definitions
Body fat estimate: The estimated percent of your body weight that is fat tissue.
U.S. Navy estimate: A body fat estimate based on height and tape measurements. In this calculator, it is the main result when the needed measurements are valid [2].
BMI-based estimate: A comparison estimate based on BMI, age, and sex from the Deurenberg adult equation [1].
Difference between methods: The absolute gap between the U.S. Navy estimate and the BMI-based estimate, measured in percentage points.
Fat mass: The estimated part of your body weight that is fat tissue.
Lean mass: The estimated part of your body weight that is not fat tissue, including muscle, bone, organs, and body water.
BMI: Body mass index, calculated from weight and height. It helps with the comparison method but does not directly measure body composition.
Body fat category: A broad reference label based on your estimated body fat percentage and sex.
Estimate note: A short message that flags low confidence, unusual results, or measurements that should be checked again.
Common mistakes and quick fixes
Mistake: Mixing unit systems, such as entering centimeters while Units is set to US.
Fix: Make sure Units matches every visible measurement field you fill in.
Mistake: Entering total height only in Height (inches) when US units are selected.
Fix: Put the feet part in Height (feet) and only the leftover inches in Height (inches) .
Mistake: For men, entering a Waist circumference (in) or Waist circumference (cm) that is not larger than the matching neck measurement.
Fix: Recheck the tape and make sure waist is measured at the correct spot and is greater than neck for the Navy formula.
Mistake: For women, leaving Hip circumference (in) or Hip circumference (cm) blank.
Fix: If Sex is female, enter hip circumference at the widest part of the hips.
Mistake: Treating BMI-based estimate as the same thing as a direct body composition measurement.
Fix: Use U.S. Navy estimate as the main home estimate here, and use the BMI result as a comparison check.
Mistake: Ignoring a large Difference between methods .
Fix: Read the Estimate note , remeasure neck, waist, and hip carefully, and treat the result more cautiously if the gap stays large.
Limitations & Key Assumptions / Boundary Conditions
- This calculator is for adults only. Children and teens need different body fat methods.
- The U.S. Navy method depends on careful tape measurements. Small tape errors can noticeably change the result.
- The BMI-based estimate is a comparison tool, not a direct measurement of body composition, and it can misread very muscular or unusual body types.
- If the U.S. Navy and BMI-based results are far apart, the estimate should be treated more cautiously rather than averaged blindly.
- For men, the Navy formula requires waist to be greater than neck. For women, it requires waist plus hip to be greater than neck.
- If a calculated body fat result falls below 0% or above 75%, the formulas are likely not suitable for the measurements entered or the measurements need rechecking.
- Body fat categories are broad reference ranges, not medical diagnoses or performance standards.
Methodology
How the calculator works
The calculator first converts your entries into consistent units. BMI uses kilograms and meters. The U.S. Navy formula uses inches, so metric tape measurements are converted to inches before that step.
Formulas used
BMI = weight_kg / (height_m^2)
BF_BMI = 1.20 x BMI + 0.23 x age - 10.8 x sex_male - 5.4
BF_Navy_male = 86.010 x log10(waist_in - neck_in) - 70.041 x log10(height_in) + 36.76
BF_Navy_female = 163.205 x log10(waist_in + hip_in - neck_in) - 97.684 x log10(height_in) - 78.387
fat_mass = weight x (body_fat_primary_pct / 100)
lean_mass = weight - fat_mass
method_difference_pct_points = abs(BF_Navy - BF_BMI)
Primary result choice
The main Body fat estimate uses the U.S. Navy method when its required tape measurements are valid, because it uses more body-size information than BMI alone [2]. If BMI comparison is turned on, the calculator also shows the BMI-based estimate so you can see how closely the methods agree [2].
Worked mini-example
Example: male, age 30, height 5 ft 10 in, weight 170 lb, neck 15 in, waist 34 in.
BMI = 77.1107 / (1.778^2) = 24.392
BF_BMI = 1.20 x 24.392 + 0.23 x 30 - 10.8 x 1 - 5.4 = 18.672%
BF_Navy_male = 86.010 x log10(34 - 15) - 70.041 x log10(70) + 36.76 = 17.863%
fat_mass = 170 x 0.17863 = 30.367 lb
lean_mass = 170 - 30.367 = 139.633 lb
difference = abs(17.863 - 18.672) = 0.809 percentage points
Since the Navy estimate is valid, it becomes the primary result in this example.
How to interpret disagreement
A small gap between methods suggests the two estimates are telling a similar story. A larger gap does not prove one is right and one is wrong, but it is a useful sign to recheck tape placement, tape tension, and entered values before trusting the result too much.
Assumptions used
The calculator assumes adult users, accurate self-measurements, and correct sex-specific equation choice. Results can differ from lab methods such as DEXA, hydrostatic weighing, or other clinical assessments because these formulas are estimates, not direct scans.