Standardize the tape
Measure the neck just below the larynx. For the male Navy equation use the abdomen at the navel; for the female equation use waist and the widest hip circumference.
Circumference body-composition estimate
Compare three body-fat estimates, not one magic number.
Run the U.S. Navy circumference equation beside relative fat mass and an adult BMI-based equation, then translate the Navy estimate into fat mass, lean mass and a target-weight scenario that holds lean mass constant.
Keep the tape horizontal and snug without compressing skin. Take two or three readings under similar conditions and use their average. Small tape-placement changes can materially affect the estimate.
Measurement and method guide
Body-fat estimates are most useful as repeatable trends
Circumference equations are accessible, but they infer composition from body shape. Hydration, tape placement, population differences and equation choice all matter.
Respect method spread
The Navy, RFM and BMI equations use different predictors. A wide spread is a warning against false precision and a reason to focus on repeatable conditions.
Escalate when accuracy matters
Clinical or performance decisions may justify a professionally administered method. Even instrument-based methods have assumptions and test–retest variation.
Navy equation
The displayed calculation uses the published inch-based logarithmic circumference equations and converts metric inputs before calculation.
BMI comparison
The adult BMI estimate is 1.20 × BMI + 0.23 × age − 16.2 for males or − 5.4 for females. It is a population equation, not a direct measurement.
Safety
Very low or high estimates, rapid changes, menstrual disruption, weakness or eating concerns should be discussed with a qualified healthcare professional.
Simple by design
Three steps to a clearer number.
Enter your details
Use realistic values in each field. You can change them anytime.
Calculate instantly
The formula runs locally, so there is no account or waiting time.
Use the result
Treat the answer as a practical estimate for your next decision.