After you weigh yourself (ideally first thing in the morning on an empty stomach), grab a tape measure and record the circumference of your neck, waist, and (for females) hips. Once you have your numbers, enter your measurements (in inches or centimeters) into the Body Fat Calculator above to estimate your body fat percentage.

Neck (Males & Females)

Wrap the tape measure horizontally around your neck, placing it directly on the skin just below the larynx (Adam’s apple). Keep your shoulders relaxed and your head neutral to ensure the most accurate reading.

Waist (Males)

Position the tape measure at your belly button, then wrap it horizontally around your waist with the tape touching the skin. Take the measurement at the end of a normal exhale, then record the result.

Waist (Females)

Measure around your natural waist, usually found about halfway between your belly button and your sternum (breastbone). Keep the tape level and take the measurement on a normal exhale.

Hips (Females Only)

Wrap the tape measure horizontally around the widest/fullest part of the buttocks, ensuring it stays level as it passes around to the front of the pelvis. Record the measurement without pulling the tape too tight.

BMI Calculator vs. Body Fat Calculator

A BMI (Body Mass Index) calculator and a Body Fat calculator can both be useful—but they answer different questions, and they should not be treated as interchangeable.

What BMI tells you (and what it doesn’t)

BMI is calculated using only height and weight. Because of that, BMI is best viewed as a quick screening tool that helps categorize weight status at a population level and flag potential risk that may warrant a closer look. CDC+1

However, BMI does not directly measure body fat. It also cannot distinguish between:

• Fat mass

• Lean mass (muscle, bone, organs)

• Differences in fat distribution (where fat is stored in the body) CDC+1

Because of these limitations, BMI can be misleading for individuals—especially those with higher amounts of muscle mass or people whose health risk is driven more by where fat is carried (for example, more abdominal/visceral fat). NCBI+1

Why BMI can misclassify muscular people

Someone who trains regularly and carries a lot of lean mass can weigh more for their height, pushing their BMI into “overweight” or even “obese” categories—despite having a healthy (or even low) body fat percentage. This is a known limitation of BMI when applied to athletic or muscular physiques. PMC+1

What a Body Fat calculator adds

A Body Fat calculator (especially circumference-based methods using a tape measure) aims to estimate your body fat percentage, which then allows you to infer your lean body mass vs. fat mass. That matters because body composition is often a more meaningful fitness metric than weight alone—especially for goals tied to performance, strength, metabolic health, and long-term body recomposition. NCBI

It’s still an estimate (not a perfect measurement like a DXA scan), but it provides more actionable insight than BMI alone for many individuals—particularly those who lift, play sports, or have above-average muscle mass. NCBI+1

The best way to use both tools

Think of it like this:

• BMI = fast screening for “where your weight falls relative to height,” but not a direct read on body composition. CDC+1

• Body Fat % = closer look at composition, helping you track whether changes in weight are coming from fat tissue vs. lean mass. NCBI

For the most accurate picture, many clinical and public-health groups recommend using BMI in combination with other markers—such as waist circumference, body composition, and other risk indicators—rather than relying on BMI alone. American Medical Association+1

Important Notice

This calculator provides educational estimates—not medical advice. If you have (or suspect) kidney disease, liver disease, uncontrolled hypertension, or other medical conditions, consult a qualified clinician before adopting a high-protein diet. PMC+1

References

1. CDC — BMI overview and limitations (BMI does not distinguish fat vs. muscle/bone; does not indicate fat distribution). CDC

2. CDC — BMI FAQ (BMI is not a direct measure of body fat; cannot separate fat mass from lean mass). CDC

3. NCBI Bookshelf (2023) — BMI is associated with body fat but is not a direct measure; limitations by age/sex/ethnicity and fat distribution. NCBI

4. American Medical Association (2023) — BMI alone is an imperfect clinical measure; should be used alongside other valid measures. American Medical Association+1

5. Etchison et al. (2011) — Discussion of BMI limitations and misclassification in muscular/athletic populations. PMC