Measurement map
Use the same landmarks every time
Keep the tape level, snug without compressing skin, and use the exact landmark required by the chosen equation.
What the percentage means
An estimated 25% body-fat result means the method predicts that about one quarter of total mass is fat and the rest is fat-free mass. Fat-free mass includes muscle, bone, organs, water, and other tissue—not only muscle.
The result does not identify where fat is stored, how fit someone is, or whether a particular percentage is medically appropriate. It also cannot separate a real change from a measurement change unless the method is repeated carefully.
How the circumference method works
The body-fat calculator implements Navy-style equations using height and selected circumferences. The male-equation version combines waist minus neck with height. The female-equation version combines waist plus hip minus neck with height. Logarithms transform those measurements into a percentage estimate.
Female-style equation: 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
The implementation converts centimeters to inches before applying these published constants. All circumference inputs must use the same unit.
A repeatable tape-measure protocol
- Use a flexible, non-stretch tape on bare skin or thin clothing.
- Stand naturally and breathe normally; do not pull the abdomen in.
- Keep the tape horizontal and snug without indenting the skin.
- Measure neck just below the larynx, following the method’s instructions.
- Measure the required waist landmark consistently. Do not switch between the narrowest point and another site across sessions.
- When the equation requires hips, measure the widest point with the tape level.
- Repeat each measurement. If repeats differ meaningfully, reposition and try again.
Time of day, meal volume, posture, breathing, tape tension, and who holds the tape can all alter the result.
Worked male-equation example
Example inputs
Height 70 in, waist 36 in, neck 15 in.
- Waist − neck = 21 in
- 86.010 × log₁₀(21) = 113.72
- 70.041 × log₁₀(70) = 129.23
- 113.72 − 129.23 + 36.76 = about 21.3%
Small differences may appear from intermediate rounding. The live tool retains full precision until the displayed result.
Why methods disagree
| Method | Useful feature | Important limitation |
|---|---|---|
| Circumference equation | Low cost and repeatable at home | Sensitive to landmarks and body proportions |
| Skinfolds | Portable and can track sites | Requires trained technique and prediction equations |
| Bioelectrical impedance | Fast and widely available | Hydration and device algorithms affect output |
| DXA | Detailed regional composition estimate | Cost, access, calibration, and model assumptions |
| Air displacement or underwater weighing | Laboratory-based density estimate | Access and assumptions about tissue density |
Different outputs do not necessarily mean one device is broken. Each method estimates composition through a different model.
Track the method, not the decimal
For trend tracking, keep the tape, landmarks, conditions, and person measuring as consistent as possible. Compare changes over multiple sessions rather than reacting to a single decimal. Record the raw measurements as well as the calculated percentage; they make it easier to identify a misplaced tape or entry error.
This calculator is not designed for pregnancy and may be a poor fit for unusual body proportions, major recent fluid shifts, or situations where body measurements could worsen disordered eating.
Frequently asked questions
How accurate is a tape body-fat estimate?
It can be useful for a repeatable trend, but individual error may be meaningful. Technique and body proportions affect the result.
Why did a smart scale give a different percentage?
Smart scales generally use bioelectrical impedance and a device-specific model, while this calculator uses circumferences. They estimate through different inputs.
Should I round measurements?
Record the tape measurement as consistently as you can and let the calculator round the final output. False precision is still possible even with precise inputs.
Can body fat fall while scale weight stays similar?
Yes. Fat, lean tissue, water, and glycogen can change in different directions.
Sources
References and further reading
- Naval Health Research Center — Body-fat prediction for U.S. Navy men (archived technical report)
- Naval Health Research Center — Body-fat prediction for U.S. Navy women (archived technical report)
Method and sources reviewed: July 23, 2026