Body Fat Calculator
Estimate body-fat percentage from height and tape measurements using the historical Navy circumference equation. Add weight for estimated fat mass, fat-free mass and BMI context.
Circumference estimate only — this is not a scan, a health diagnosis or an official military body-composition assessment.
Use the calculatorEstimate Body Fat from Tape Measurements
Adult circumference estimates with optional mass calculations.
Your estimate will appear here
Choose an equation group and enter the relevant tape measurements. Weight is optional.
Body-fat estimate
A circumference-based prediction, without a health-category label.
Optional mass breakdown
Add weight for estimated fat and fat-free mass; neither is a muscle measurement.
Measurement sensitivity
Compare the arithmetic effect of a 1 cm change at the abdomen or waist site.
Your circumference estimate
estimated body fat · no diagnosis or fitness classification
Medical disclaimer: This is an educational circumference estimate, not a scan, diagnosis or personalised body-fat target. It cannot assess visceral fat, muscle mass or a medical condition. It is not intended for children, pregnancy, breastfeeding, eating disorders or supervised body-composition assessment. Seek individual advice from a qualified healthcare professional when clinical accuracy or nutrition decisions matter.
What a 1 cm tape difference does to the calculation
| Abdomen / waist input | Body-fat estimate | Change from entered value |
|---|
Check the inputs used in the equation
| Equation input | Metric value | Inches used internally |
|---|
The measurement site is part of the formula
A natural waist and an abdomen measured at the navel are different inputs.
| Measurement | Where to measure | Avoid this mismatch |
|---|---|---|
| Height | Standing upright without shoes. | Do not use a height in metres in a centimetre field. |
| Neck · both groups | Just below the larynx, following the neck contour without compressing skin. | Do not measure directly across the larynx or tighten the tape. |
| Male abdomen | At the navel after normal exhalation, with the abdomen relaxed and tape level. | Do not substitute the narrowest waist or hold the abdomen in. |
| Female waist | At the narrowest natural waist, with the abdomen relaxed and tape level. | Do not automatically use the male navel site. |
| Female hip | Widest circumference over the buttocks, with tape level. | Do not use a trouser size or measure only the hip bones. |
Repeat the procedure, not just the number
Use a flexible, non-stretch tape. Reposition it and repeat each measurement; investigate a noticeable discrepancy instead of choosing the smallest reading. Keep sites, posture, tape tension and measurement conditions consistent.
Enter your representative reading in centimetres or inches. This tool does not apply military tape rounding, age standards or pass/fail rules.
Neck size is an equation input
A larger neck lowers the mathematical result when other inputs stay fixed. That does not prove the difference is muscle or that health has improved. The equation cannot identify the tissue at a measurement site.
Which circumference equation is being calculated?
The historical Navy direct-percentage form uses base-10 logarithms and inches.
| Equation group | Direct body-fat percentage equation |
|---|---|
| Male | 86.010 × log₁₀(abdomen − neck) − 70.041 × log₁₀(height) + 36.76 |
| Female | 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387 |
The 2023 research methods print both equations with centimetre-to-inch conversion. This tool uses that direct-percentage form, not a separate body-density/Siri calculation. Results from different equation forms need not be identical.
Male tape measurements
Height 175 cm, abdomen 90 cm and neck 38 cm give approximately 20.8% body fat. With weight 75 kg, estimated fat mass is 15.6 kg and fat-free mass is 59.4 kg.
Female tape measurements
Height 165 cm, natural waist 75 cm, hip 100 cm and neck 33 cm give approximately 29.7%. With weight 65 kg, estimated fat mass is 19.3 kg and fat-free mass is 45.7 kg.
Weight changes mass, not percentage
Keeping Example 1’s tape measurements but entering 85 kg leaves the equation percentage at about 20.8%. Calculated fat mass becomes 17.7 kg. This arithmetic cannot establish what tissue caused a real weight change.
Fat-free mass is not muscle mass
The mass calculations use weight × estimated fat fraction and weight minus estimated fat mass. The remainder includes water, organs, bone and other non-fat tissues, not just skeletal muscle. They inherit the error of the percentage estimate.
Weight is optional and is not part of the circumference percentage. BMI is also optional context and cannot validate the body-fat estimate.
A tape estimate cannot certify a change
A 2023 training study found that circumference estimates did not accurately capture body-composition changes against DXA in its study population. That is not a universal error percentage, but it is a reason to avoid treating a small calculated change as measured fat loss.
This tool does not label results as athletic, obese, healthy or an ideal target. Individual health interpretation needs more than a tape estimate.
A mathematically valid result is not evidence that this equation fits your body. It does not assess visceral fat, regional fat distribution, hormone treatment, swelling or unusual body composition. Use professional assessment when a health, nutrition or official eligibility decision depends on the result.
Helpful resources
Related body-measurement tools and primary research for checking the method.
Useful resources
Sources support verification, not endorsement or a clinical review of this page. This tool does not implement current military eligibility rules.
Questions about tape-based body-fat estimates
Measurement sites, optional weight and limits of interpretation.
Is this a body-fat measurement or an estimate?
An estimate from tape measurements. It is not a DXA scan, an individual accuracy guarantee or a diagnosis. The number of decimal places does not establish biological precision.
Which Navy equation form does this tool use?
The historical direct-percentage circumference equations, with base-10 logarithms and all dimensions converted to inches. It does not use a separate density/Siri calculation or apply military rounding and pass/fail standards.
Where should I measure the abdomen or waist?
For the male equation, measure the relaxed abdomen at the navel. For the female equation, use the narrowest natural waist and also the widest hip circumference over the buttocks. These sites should not be substituted for one another.
Why is weight optional and why is there no age field?
Weight is used only for estimated fat mass, fat-free mass and BMI context. Neither weight nor age is a term in this circumference percentage equation. The adult-use checkbox limits the tool’s intended audience; it is not a clinical assessment.
Does fat-free mass tell me my muscle mass?
No. It is the calculated remainder after estimated fat mass is subtracted from body weight. It includes water, organs, bone and other non-fat tissues as well as muscle.
Is the 1 cm comparison an error margin?
No. It changes only the abdomen or waist input and holds the other values fixed. The change is reported in percentage points. It is a mathematical sensitivity example, not a confidence interval or proof of fat loss.
Can this show my ideal body-fat target or military eligibility?
No. This page does not assign health categories, personalised targets or official military pass/fail status. Seek qualified advice when clinical or official decisions depend on body composition.
Are my measurements saved?
This calculator script processes entries in browser memory. It does not upload entries or save them to browser storage. Other website scripts and services are outside the calculator’s scope.