Calculate body fat percentage with a tape measure
Estimate your body fat percentage, lean mass and FFMI from your height, weight, and neck, waist and hip measurements.
Short answer
Using just a tape measure, the US Navy method estimates your body fat percentage from your neck and waist measurements, plus hip measurements for women. It is an estimate with a margin of error of a few percentage points: its main use is tracking your progress, measurement by measurement.
Calculator
Validated inputsYour body measurements
Take your measurements in the morning, in a fasted state, ideally under the same conditions each time.
Measurement tip: measure your neck just below the Adam's apple, your waist consistently at the narrowest point or at the navel, and your hips at the widest point. For women, hip circumference is needed for the estimate.
Body fat
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% of body weight
Total fat mass
--
kg of fat
Lean mass
--
kg of fat-free mass
FFMI
--
Fat-Free Mass Index
Normalized FFMI
--
reference 1.80 m
Current weight
--
kg on the scale
Estimated optimal weight
Estimate assuming you maintain your current lean mass and mainly change your body fat level.
Cutting
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Target body fat
Athletic
--
Target body fat
Form
--
Target body fat
Quick interpretation
What you can do with it
- • Track a cutting phase without relying solely on weight.
- • Check whether your bulking phase is mainly increasing lean mass.
- • Compare two periods under the same measurement conditions.
- • Adjust your calories and protein using a more precise reference point than BMI.
How to read the FFMI
FFMI allows you to compare the amount of lean mass relative to height. In practice, a low FFMI often suggests room for muscle growth, while a higher FFMI reflects greater muscular development. However, it remains influenced by body build, training level and measurement quality.
Measurement reference points
Neck circumference
Measure at the narrowest point without pulling the tape tight.
Waist circumference
Measure while relaxed, always in the same place.
Hip circumference
Required for the female estimate.
What the tool measures
The scale doesn't tell you what you're losing or gaining
Body weight lumps everything together: fat, muscle, water, digestive contents. If you do strength training while losing fat, the scales may stay steady even as your shape changes.
This calculator uses the US Navy equation, developed by the Naval Health Research Center and used by the US Navy to monitor its personnel's body composition. It predicts body fat percentage from height and tape measurements of body circumferences. It is less accurate than a DEXA scan or hydrostatic weighing, but free and easy to repeat.
The calculator then estimates your lean mass and your FFMI, a fat-free mass index adjusted for height.
Interpreting the result
Interpret without overpromising
- Body fat percentage: around 14 to 24% is common in active adult men; around 21 to 31% in women. The displayed reference ranges are indicative.
- FFMI: around 17 to 20 in the average man. In athletes who have never taken steroids, it rarely exceeds 25.
- Target body weight: the weight you would have at a target body fat percentage while keeping your current lean mass.
The main benefit is tracking: measure yourself every 2 to 4 weeks, under the same conditions, and watch the trend.
Calculation and assumptions
Methodology
- Men: body fat % = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76, in inches.
- Women: body fat % = 163.205 × log10(waist + hips − neck) − 97.684 × log10(height) − 78.387, in inches.
- Lean mass = weight × (1 − body fat %).
- FFMI = lean mass ÷ height²; normalized FFMI = FFMI + 6.3 × (1.80 − height in m), according to Kouri and colleagues.
Sources consulted
Sources
- Peterson (2015) : History of the U.S. Navy Body Composition program
History of the circumference-based US Navy equations and their limitations.
- Shake et al. (1993) : Predicting percent body fat from circumference measurements
Validation against hydrostatic weighing: the equation makes more errors at extreme values, with 6.8–18% false positives for excess body fat.
- Kouri et al. (1995) : Fat-free mass index in users and nonusers of anabolic-androgenic steroids
Definition of normalized FFMI: in steroid-free athletes, it rarely exceeds 25.
Scope of validity
Limitations to know
- The equation is less reliable in people who are very lean, very muscular or have severe obesity.
- A one-centimeter error in waist circumference changes the result by around one percentage point: always measure at the same spot, in the morning.
- It does not apply to children or during pregnancy.
- Body fat percentage alone cannot assess health: waist circumference alone is already a useful indicator of cardiometabolic risk.
Frequently asked questions
Before using the result
How do you calculate your body fat percentage?
Using a tape measure, the US Navy method uses your height and neck and waist circumferences, plus hip circumference for women. More precise methods include DEXA or hydrostatic weighing.
What is a normal body fat percentage?
In adults, around 14 to 24% for men and 21 to 31% for women are common values. Athletes go lower, and normal values rise slightly with age.
Is the US Navy method reliable?
It gives a rough estimate, with an error of a few percentage points, larger at the extremes. It's mainly useful for tracking changes with measurements taken the same way.
What is FFMI?
It's lean mass divided by height squared, like a BMI for lean mass. In steroid-free athletes, normalized FFMI rarely exceeds 25.
Is a bioelectrical impedance scale more accurate?
Not necessarily. Consumer scales vary with hydration and the time of measurement. As with a tape measure, what matters is the trend under the same conditions.