Body fat percentage estimates how much of your mass is fat versus lean tissue. Tape-based methods are not lab scans, but the U.S. Navy circumference method (Hodgdon–Beckett) is a transparent, widely used estimate from height plus a few girths. This guide walks through the same centimeter equations used on our Body Fat Calculator. It is general fitness education for healthy adults, not medical advice or a clinical diagnosis.
The formulas (metric centimeters)
Our calculator converts imperial inputs to centimeters, then applies these Navy-style density equations and the classic Siri-style conversion %BF = 495 ÷ density − 450.
Men (height, neck, waist at the navel):
circumference = waist − neck(cm; must be positive)density = 1.0324 − 0.19077 × log₁₀(circumference) + 0.15456 × log₁₀(height)% body fat = 495 ÷ density − 450
Women (height, neck, waist at the narrowest point, hips at the widest):
circumference = waist + hip − neck(cm; must be positive)density = 1.29579 − 0.35004 × log₁₀(circumference) + 0.221 × log₁₀(height)% body fat = 495 ÷ density − 450
Official Navy assessment tables often use an inches form of the same Hodgdon–Beckett relationship. Either path is an estimate; stick to one unit system and measure consistently.
How to measure
Keep the tape snug but not compressing soft tissue. Measure after a normal exhale; do not suck in.
| Site | Men | Women |
|---|---|---|
| Neck | Just below the larynx (Adam’s apple) | Same |
| Waist | At the navel | At the narrowest point of the torso |
| Hip | Not used | At the widest point of the hips/glutes |
| Height | Standing tall, no shoes | Same |
Optional bodyweight lets the calculator report approximate fat mass and lean mass:
fat mass ≈ bodyweight × (%BF ÷ 100)lean mass ≈ bodyweight − fat mass
ACSM-style adult categories
These match the category bands shown on the calculator (population labels, not prescriptions):
| Category | Men | Women |
|---|---|---|
| Essential fat | < 6% | < 14% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Above average | 25%+ | 32%+ |
Navy retention cutoffs used in service policy are a different topic from these ACSM-style fitness labels. For training, treat the percentage as context next to strength progress, waist trends, and how you feel in the gym.
Worked example (men)
Height 180 cm, neck 38 cm, waist 85 cm, optional weight 80 kg:
circumference = 85 − 38 = 47density ≈ 1.0324 − 0.19077 × log₁₀(47) + 0.15456 × log₁₀(180) ≈ 1.062%BF ≈ 495 ÷ 1.062 − 450 ≈ 16.1%→ fitness band for adult men- Fat mass ≈
80 × 0.161 ≈ 12.9 kg; lean mass ≈67.1 kg
Small tape errors change the log terms — remeasure a few times and average if the number jumps around.
How to use the body fat calculator
- Open the Body Fat Calculator.
- Choose sex and metric (cm, kg) or imperial (in, lb).
- Enter height, neck, and waist (plus hip for women). Optionally add weight for fat/lean mass.
- Read the estimated percentage, category label, and mass split if you entered weight.
- Pair the estimate with training and nutrition tools rather than chasing a single decimal.
Useful companions:
- BMI Calculator and How to Calculate BMI — height/weight screening when you want a simpler index
- Protein Intake Calculator and How Much Protein Do You Need — bodyweight × g/kg targets
- One-Rep Max Calculator and How to Calculate Your One-Rep Max for load planning
- RPE & Rest Calculator and How to Use RPE for Working Sets for set difficulty
- Simple programs: 5x5 for Beginners or Push/Pull/Legs Split Explained
- Library anchors such as Full Squat, Deadlift, Bench Press, and Pull-Up
Why the tape can disagree with the mirror (or BMI)
Circumference equations assume average fat distribution. People who store more fat internally, carry unusual muscle on the neck/waist, or measure inconsistently can see several percentage points of error versus DEXA, Bod Pod, or hydrostatic weighing. BMI can flag “overweight” for a muscular lifter while Navy %BF sits in a fitness band — or the reverse for someone with lower scale weight but a larger waist. Use trends over weeks, not one reading, and keep training progressive.
Common mistakes
- Measuring waist at different landmarks each time (navel vs. narrowest vs. belt line)
- Pulling the tape tight enough to dent soft tissue
- Comparing a morning fasted tape to an evening post-meal tape as if both were “true”
- Treating ACSM-style labels as medical cutoffs
- Ignoring strength, sleep, and protein while obsessing over a 0.5% change
Limitations
- Typical error versus lab methods is often on the order of a few percentage points
- Equations are for adults; they are not pediatric tools
- Sex-specific equations matter — do not mix the male and female formulas
- This page and calculator do not diagnose obesity, eating disorders, or metabolic disease
- Calorie planning tools (TDEE / macros) are better for diet math once those pages are live — until then, protein targets and training structure are the most actionable next steps on this site
Quick links
- Body Fat Calculator — U.S. Navy / Hodgdon–Beckett circumference method
- BMI Calculator — WHO kg/m² screening estimate
- How to Calculate BMI — formula, categories, and lifter caveats
- Protein Intake Calculator — bodyweight × g/kg presets
- How Much Protein Do You Need — companion g/kg guide
- How to Calculate Your One-Rep Max — Epley and Brzycki load estimates
- How to Use RPE for Working Sets — effort rating for strength training
- How to Use Pace and Heart Rate Zones — easy cardio alongside lifting
- How to Read Exercise Instructions — demos, steps, and library anatomy
- Push/Pull/Legs Split Explained — sample weekly lifting structure
- 5x5 for Beginners — short full-body strength template
- Best Barbell Exercises — squat, hinge, press, and pull shortlist
- Best Dumbbell Exercises — press, pull, squat, hinge, and carry shortlist
- All fitness calculators
- All training guides
- Exercise library