Body Fat Calculator
Find your estimated body fat percentage with just a tape measure — plus your fat mass, lean mass, and fitness category.
Measure your body fat percentage
Tape-measure estimate — typically within ±3–4 percentage points of a DEXA scan. Track the trend, not a single reading.
How It Works: The U.S. Navy Method
You don't need calipers, a lab, or an expensive scan to get a useful body fat estimate. The U.S. Navy method needs only a flexible tape measure and your height. It was developed in 1984 by researchers Hodgdon and Beckett at the Naval Health Research Center, who needed a way to check the body composition of thousands of sailors quickly and cheaply.
The idea is simple: where your body stores fat changes your shape, and your shape can be measured. For men, the formula compares waist minus neck against height; for women it uses waist plus hips minus neck. Those ratios feed a logarithmic equation that estimates your body density, and the Siri equation converts density into a body fat percentage:
The original research validated the method against hydrostatic (underwater) weighing — the gold standard of its day — with a correlation around r = 0.90. Later comparisons against DEXA scans put the typical error at about ±3–4 percentage points. That's not lab precision, but it's remarkably good for a $3 tape measure, and it's more than accurate enough to track whether your training and diet are working.
How to Measure Correctly (Tape Guide)
A sloppy measurement can swing the result by several points, so take 30 seconds to do it right:
- Use a flexible tape (fabric or vinyl), snug against skin but not compressing it. Measure over bare skin, not clothing.
- Neck: just below the Adam's apple, tape flat and level. Look straight ahead — don't crane your neck up or down.
- Waist (men): horizontally around the abdomen at navel level, relaxed — breathe out normally, don't suck in.
- Waist (women): at the narrowest point of the torso, usually just above the navel.
- Hips (women): around the widest part of the buttocks, feet together.
- Measure in the morning before eating, and always at the same time of day — your waist can vary 1–2 cm through the day.
- Take each measurement twice and average them. If the two differ by more than 1 cm, measure a third time.
Body Fat Percentage Categories Chart
The American Council on Exercise (ACE) reference ranges below are the most widely used. Note that healthy ranges differ a lot between men and women — women naturally carry more essential fat for hormonal health.
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25%+ | 32%+ |
For most people, the "fitness" range is a great long-term target: lean enough to look and feel athletic, with enough reserve to stay healthy, perform, and (for women) keep hormones regular. Dropping into the athlete range is a legitimate goal for competitors, but staying there year-round takes serious dietary discipline.
Worked Example
Profile: man, 178 cm, waist 95 cm, neck 40 cm, weight 80 kg
Step 1 — girth term: waist − neck = 95 − 40 = 55 cm.
Step 2 — body density: 1.0324 − 0.19077 × log₁₀(55) + 0.15456 × log₁₀(178) = 1.0324 − 0.3320 + 0.3478 = 1.0482 g/mL.
Step 3 — Siri conversion: 495 ÷ 1.0482 − 450 = 22.2% body fat ("Average" for men).
Step 4 — breakdown: fat mass = 80 × 0.222 = 17.8 kg; lean mass = 80 − 17.8 = 62.2 kg.
Limitations vs DEXA Scans
An honest tool tells you what it can't do. The Navy method is a statistical estimate, not a measurement:
- ±3–4 point error is normal. If it says 22%, your true value is probably somewhere between 18% and 26%. Use it to track change over weeks, not to declare an exact number.
- Unusual builds fool it. Very muscular people (thick necks, small waists) can read artificially low; people who store fat mostly in the limbs rather than the trunk can read off in the other direction.
- It can't see visceral fat. Two people with the same waist can carry different amounts of the deeper abdominal fat that matters most for health.
- DEXA isn't perfect either — hydration, meal timing, and even the machine brand shift DEXA readings by a couple of points. No method short of a research lab is exact.
The practical approach: measure the same way, at the same time of day, every 2–4 weeks. If the trend is falling while your strength holds, your plan is working — whatever the absolute number says.
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Frequently Asked Questions
How accurate is the Navy body fat method?
The Navy method was validated against hydrostatic weighing with a correlation around r = 0.90. Compared with DEXA scans, expect a typical error of about ±3–4 percentage points. That makes it excellent for tracking change over time, though not lab-grade for a single exact number.
What is a healthy body fat percentage?
For men, the "fitness" range is roughly 14–17% and "average" is 18–24%. For women, "fitness" is about 21–24% and "average" is 25–31%. Women need more essential fat (10–13%) than men (2–5%) for hormonal health. Athletes sit lower, but staying extremely lean year-round is hard and not necessary for health.
How do I measure for body fat percentage?
Use a flexible tape on bare skin: neck just below the Adam's apple, waist at the navel (men) or narrowest point (women), and hips at the widest point (women). Keep the tape snug but not tight, measure in the morning before eating, and average two readings.
Is body fat % better than BMI?
For most people, yes. BMI only uses height and weight, so it calls muscular people "overweight" and misses "skinny fat" — normal weight with high body fat. Body fat percentage measures what you actually care about: how much of you is fat versus lean tissue. BMI is still useful as a quick population-level screening tool.
How often should I check my body fat?
Every 2–4 weeks is plenty. Body fat changes slowly, and daily fluctuations in water and food will just add noise. Measure the same way at the same time of day, and judge your plan by the trend over a month or more.
Sources
- Hodgdon JA, Beckett MB (1984) — Prediction of percent body fat for U.S. Navy men and women (methodology summary)
- Frontiers in Physiology (2022) — Circumference-Based Predictions of Body Fat Revisited (peer-reviewed)