Health
Body Fat Calculator
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How to use it
Using the body fat calculator
- 01
Measure with a flexible tape
Neck below the larynx; waist at the navel for men, narrowest point for women; hips at the widest point (women).
- 02
Keep the tape snug, not tight
Parallel to the floor without compressing skin; repeat each measurement twice and average.
- 03
Track the trend, not the decimal
Monthly remeasurements reveal direction reliably even where absolute precision is ±3%.
Good to know
Why circumference beats BMI for composition
Because tape measures respond to where fat actually sits, the Navy method distinguishes a muscular waist-thick torso from a soft one far better than weight-for-height ratios ever can. It costs nothing and takes ninety seconds.
Reading your number honestly
Essential fat sits around 2–5% for men and 10–13% for women; athletic populations commonly measure 6–13% and 14–20%; average healthy ranges extend to roughly 24% and 31% respectively. Lower is not better once essential fat is approached.
Improving measurement reliability
- Same time of day, ideally mornings before food.
- Same measurer or careful mirror positioning each session.
- Average of two or three takes per site, recorded verbatim.
How it's calculated
The math behind this calculator
Men: %BF = 495 / (1.0324 − 0.19077·log₁₀(waist−neck) + 0.15456·log₁₀(height)) − 450
Women: %BF = 495 / (1.29579 − 0.35004·log₁₀(waist+hip−neck) + 0.221·log₁₀(height)) − 450
(circumferences in inches)The Hodgdon & Beckett (1984) circumference equations, adopted by the US Navy, predict body density from log-transformed tape measurements taken at standardized sites, then convert density to fat percentage via the Siri-equivalent 495/D − 450 relation.
Logically, a bigger waist raises estimated fat while thicker necks (muscular, not fatty tissue) lower it. Measurements must follow the same landmarks each time for trend-tracking to mean anything.
Assumptions & limitations
- Adults with typical fat distribution; accuracy degrades at extremes.
- Typical error versus DEXA is ±3–4 percentage points.
- Hip field exists for both sexes but enters only the female equation.
Worked example
A 180 cm man with a 38 cm neck and 85 cm waist measures around 9.8% body fat by the Navy method; the athletic end for his frame.
FAQ
Frequently asked questions
- Why do men not need the hip measurement?
- The male regression uses only waist and neck because male fat pattern concentrates centrally; female equations incorporate hips to capture the gluteofemoral depot. The hip box stays visible so women have everything in one form.
- How close is this to a DEXA scan?
- Group-level correlation is good (r ≈ 0.8–0.9) but individuals can differ from DEXA by 3–4 points either way; use it for trends.
- Can I be too lean?
- Yes. Sustained measurement near essential-fat levels disrupts hormones, immunity and (in women) menstrual function. Performance suffers before health collapses; reverse course early.
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