Body Fat Percentage Calculator

What this Body Fat Percentage Calculator does

This English-language body fat percentage calculator applies the U.S. Navy circumference method — developed by Hodgdon and Beckett in 1984 — to estimate your body fat index from a handful of tape measurements. Enter your height, neck, and waist (plus hip, for women), and the tool returns your body fat estimate, lean body mass, and fat mass, then plots your result against the ACSM healthy-range bands for your sex. Because women carry higher essential fat (10–13%) than men (2–5%) for reproductive biology, the calculator uses sex-specific formulas and separate category thresholds throughout. Notably, the WHO BMI classification and body fat percentage measure different things: a muscular athlete may carry high BMI but low body fat, while a sedentary person can show normal BMI with an unhealthy fat rate — a gap this body fat index calculator helps you see. 100% client-side — your data never leaves your browser. No uploads, no tracking, no server logs.

Features

Navy formula with transparent math. Uses the validated Hodgdon-Beckett circumference equations, cited back to the original 1984 Naval Health Research Center technical report, so you know exactly what's being calculated.
Metric and imperial inputs. Switch between centimeters/kilograms and inches/pounds with a single toggle. All unit conversions happen client-side — no page reload required.
Sex-specific ACSM category bands. Results are mapped against ACSM 10th-edition ranges — essential, athlete, fitness, acceptable, and obese — separately for males and females, reflecting real biological differences in fat distribution.
Lean body mass and fat mass breakdown. Optionally enter your weight to see lean body mass and fat mass in kg or lb — useful as inputs for TDEE or protein-target calculations. Pair this with the [TDEE calculator](/en/tdee-calculator/) for a complete picture.
Prominent YMYL disclaimer. The tool surfaces the ±3.7% standard error of estimate upfront, distinguishes screening from clinical diagnosis, and directs you to a healthcare professional for any medical concern.
Privacy-first, zero server calls. Every calculation runs in JavaScript in your browser tab. Nothing is transmitted, stored, or logged — not your measurements, not your results.

How to use the Body Fat Percentage Calculator

Five inputs are all you need for a body fat estimate. Measure with a flexible tape and enter the values below.

Choose your unit system. Select Metric (cm, kg) or Imperial (in, lb) — the labels and placeholder values update automatically.
Select your sex. The hip circumference field appears only for female, matching the Navy formula's sex-specific inputs. The category thresholds also switch accordingly.
Take and enter your measurements. Measure neck just below the larynx; waist at the navel (men) or narrowest point (women); hip at the widest point. Enter height in the same unit system. For example, a 5 ft 10 in male with a 15 in neck, 33 in waist would type 15 and 33 in the respective fields.
Add weight (optional). If you enter your weight, the calculator derives lean body mass and fat mass in addition to your body fat percentage — useful when setting protein targets with the [Macro Calculator](/en/macro-calculator/).
Read your results. The tool displays body fat %, your ACSM category, and — if weight was provided — lean mass and fat mass. Compare against the reference table to understand where your fat index sits relative to healthy ranges.

Common use cases

Tracking a body recomposition program. Scale weight can stay flat while fat drops and muscle grows. A weekly body fat estimate gives you a more honest signal than mass alone when you're in a recomp phase.
Estimating lean mass for nutrition targets. Protein recommendations are often expressed as grams per pound of lean body mass. Use this calculator to get that number, then feed it into a macro or TDEE calculation without needing a gym's body-fat scale.
Personal health screening alongside BMI. Neither BMI nor body fat percentage alone tells the full story. Checking both — the [BMI calculator](/en/bmi-calculator/) and this body fat index calculator — gives you a more complete screening picture between annual checkups.
Education and fitness coaching. Coaches working with clients in San Francisco, Dublin, or anywhere else can walk through the ACSM ranges live, explaining why essential fat, athlete, and fitness categories differ by sex without needing specialized equipment.
Comparing methods without lab costs. If you're curious how circumference-based body fat estimates compare to what a DEXA scan might show, this tool lets you calculate a baseline for free. The displayed standard error (±3.7%) sets honest expectations.

Frequently asked questions

How accurate is the Navy circumference method?

The Hodgdon-Beckett formula has a standard error of estimate of about ±3.7% body fat compared to hydrostatic (underwater) weighing, which is the reference standard. DEXA scans are another clinical gold standard. That means your true body fat percentage could realistically be 3–4 percentage points higher or lower than what this body fat estimate shows. For tracking trends over time — using consistent measurement technique — the method is reliable even if the absolute number carries some uncertainty.

Is my data private? Does anything get sent to a server?

No data leaves your device. This calculator runs entirely in your browser — there are no API calls, no analytics payloads containing your measurements, and no server logs. You can even disconnect from the internet after the page loads and it will still calculate. Your height, circumferences, and weight are never transmitted anywhere.

Why is body fat percentage different from BMI?

BMI divides your weight by the square of your height — it says nothing about how much of that weight is fat versus muscle. A powerlifter may register as 'obese' by BMI while carrying 10% body fat. Conversely, a sedentary person can have a 'normal' BMI but a high fat rate — sometimes called skinny-fat. The IRS uses BMI bracket tables only as a flag for further screening, never as a sole diagnostic, which illustrates exactly why it shouldn't be used in isolation. Body fat percentage is a more direct measure of body composition.

Where exactly should I measure my waist and neck?

For men, measure your waist at the level of the navel, not at the narrowest point. For women, measure at the narrowest point of the torso, typically just above the belly button. Neck circumference is measured just below the larynx (Adam's apple) for both sexes. Hip circumference (women only) goes at the widest point of the hips and buttocks. Consistent tape placement across measurements matters more than absolute precision — small placement errors compound into larger fat-estimate errors.

What body fat percentage is considered healthy?

According to ACSM guidelines, the 'acceptable' range is 18–24% for men and 25–31% for women. 'Fitness' ranges are 14–17% (men) and 21–24% (women). Women's essential fat alone sits at 10–13% due to hormonal and reproductive fat stores, compared to 2–5% for men — so a woman at 15% body fat is well into athletic territory, not 'low' by male standards.

Can I use this calculator if I'm an athlete, pregnant, or elderly?

The Navy formula was validated on a military population and may be less accurate for athletes with significant muscle mass, pregnant or postpartum women, older adults with age-related muscle loss, or people with notable edema. The circumference method will still produce a number, but treat it with extra skepticism. For these groups, DEXA or a clinical body composition assessment is a better fit.