How accurate is the US Navy formula?

Within ±3% of a DEXA scan for most people. Good enough for tracking progress over time. Not accurate for very lean or very obese individuals — those should use DEXA, BodPod, or hydrostatic weighing.

Why does the formula need different measurements for men and women?

Body fat distribution differs. Men tend to carry abdominal fat (waist measurement captures it). Women's body fat is more spread (waist + hip captures it). The formula was developed from US Navy data and validated against hydrostatic weighing.

What's a healthy body fat percentage?

Men: 6-13% (athletes), 14-17% (fit), 18-24% (average), 25%+ (obese). Women: 14-20% (athletes), 21-24% (fit), 25-31% (average), 32%+ (obese). Essential fat (3-5% men, 10-13% women) is the minimum for health.

US Navy formula · tape measure · local

Estimate body fat % from tape measurements

US Navy formula using neck, waist (and hip for women) plus height. No skin calipers needed — a soft measuring tape is enough. Runs entirely in your browser.

Body Fat Calculator

Body fat

16.9%

Fitness

Fat mass

12.7 kg

weight × %fat

Lean mass

62.3 kg

remainder

Status

ready

live

input · measurements

AgeWeight (kg)Height (cm)Neck (cm)Waist (cm)

output · body fat

Body fat (US Navy formula)

16.9%

Fitness

Fat mass: 12.7 kgLean mass: 62.3 kg

Quick Start

How to estimate body fat

The US Navy formula uses circumference measurements at the neck, waist (and hip for women) plus height to estimate body density, then converts via Siri's equation.

01
Pick sex & units
Female form adds hip; units convert internally to cm/kg.
Inputs are treated as UTF-8 throughout the pipeline and validated client-side as you type — nothing dispatches until the value is well-formed.
forminpututf-8client-sidevalidated
sex · textmale · set
```
male · metric
```
loaded · parsedclient-side
02
Measure neck
Just below the larynx, looking straight ahead.
Each toggle re-computes through a memoised hook, so flipping options never resets your input and the UI stays snappy on large inputs.
neckoptionslivememoisedtoggles
neck · memoisedcm · live
```
38 cm
```
options set · memoisedlive
03
Measure waist
At the navel (men) or narrowest point (women).
Randomness — where used — comes from crypto.getRandomValues, never Math.random. Heavy work sits in useMemo.
waistcomputeinstantlocaldeterministic
waist · instantcm · computed
```
85 cm
```
computed · instantinstant
04
Read result
Percentage and category appear color-coded.
Copy writes through navigator.clipboard and downloads use a Blob — zero network traffic. Same input plus same options always yield the same bytes.
bfcopydownloadzero-networkreproducible
ok · copiedfit · copied
```
16% — Fitness
```
ready · ready to shipsuccess

Pro tips

Pro tips for everyday utilities

Reach for crypto.getRandomValues

Cryptographically random for IDs, tokens, salts. Math.random is for animation and games, not security.

Sort IDs lexicographically

UUID v4 has no time ordering. Use ULID or UUID v7 if you need sorted IDs.

Mock data with seeds

Faker-style libraries can be seeded so tests are reproducible. Embed the seed in the fixture file.

Reference cards beat memory

Cheatsheets for Vim, Git, Docker, Linux signals are faster than man pages when you’re mid-flow.

Browser APIs vary

Touch events, gamepads, and clipboard differ across vendors. Feature-detect before use.

Persist in the right place

localStorage for user preferences, sessionStorage for transient state, IndexedDB for structured data.

Use Cases

When body fat % is useful

Cuts, bulks, recomp, healthy aging.

Cutting phase

Track shrinking waist while preserving muscle. Random values come from crypto.getRandomValues — cryptographically secure for tokens, salts, and session IDs. Math.random is deterministic enough for animations but predictable enough to be a security bug.

CSPRNG-backed

Bulking phase

Watch for crossing into 'average' bf %. Pair with hashids, ULID, or UUID v7 when you need both random and sortable — useful for primary keys, public-facing IDs, and event log ordering without a sequence column.

reproducible

Body recomp

BMI stays flat while bf % drops. Output is deterministic given the same seed, so fixtures and tests stay reproducible across CI runs and contributor machines. Embed the seed in the fixture file for full replay.

snappy

Endurance training

Confirm runners aren't dropping into 'essential'. Heavy work runs through useMemo so the UI stays snappy on large inputs. Same input + same options yields identical bytes — the kind of stability snapshot tests rely on.

feature-detected

Healthy aging

Lean mass loss after 40 — watch the trend. Storage tools persist to the right backend: localStorage for user preferences, sessionStorage for transient state, IndexedDB for structured data. Pick the right shelf and quota worries disappear.

deterministic

Coaches

Quick weekly check-in without skin calipers. Browser feature detection: clipboard, fullscreen, gamepad, and Web Bluetooth APIs gracefully fall back when unavailable. No surprise white screens on Safari or older Firefox.

production-ready

FAQ

Frequently asked questions

How accurate is the Navy formula?

About ±3% vs DEXA on most builds. Less reliable on extreme physiques. It's a tracker for change, not a clinical reading.

Why measure circumference and not skinfolds?

Tape-measure inputs are easier to do alone and reproduce. The formula was specifically calibrated against hydrostatic weighing.

Should I tense or relax?

Relax. Stand upright with arms at sides — same posture each time.

Why is hip only used for women?

Women carry more fat in the hips/glutes; the female formula includes hip circumference, the male formula doesn't.

Is anything sent to a server?

No. Calculation runs entirely in your browser.

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