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Body Fat Calculator

Name: ForgePlug
Author: ForgePlug

ForgePlug's Body Fat Calculator is a premium body-composition dashboard. Estimate your body-fat percentage with the US Navy circumference method or the Jackson-Pollock skinfold method (3-site or 7-site, with the Siri equation), compare against a BMI-based estimate, and instantly see lean body mass, fat mass, and your ACE health category. Interactive charts — a live gauge, body-composition donut, category progress bar, and method comparison — update as you type. Includes an interactive measurement guide, smart insights, education, and export (PDF, CSV, copy, print). Works fully offline after first load, supports metric & imperial units, and never uploads your data.

Your Measurements

Estimates body fat from neck, waist, and (for women) hip circumferences compared with height. The standard field method used by the US military.

Units

Sex

Age(years)Optional — for BMI comparison only

Weight(kg)

Height(cm)

Circumference Measurements

Neck(cm)

Waist(cm)

All calculations happen entirely inside your browser. Your measurements are never uploaded or stored on our servers.

Body Fat

17.3%

Average

Lean Body Mass

64.5kg

83% of weight

Fat Mass

13.5kg

17.3% of weight

Healthy Category

Average

ACE classification (male)

This calculator provides an estimate using the U.S. Navy circumference method. Results are typically accurate within approximately ±3–4% compared with laboratory methods such as DEXA scans. It is intended for educational purposes only and should not be considered a medical diagnosis. Consistent measurements over time are generally more meaningful than a single reading.

Body Fat Gauge

17.3%

average category

Body Composition

17.3%

body fat

Lean Mass

83%

Fat Mass

17.3%

Lean vs Fat (kg)

64.5 / 13.5 kg

ACE Category Position

Essential Fat2–5%Athletes6–13%Fitness14–17%Average18–24%Obese25–60%

Method Comparison

US Navy MethodCurrent

17.3%

Direct measurement estimate

BMI Estimate

20.2%

From BMI, age & sex

Ideal (Fitness)

—

Fitness range: 14–17%

Athlete

—

Athlete range: 6–13%

The US Navy Method and BMI estimate differ by 2.9 points because BMI cannot distinguish muscle from fat — body composition can vary widely at the same BMI.

Smart Insights

Within the Average range

Your estimated body fat of 17.3% is in the Average category for your sex. Consistent tracking over time is more meaningful than any single reading.

Lean body mass

Lean body mass represents approximately 83% of your total body weight (≈64.5 kg).

Method comparison

The BMI estimate reads 2.9 points higher than the Navy method — expected when muscle mass is higher than average, since BMI cannot distinguish muscle from fat.

Consistency beats perfection

Body-fat estimates are most useful when tracked consistently over time with the same technique. If your goal is fat loss, prioritise preserving lean mass through protein and strength training.

Educational estimate

This tool provides an estimate (US Navy circumference, Jackson-Pollock skinfold, or BMI). It is educational and never a substitute for a medical diagnosis.

Export Report

Summary Preview

Body Fat Calculator — Summary
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Method: US Navy Method
Sex: male · Age: 30 years
Height: 178 cm · Weight: 78 kg
Neck: 38 cm · Waist: 86 cm

Body fat: 17.3%
Category (ACE): Average
Fat mass: 13.5 kg
Lean body mass: 64.5 kg

BMI comparison
BMI: 24.6 (Healthy)
BMI body-fat estimate: 20.2%
Difference (Navy − BMI): -2.9 points

All calculations happen entirely inside your browser. This is educational information, not medical advice.

Measurement Guide

Accurate, consistent measurements are the key to a trustworthy estimate.

1Stand straight with shoulders relaxed and head facing forward.
2Place the tape just below the larynx (Adam's apple).
3Keep the tape level all the way around, sloping slightly downward at the front.
4Record the measurement at the end of a normal breath.

💡 Don't pull the tape tight enough to compress the skin.

💡 Look straight ahead — tilting your chin changes the reading.

Example: most adults measure 30–45 cm (12–18 in).

Your Measurements

Estimates body fat from neck, waist, and (for women) hip circumferences compared with height. The standard field method used by the US military.

Units

Sex

Age(years)Optional — for BMI comparison only

Weight(kg)

Height(cm)

Circumference Measurements

Neck(cm)

Waist(cm)

All calculations happen entirely inside your browser. Your measurements are never uploaded or stored on our servers.

Health Education

Understand what these numbers mean — and why they matter.

Body-fat percentage is the mass of fat tissue divided by total body mass. It gives a far clearer picture of health than weight alone.

Fat is essential — it stores energy, insulates the body, and supports hormones. Essential fat levels are roughly 2–5% for men and 10–13% for women.

Beyond essential levels, excess body fat is associated with increased cardiometabolic risk, while extremely low levels can impair hormonal and immune function.

Frequently Asked Questions

Everything you need to know about body fat, the Navy method, and accuracy

How accurate is the US Navy body fat method?

The US Navy circumference method is typically accurate to within approximately ±3–4% compared with laboratory methods such as DEXA scans or hydrostatic weighing for most adults. Accuracy depends heavily on taking consistent, level measurements with the tape snug but not compressing the skin. It's an excellent tool for tracking changes over time, not a lab-grade diagnosis.

What is a healthy body fat percentage?

Using official ACE classifications: essential fat is 2–5% for men and 10–13% for women; athletes typically sit at 6–13% (men) and 14–20% (women); fitness levels are 14–17% (men) and 21–24% (women); average is 18–24% (men) and 25–31% (women); and obese begins at 25% (men) and 32% (women). Individual health depends on far more than a single number.

How do I measure my neck, waist, and hips correctly?

Neck: measure just below the larynx (Adam's apple), with the tape sloping slightly downward at the front. Waist (men): measure at the navel with normal breathing — don't suck in. Waist (women): measure at the narrowest point, which is not necessarily the belly button. Hips (women): measure at the widest point around the glutes with the tape parallel to the floor. Keep the tape snug but not tight.

Why does the BMI estimate differ from the Navy method?

The BMI formula (1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4) only uses height, weight, age, and sex — it assumes an average body composition. The Navy method measures actual circumference at key sites. They differ because BMI can't see muscle: an athletic person will typically score higher via BMI than via circumference measurements.

What is the Jackson-Pollock skinfold method?

The Jackson-Pollock method uses skinfold calipers to measure fat thickness at standardized sites — three (men: chest, abdomen, thigh; women: triceps, suprailiac, thigh) or seven (add midaxillary, subscapular, and the remaining sites). The sum is plugged into published equations (Jackson & Pollock 1978; Jackson, Pollock & Ward 1980) to predict body density, which is then converted to body-fat percentage with the Siri equation: %BF = (495 ÷ density) − 450. It's one of the most widely validated field methods for body composition.

Do I need skinfold calipers to use this method?

Yes — the Jackson-Pollock method requires a skinfold caliper (a few dollars online). Pinch a fold of skin and fat at each site with your thumb and forefinger, place the caliper jaws about 1 cm from your fingers, and read the thickness after about 2 seconds. Take each site 2–3 times and use the average. Sites are always measured on the right side of the body.

Which method is most accurate: Navy, skinfold, or BMI?

For most adults, the Jackson-Pollock skinfold method and the US Navy circumference method are both within roughly ±3–4% of laboratory methods like DEXA when measured correctly. The BMI-based estimate is the least accurate because it assumes average body composition and can't see muscle. Track with one method consistently over time — changes matter more than a single reading.

Is my body fat data private?

Absolutely. Every calculation runs entirely in your browser. Your measurements are never uploaded, stored on our servers, or shared with anyone. You can optionally save your session to your own device's localStorage so your numbers persist between visits — and clear them at any time.

Does this calculator work on mobile?

Yes — the Body Fat Calculator is designed mobile-first. On small screens you get a guided flow with a sticky live-results bar and tabbed navigation; on desktop the full analytics dashboard appears side-by-side. Metric and imperial units are both fully supported.

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