Body Fat Calculator
Estimate body fat percentage using the Navy formula
100% free. Runs entirely in your browser - your files and data never leave your device and nothing is uploaded to any server.
About this tool
Body fat percentage tells you something BMI cannot: how much of your weight is actually fat rather than muscle, bone and water. Two people at identical BMI can have very different compositions, which is the well-known limitation of weight-and-height measures. This calculator offers two approaches. The US Navy method uses tape measurements - neck and waist for men, plus hips for women - and is the more accurate of the two, typically within 3-4 percentage points of a clinical scan when measured carefully. The BMI method needs only your BMI and age, which makes it quick but noticeably rougher, since it is still ultimately derived from weight and height. Neither replaces a DEXA scan or hydrostatic weighing. What both do well is track change: if you measure the same way each time, the trend over weeks is genuinely informative even when the absolute number carries a margin of error. Measurement technique matters more than which method you pick. Everything runs in your browser.
How to use this tool
- Choose your methodNavy for accuracy if you have a tape measure; BMI for a quick estimate if you do not.
- Select your genderThe formulas differ because fat distribution differs, and using the wrong one gives a meaningfully wrong result.
- Enter your measurementsNavy needs height, waist at the navel and neck below the larynx, plus hips at the widest point for women.
- Read the percentage and categoryThe result is shown against the standard ranges for your gender, from essential fat through to obese.
Key features
- Distinguishes fat from lean mass, which weight and BMI alone cannot do.
- Two methods, so it works with or without a tape measure.
- Shows the category ranges for your gender, so the number has context.
- Runs in your browser - body measurements are personal and are never transmitted.
- Free with no account, unlike most body composition services.
Common uses
- Tracking composition change during a training or weight-loss programme.
- Understanding why your BMI reads high despite regular strength training.
- Setting a realistic body composition target rather than a weight target.
- Checking progress when scale weight has stalled but shape is changing.
- Getting a baseline before starting a fitness plan.
Tips for better results
- Measure at the same time of day, ideally first thing in the morning before eating. Waist measurements vary noticeably through the day.
- Keep the tape snug but not compressing the skin, and level all the way round. Inconsistent tension is the main source of error.
- Measure the waist at the navel and the neck just below the larynx - the Navy formula assumes those specific points.
- Track the trend, not one reading. A single measurement carries error; four weeks of readings taken the same way does not.
- Prefer the Navy method where possible. The BMI-based estimate is convenient but distinctly rougher.
Mistakes to avoid
- Measuring the waist at the trouser line rather than the navel, which the formula does not expect.
- Pulling the tape tight enough to compress, which understates every measurement.
- Comparing a reading from this method against a DEXA result and treating the difference as an error - they are different techniques with different margins.
- Measuring at different times of day and reading the fluctuation as real change.
- Chasing very low body fat. Essential fat exists for a reason, and going below it carries genuine health consequences.
Frequently asked questions
It's a reasonably reliable estimation method for general tracking purposes, though it's less precise than clinical methods like DEXA scans or hydrostatic weighing - it's best used for tracking trends over time rather than as an exact clinical measurement.
Typically neck and waist circumference for men, with hip circumference added for women, plus height - measured with a flexible tape measure.
Because fat distribution differs physiologically, and healthy ranges differ with it. The female formula includes hip circumference and the healthy ranges sit higher - essential fat is around 10-13% for women against 2-5% for men. Selecting the wrong option gives a meaningfully wrong result.
Navy, if you have a tape measure. It uses actual body circumferences and is typically within 3-4 percentage points of a clinical scan. The BMI method is quicker but rougher, since it is still derived from weight and height rather than measuring your shape.
Broadly, 14-24% for men and 21-31% for women covers the fitness and average ranges, with athletes below that. These are guidelines rather than targets, and they shift with age. A doctor can advise on what is appropriate for you specifically.
No. Everything is calculated in your browser and nothing is transmitted - which matters for personal body measurements.
Yes. Body Fat is free for normal use with no account required, and ToolBox does not add a watermark to your result.
Most tools in this category run in your browser so the file stays on your device. If a tool needs a temporary server job, files are handled for that job only and are not kept as a lasting archive.
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