Body Fat Calculator
Estimate body fat with the US Navy method or the BMI formula, plus category, fat mass and lean mass
How to use
- Pick your sex, then enter age, height and weight. Results refresh as you type.
- The Navy method is the default. Add waist and neck, plus hip for women. Men can leave hip alone.
- Switch to the BMI method when you have no tape. Age is all it needs on top of height and weight.
- The result box shows body fat, category, fat mass and lean mass, recalculated on every change.
- For tracking, keep the same method and the same tape routine so your trend stays comparable.
Tapes and accuracy
The two methods disagree a lot. Which one should I trust?
How do I measure the tapes accurately?
Why does the hip measurement matter for women?
Common questions
What body fat percentage is healthy?
How is body fat different from BMI?
Does the result include visceral fat?
Is my input uploaded anywhere?
Screenshots
How to use
- Pick sex and basics
Choose male or female, then enter age, height and weight. The page recalculates as you type, so there is no button to press.
- Add the tape values
The Navy method needs waist and neck, plus hip for women. Keep the tape snug and level, and measure at the marked body points.
- Switch to BMI when needed
Without a tape, use the BMI method. Age, sex, height and weight are enough for a rough body fat estimate in seconds.
- Read results and track
The result box shows body fat, category, fat mass and lean mass. Keep the same method for tracking so the trend stays honest.
About this tool
Body fat percentage is the share of your weight that comes from fat. Everything else is lean mass: muscle, bone, water and organs. A scale cannot tell the two apart, so two people with the same height and weight can look very different. During a cut, body fat tells you whether you are losing fat or muscle, which a scale never shows. Humans also need a baseline of fat for hormones and temperature control. Real measurement options range from DXA scans and underwater weighing to bioelectrical impedance and tape estimates, each with its own cost and accuracy. This page covers the two cheapest routes: tape estimation and the BMI regression.
This tool implements the US Navy circumference equations and a BMI based regression side by side. The Navy path estimates body density from neck, waist and, for women, hip tapes together with height, then converts density into a fat percentage. The BMI path uses BMI, age and sex only. Categories follow American Council on Exercise bands, split by sex, and fat plus lean mass come from applying the percentage to your weight. Everything recalculates on every keystroke, runs locally in the browser and stores your last inputs on this device only.
How it works
The Navy equations come from a 1984 study by Hodgdon and Beckett for the United States Navy. The study regressed body density on circumference tapes, and the Siri equation turns density into fat percentage. The conversion is 495 divided by density minus 450. Men use neck and waist, women use neck, waist and hip, and the coefficients in the formula are the fitted values from that sample. The BMI method comes from Deurenberg and colleagues in 1991 as a linear model on BMI, age and sex. Male counts one and female counts zero. Both are population models, so an individual result carries real error. Cleaner tape technique narrows that error.
Error comes mostly from measurement and from the models themselves. Tape tightness, placement drift and normal daily shifts in your body all feed into the result. Very muscular people with small waists can also sit outside the model. The BMI method cannot separate muscle from fat, so it skews high for lifters. It also drifts for older adults, pregnant women and across ethnic groups. Pregnancy invalidates both routes. Treat the numbers as a trend line rather than a lab report. Remember that this page is an estimate and not medical advice. Confirm health decisions with a doctor and proper testing.
When to use it
- Track a cutting phase
Weigh and tape yourself on the same weekday each week, in the morning before food. Log body fat and lean mass together. If lean mass holds while body fat drops, the deficit is burning fat rather than muscle. That is the real quality signal of a cut and far more useful than scale weight alone.
- Normal weight, soft shape
Many people land inside the normal BMI band while the waist stays large and muscles stay soft. One tape measurement often reveals a body fat percentage above the fitness band. That shifts the goal from losing weight to gaining muscle and controlling sugar, because the scale alone was hiding the problem.
- Baseline for training
Take one set of measurements before starting a strength program, then repeat every four to six weeks. Rising lean mass with flat or falling body fat means the program works. Numbers like these describe body composition changes that a mirror or a scale can easily miss.
Tips & tricks
- Measure in the morning before food and after the bathroom, when water and food shifts matter least.
- Keep the tape snug without pinching and average three readings per site to cut single shot error.
- Log weekly numbers and read the month level trend, since one week of noise means little.
Things to watch
- Watching only the scale
Scale weight can drop from water and muscle while body fat barely moves. Log body fat and lean mass together during a cut. If lean mass falls clearly, raise protein or shrink the calorie gap so the metabolism does not pay the bill.
- Tapes in the wrong place
A waist measured above the navel or a neck pressed against the larynx shifts the whole result. Confirm the anatomical points first, keep the tape level and snug, repeat three times, and use the same routine when different people take the measurements.
- Chasing ultra low fat
Staying under 6% for men or 14% for women for long stretches can disturb hormones and immunity. Symptoms include feeling cold, poor sleep or lost periods. Outside contest season even athletes relax, and the fitness band is the sensible target for everyone else.