ReckonBoxHow this calculator works
Body fat percentage is the share of your weight that is fat rather than muscle, bone, and water. It tells you something a bathroom scale can't: whether the weight you carry is lean. This tool uses the US Navy tape method, which estimates body fat from a few circumference measurements and your height. No calipers, no scan, no appointment.
Measure your neck below the larynx, your waist at the navel, and, for women, your hips at the widest point. The calculator feeds those into the Navy equation and reports your category, your fat mass, your lean mass, your fat-free mass index, and a second estimate from the BMI method for comparison.
Measure twice. A single centimetre at the waist moves the result by roughly half a point.
The US Navy formula
The method takes the base-10 logarithm of your measurements, in centimetres.
- Men:
%BF = 495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450 - Women:
%BF = 495 / (1.29579 - 0.35004 x log10(waist + hip - neck) + 0.22100 x log10(height)) - 450
Women add the hip measurement because fat distributes differently, and the extra term keeps the estimate honest.
You'll also see a second version of this equation elsewhere, written as 86.010 x log10(waist - neck) - 70.041 x log10(height) + 36.76 with the measurements in inches. Calculator.net, LibreTexts and RippedBody publish that one; Omni Calculator and this page publish the version above. They look nothing alike and they're the same method. Run both across a range of bodies and they land between 0.03 and 0.30 percentage points apart, so if you've found a discrepancy between two calculators, this isn't it.
What the accuracy figure actually is
Nearly every body fat calculator says the Navy method is accurate to "about 3 to 4 percent" and then declines to say where that came from. Eight of the ten pages checked for this page on 8 September 2026 did exactly that.
The number has a source. Per BodyTypeCalc's measurement accuracy guide, it traces to the Hodgdon-Beckett formulas developed at the Naval Health Research Center in 1984, validated against underwater weighing on 1,022 Navy personnel, giving a standard error of estimate of 3.52% for men and 3.72% for women.
Standard error of estimate is worth reading slowly. It means your true body fat sits within about 3.5 points of the number above roughly two thirds of the time. On a reading of 19.8%, that's a real range of roughly 16% to 23%. The tape is good. It isn't a measurement.
The disagreement nobody mentions
Three of the ten pages checked make three incompatible claims about how the Navy method treats muscular bodies, which is a problem for exactly the people most likely to be running a body fat calculator:
| Source | Claim |
|---|---|
| BodyTypeCalc | Underestimates by 2.5 to 4.5% for muscular individuals with a BMI above 27 |
| RippedBody | Overestimates for someone with well-developed abs, reading "higher than reality" |
| Bony to Beastly | Neither. It "works well for people who exercise and lift weights" |
Under, over, and fine. None of the three acknowledges that anyone says otherwise.
The reason they can all be describing something real is that a circumference method doesn't know what the circumference is made of. Thick abdominal muscle widens the waist and reads as fat. A heavily trained neck widens the neck and reads as leanness, because the formula subtracts neck from waist. Which effect wins depends on which one you've trained harder, so the direction of the error isn't fixed the way a simple bias would be.
What that means in practice: if you lift seriously, this number tracks direction well and settles very little on its own. The change over eight weeks carries more information than any single reading.
A worked example
The defaults: a man, 180 cm, 80 kg, 38 cm neck, 90 cm waist, age 30.
| Output | Value |
|---|---|
| Body fat, Navy method | 19.8% |
| Fat mass | 15.8 kg |
| Lean mass | 64.2 kg |
| FFMI | 19.8 |
| BMI | 24.7 |
| Body fat, BMI method | 20.3% |
| Ideal for age 30 | 12.7% |
The two methods land half a point apart on this body, which is what you'd expect on someone close to average. Put a muscular body through the same pair and they diverge hard, because the BMI method reads muscle as weight and the tape reads shape. When your two numbers separate, that gap is the signal.
Body fat categories
The American Council on Exercise classification is the common standard, and it splits by sex because women carry more essential fat.
| Category | Men | Women |
|---|---|---|
| Essential fat | 2 to 5% | 10 to 13% |
| Athletes | 6 to 13% | 14 to 20% |
| Fitness | 14 to 17% | 21 to 24% |
| Average | 18 to 24% | 25 to 31% |
| Obese | 25% and above | 32% and above |
Healthy ranges drift up a little with age, so read your number alongside your age. What the BMI screen says on its own is a useful companion reading.
Your ideal, and the distance to it
The result shows an ideal body fat for your age, from the Jackson and Pollock reference, and the fat you'd shed to reach it. The ideal climbs with age for both sexes, because a little more fat is normal and healthy as you get older. A 30 year old man sits near 12.7 percent, a 30 year old woman near 19.3 percent.
The "fat to lose" figure assumes your lean mass stays put. That assumption is doing a lot of work, and holding muscle through a deficit is largely a protein question, which is where splitting your calories into daily macros gets decided.
What FFMI tells you
Fat-free mass index is your lean mass divided by your height in metres squared. It does for muscle what BMI does for weight, and it's the better progress marker once you're training. Add muscle and your FFMI climbs even when your weight holds flat.
None of the ten calculators checked for this page reports it. Calculator.net comes closest on outputs, printing fat mass, lean mass, an age-based ideal and a BMI comparison, and still leaves FFMI out. Omni Calculator has an FFMI tool sitting on a different page, so you'd run two calculators and carry a number between them.
A natural, drug-free lifter usually tops out around 22 to 25. Values well clear of that are a clue that something beyond training is involved.
What this calculator can't do
A tape measure reads circumference, so it can't see where the fat sits. Visceral fat around the organs carries different health implications from subcutaneous fat, and no circumference method distinguishes them.
DEXA, hydrostatic weighing and a Bod Pod are the reference methods. They cost money and a trip, and they're what you'd use if a number needed to be right rather than close.
Results here are estimates and not a diagnosis or medical advice. For decisions about your health, training or diet, talk to a doctor or a registered dietitian.
Frequently asked questions
How accurate is the US Navy body fat method? Per BodyTypeCalc's accuracy guide, the underlying Hodgdon-Beckett formulas were validated at the Naval Health Research Center in 1984 against underwater weighing on 1,022 people, with a standard error of estimate of 3.52% for men and 3.72% for women. That means a reading of 19.8% carries a true range of roughly 16% to 23%, so treat it as a good estimate.
Does the Navy method over or underestimate body fat for muscular people? The sources contradict each other. BodyTypeCalc reports it underestimating by 2.5 to 4.5% above a BMI of 27, RippedBody reports it reading higher than reality for someone with well-developed abs, and Bony to Beastly reports it working well for people who lift. A circumference method can't tell muscle from fat, so thick abs push the number up while a trained neck pushes it down, and which wins depends on your training.
What is a healthy body fat percentage? For men the fitness range is roughly 14 to 17 percent and average is 18 to 24. For women fitness is about 21 to 24 percent and average is 25 to 31. Some essential fat is required for health, which is why the scale doesn't start at zero.
What is FFMI? Fat-free mass index is your lean mass divided by height squared. It measures muscularity the way BMI measures weight, and it's a better progress marker for lifters than body fat alone. A natural lifter usually tops out around 22 to 25.
Why do two calculators show different Navy formulas? Because two algebraic forms of the same equation circulate. One is written as 495 divided by a density expression, the other as 86.010 x log10(waist - neck) minus 70.041 x log10(height) plus 36.76 in inches. Checked across a range of bodies they agree to within 0.30 percentage points.
Why does my BMI-method number differ from the tape number? The BMI method infers fat from weight and height, so it reads muscle as weight and marks a muscular person high. The tape reads your actual shape. On an average body the two land close, half a point apart on this page's defaults, and they separate as muscularity rises.
Should I use this instead of a DEXA scan? They answer different questions at different prices. DEXA, hydrostatic weighing and a Bod Pod are the reference methods for a number that has to be right. A tape tracks change over weeks for free, which is what most people actually need.
Sources
Fetched 8 September 2026.
- BodyTypeCalc measurement accuracy guide, for the Hodgdon-Beckett validation figures and the bias claim
- US Navy body fat estimation formula (Physiology, LibreTexts)
- Calculator.net body fat calculator, which publishes both unit forms of the equation
- Omni Calculator Navy body fat, for the density form of the formula
- RippedBody, for the reading-higher-than-reality claim
- Bony to Beastly, for the works-well-for-lifters claim
- American Council on Exercise: percent body fat