Your scale weight is the sum of everything you’re made of — muscle, bone, organs, water and fat, added together and reported as one number. Body fat percentage splits that number into the part that’s fat and the part that isn’t, which is usually the more useful question.
Body Fat Percentage
The US Navy tape method. Measure relaxed, at the end of a normal breath, tape snug but not compressing.
Method: US Navy circumference equations (Hodgdon & Beckett). Typical error is about 3–4 percentage points against a DEXA scan. Measurement technique matters more than the math — take each girth three times and average.
Estimates only — general education, not medical advice. Talk with your doctor before changing how you eat or train. More tools at whatshouldiweigh.com.
Why composition beats weight
Two people at 170 pounds and the same height can look and function entirely differently. One at 12% body fat carries about 150 pounds of lean mass. The other at 30% carries 119. Same scale reading, different bodies, different metabolic profiles.
It also explains the most frustrating experience in early training: someone starts lifting, eats sensibly, and the scale barely moves for six weeks. They’re gaining muscle and losing fat at similar rates. Weight is flat and composition is improving — and only one of those two things shows up on a bathroom scale.
How the tape method works
The calculator uses the US Navy circumference equations, developed by Hodgdon and Beckett for the Navy’s body composition assessment. It needs your height, neck and waist — and for women, hips.
The equations estimate body density from the ratio between those girths, then convert density to a fat percentage. The logic is that fat deposits preferentially at the waist while the neck stays relatively stable, so the gap between them carries information.
Expect it to land within about 3–4 percentage points of a DEXA scan for most people. That’s respectable for a tape measure and a piece of arithmetic.
Measuring properly matters more than the maths. A 1 cm error in your waist can shift the result by 1–2 points.
- Neck: just below the larynx, tape horizontal, looking straight ahead. Don’t flex.
- Waist: at the navel for men, at the narrowest point for women. Stand relaxed, exhale normally, don’t brace or suck in.
- Hips: at the widest point of the buttocks, tape level all the way round.
- Take each measurement three times and average. Measure at the same time of day, ideally first thing.
How the methods compare
| Method | Typical error | Cost | Notes |
|---|---|---|---|
| DEXA scan | ±1–2% | High | Closest to a reference standard; also reports bone density |
| Hydrostatic weighing | ±2% | High | Accurate, unpleasant, hard to find |
| Bod Pod | ±2–3% | Medium | Air displacement; quick and comfortable |
| Skinfold calipers | ±3% | Low | Only as good as the practitioner |
| Navy tape method | ±3–4% | Free | What this calculator uses |
| Bioelectrical impedance | ±5–8% | Low | Smart scales and handheld units; heavily affected by hydration |
The pattern worth noticing: the free method is not far behind the cheap gadget, and the smart scale that gives you a decimal place is the least reliable thing on the list. Precision in the display isn’t accuracy in the measurement.
Healthy ranges
These follow the American Council on Exercise reference categories.
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletic | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| High | 25%+ | 32%+ |
Women sit roughly 8–10 points higher than men at equivalent fitness. This is normal physiology, not a deficiency — it reflects reproductive and hormonal requirements.
Essential fat is a floor, not a target
The essential category is the minimum your body needs to function. Fat isn’t inert storage; it insulates nerves, forms part of every cell membrane, cushions organs and is required for hormone production and the absorption of vitamins A, D, E and K.
Pushing below it has consequences. In women, very low body fat commonly disrupts menstrual function and, over time, bone density. In both sexes it can suppress immune function, testosterone, thyroid output, sleep quality and mood.
Physique athletes who reach single-digit figures hold them for days around a competition, not as a way of living. The stage-ready look isn’t a maintainable state, and treating it as a goal is how people get into trouble.
What actually moves the number
Body fat percentage is a ratio, so it moves when either side changes.
Losing fat requires a sustained energy deficit. Nothing else does it — no food, supplement or exercise burns fat in the absence of one.
Building muscle lowers the percentage without losing a pound of fat, by increasing the denominator. Resistance training plus adequate protein is the mechanism.
Doing both at once is possible for beginners, people returning after a layoff, and those with higher starting body fat. For a trained lifter near their genetic ceiling it becomes very difficult, which is why experienced athletes alternate phases.
Adequate protein matters throughout. During a deficit it’s what protects lean mass — without it, a meaningful share of the weight you lose is muscle, which raises your body fat percentage even as the scale falls.
Reading your result sensibly
Measure every two to four weeks, not daily. The tape method isn’t precise enough to resolve small changes, and week-to-week noise from hydration and gut contents will swamp real signal.
Track the trend across a few measurements. If it’s moving in the direction you want over two months, the plan is working — regardless of what any single reading said.
Common questions
How accurate is the Navy method? Within about 3–4 percentage points of DEXA for most people. Your measurement technique contributes more error than the equation does.
What’s a good body fat percentage? For men, 14–17% is the fitness range and 18–24% is average. For women, 21–24% and 25–31% respectively. All of those are healthy; the lower bands take considerably more effort to hold.
Why does my smart scale give a different number? Bioelectrical impedance sends a small current through the body and infers composition from resistance, which hydration affects heavily. Readings shift with how much you’ve had to drink, when you last ate and whether you’ve trained. Its error range is roughly double the tape method’s.
Can I lose fat from one specific area? No. Spot reduction doesn’t work — fat mobilises from the whole body, and the distribution pattern is largely genetic. Where you lose it first isn’t something you get to choose.
Should I use body fat or BMI? Body fat tells you more, and takes more effort to measure. BMI is a fast screen. Using both, plus a waist measurement, gives a fuller picture than any one of them alone.
The calculators on this site are educational estimates, not medical advice. If you’re concerned about your body composition, or find yourself measuring compulsively, that’s worth raising with a doctor.