Not medical advice. This calculator does arithmetic on numbers you enter, for general education. It does not diagnose, treat, or recommend a dose or treatment for any condition, and it does not replace an evaluation by a licensed physician. A tape-measure estimate is not a body composition scan and it does not assess your health risk. Talk to your own clinician before acting on any result.
A tape measure and the U.S. Navy circumference equations give a usable body fat percentage from your height, neck, and waist, plus your hips for women. The equations were built against underwater weighing, the laboratory standard of the 1980s, and they turn four minutes with a tape into a number you can track for free.1,2
Add your weight and the calculator below splits it into fat mass and lean body mass, the figure the FFMI calculator needs.
Body Fat Percentage Calculator
Measure with a flexible tape against the skin, without compressing it, and round to the nearest half inch or centimeter. Weight is optional and adds fat mass and lean body mass.
Estimated body fat
…
Fat mass
…
Lean body mass
…
The Navy equations estimate body fat from circumferences. A result can differ from a DEXA scan or a bioimpedance scale by several percentage points, so compare it only with other tape measurements taken the same way.
How the Navy Tape Method Works
In 1984, researchers at the Naval Health Research Center measured the circumferences of Navy personnel and compared them with body fat measured by underwater weighing. The regression equations they published predict body fat from height, neck, and waist in men, and from height, neck, waist, and hips in women.1,2 The calculator prints the equation it applied under each result.
The neck measurement stands in for frame and lean mass, and the waist carries most of the fat signal. That is why the waist dominates the result: for a 5 ft 10 in man with a 15 in neck, each 2 in off the waist moves the estimate by roughly 4 percentage points.
| Waist at the navel | Estimated body fat | Fat mass at 185 lb | Lean body mass at 185 lb |
|---|---|---|---|
| 36 in | 21.3% | 39 lb | 146 lb |
| 34 in | 17.5% | 32 lb | 153 lb |
| 32 in | 13.4% | 25 lb | 160 lb |
Height 70 in and neck 15 in held constant. Because the waist does most of the work, measuring it the same way every time does the most to make the estimate repeatable.
Taking the Four Measurements
- Height. Barefoot, standing straight against a wall.
- Neck. Just below the larynx, with the tape sloping slightly down toward the front.
- Waist. For men, horizontally at the level of the navel. For women, at the narrowest point of the abdomen.
- Hips, women only. At the widest point of the buttocks, with feet together.
Measure at the end of a normal breath out, without pulling the tape tight. Take each measurement three times and use the average, and measure at the same time of day each time.
Body Fat, Lean Mass, and FFMI
With weight added, the calculator multiplies weight by body fat to get fat mass and subtracts it to get lean body mass. Lean body mass includes muscle, bone, organs, and the water held in all of them, so it is a broader figure than muscle mass alone.
Lean body mass scaled to height becomes fat-free mass index. Enter the body fat from this page into the FFMI and body composition calculator to see that figure, and the waist-to-height ratio calculator uses the same waist measurement for a central-fat screen.
Choosing a Measurement Method
A tape costs nothing and repeats well when one person measures the same way each time. DEXA scans and air-displacement pods are more accurate for a single reading. Bioimpedance results can sit several percentage points from DEXA, and in a study of 3,655 paired measurements the agreement varied with BMI.3 The FFMI calculator page compares those methods side by side.
A practical approach is one scan for an accurate baseline, then tape measurements every few weeks to follow the direction between scans. Monthly tape readings between scans show the direction of change.
Tracking a Change Over 12 Weeks
Body fat falls slowly enough that weekly readings mostly record measurement noise. Measure every 4 weeks, and read three readings in a row as the trend. A waist that drops while weight holds steady usually means fat lost and lean mass held, which is the result a good training block produces. If the waist and the scale both rise, the calorie surplus is larger than the training can use, and the calorie and macro calculator resets the target.
Body Composition and Hormones
Testosterone helps set how much lean mass a man carries, and testosterone treatment in men with low testosterone has increased lean mass and reduced fat mass in pooled observational data.4 The relationship runs both ways, because fat tissue converts testosterone to estradiol, so higher fat mass and lower testosterone tend to travel together.5 A body fat figure that keeps climbing on a sound training and nutrition plan is a reason to look at a lab panel, and our page on low testosterone symptoms lists the other signs worth noting.
The Opt Take on Body Fat Percentage
A tape measurement repeated the same way each month shows the direction your body composition is moving, and that direction is most useful beside lab results. At Opt Health, body composition care starts with a blood panel of 55+ biomarkers and a 1:1 review with an Opt Health physician. The physician looks for hormonal and metabolic causes when body composition stalls.
The Opt app connects Apple Watch, Garmin, and Fitbit data, and the Opt Performance Score updates bi-weekly from wearables, check-ins, and quarterly labs. Bring your monthly body fat readings to each physician consultation, and labs every 3 to 4 months put the trend in context.
Get started: Begin with Opt Health, where a physician reads your labs, builds your plan, and adjusts it over the loop.
Frequently Asked Questions
The Navy equations were built against underwater weighing and give a reasonable estimate for most adults, but a single reading can differ from a DEXA scan by several percentage points. The method is most useful for tracking change when one person measures the same way each time.
Men measure horizontally at the level of the navel. Women measure at the narrowest point of the abdomen. Keep the tape level, take the reading at the end of a normal breath out, and do not compress the skin.
Multiply your weight by your body fat percentage to get fat mass, then subtract fat mass from your weight. At 185 lb and 17.5 percent body fat, fat mass is about 32 lb and lean body mass is about 153 lb.
Bioimpedance scales estimate body fat from an electrical current, and hydration, food, and time of day all shift the reading. The tape method and the scale use different inputs, so compare each method only with its own earlier readings.
Every 4 weeks is enough to see a real change. Body fat moves slowly, so weekly readings mostly capture measurement noise rather than progress.
When body composition will not move on a sound plan, a physician looks at the hormones behind it. An Opt Health membership pairs a 55+ biomarker panel with a physician who reads it beside your measurements.
References
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men from body circumferences and height. Report No. 84-11. San Diego, CA: Naval Health Research Center; 1984.
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy women from body circumferences and height. Report No. 84-29. San Diego, CA: Naval Health Research Center; 1984.
- Achamrah N, Colange G, Delay J, et al. Comparison of body composition assessment by DXA and BIA according to the body mass index: a retrospective study on 3655 measures. PLoS One. 2018;13(7):e0200465.
- Corona G, Giagulli VA, Maseroli E, et al. Testosterone supplementation and body composition: results from a meta-analysis of observational studies. J Endocrinol Invest. 2016;39(9):967-981.
- Kelly DM, Jones TH. Testosterone and obesity. Obes Rev. 2015;16(7):581-606.
This content is for informational purposes and does not replace evaluation, diagnosis, or treatment by a qualified medical professional.
This article is part of our guide to body composition. See the doctor-guided body composition program at Opt Health.
Start Today
Your health, your terms. Discover how personalized care can transform not just the way you feel, but how you live.