How to Calculate Body Fat Percentage
You can measure body fat in a lab or estimate it at home. The real question is how close each method gets to the truth, so this guide scores them against a DXA scan using a study that tested seven methods on the same 437 people.
To calculate your body fat percentage at home, use skinfold calipers, a tape measure (the Navy method) or a BMI-based formula; for a true measurement, get a DXA scan. In a study of 437 college students, underwater weighing, the Bod Pod, a lab impedance analyzer and 7-site skinfolds all landed within 1.0–1.6 points of DXA on average, while a hand-held impedance device was off by 4.9 points.
Source: Burns, Fu & Constantino, PLOS ONE 2019. Try the home methods in our body fat calculator.
Every method compared with DXA
Comparisons often mix different studies, people and reference methods; the numbers below come from one study where every method was tested on the same people against the same DXA scanner. The researchers set a strict test: a method counted as “equivalent” to DXA only if its average result stayed within 10% of the DXA average, which in this group meant ±2.7 body fat points.
| Method | Where | What it measures | Average difference from DXA* | Verdict* |
|---|---|---|---|---|
| DXA scan | Clinic or research lab | X-ray scan separating fat, lean tissue and bone | Reference method in this comparison | CDC calls it the most widely accepted method |
| Underwater (hydrostatic) weighing | University or sports lab | Body density from your weight under water | 1.0 points | Equivalent to DXA (±10%) |
| Bod Pod (air displacement) | Lab, some gyms | Body density from the air you displace | 1.6 points | Equivalent to DXA (±10%) |
| Lab impedance analyzer, 4 electrodes | Clinic or lab | Electrical resistance through hand and foot electrodes | 1.6 points | Equivalent to DXA (±10%) |
| Skinfold calipers, 7 sites | Home or gym, trained measurer | Pinch thickness at seven sites, converted to density | 1.4 points | Equivalent; lowest individual error |
| Smart scale, foot-to-foot | Home | Electrical resistance between the feet | Not reported in text | Not equivalent at ±10% (equivalent at ±15%) |
| Hand-held impedance device | Home | Electrical resistance between the hands | 4.9 points | Not equivalent; highest individual error |
| Near-infrared device | Gym | Light reflected from the upper arm | Not reported in text | Not equivalent, even at ±15% |
| Tape measure (Navy method) | Home | Neck, waist and hip circumferences | Not in this study | Wide individual differences in a smaller study |
| BMI formula (Deurenberg) | Home | BMI, age and sex only | Not in this study | Standard error 4.1 points in its original study |
* From Burns et al. 2019 unless noted. The foot-to-foot scale and near-infrared device were tested, but the study reports their averages only in a table image, so we give the verdict without a number. Tape and BMI rows: see the at-home section.
Two caveats matter. The volunteers were young, fit exercise-science students, so errors may be larger in older people or people with more body fat; the authors also found that every method’s error grew as body fat went up. And an average difference of 1.5 points can still hide bigger misses for individuals: even the best home method, skinfolds, had an average individual error of 11.7% of the DXA value.
Lab methods
DXA scan
A low-dose X-ray scan that separates fat, lean tissue and bone. CDC calls it the most widely accepted method of measuring body composition and uses it in its national survey (CDC). Settings matter: CDC’s survey setting adds 5% of lean mass to fat mass.
Underwater weighing
You exhale fully while seated under water; your underwater weight gives body density, which an equation converts to body fat. Closest to DXA in the study (1.0 point), but slow and demanding.
Bod Pod
You sit in a sealed egg-shaped chamber that measures the air you displace, giving body density. 1.6 points from DXA on average in the study, and far less demanding than going under water.
At-home methods
Skinfold calipers
The best home method in the study: 1.4 points from DXA on average with 7 sites, measured by trained students. A 3-site version is quicker; our calculator uses the Jackson–Pollock 3-site equations. The catch is skill: results depend on the person pinching.
Tape measure (Navy method)
Neck and waist (plus hips for women) and your height. Easy to do, but not tested in the DXA study above. In a smaller study of 69 active young adults, the 95% limits of agreement between the military tape method and underwater weighing, impedance and skinfolds were wider than ±3.5 points, so individual results could differ by more than that either way (conference abstract).
BMI formula
Deurenberg’s formula needs only BMI, age and sex. Its standard error is 4.1 points, and it slightly overestimates body fat in people with obesity (Deurenberg et al., 1991). Fine as a first rough guess.
Run the home methods side by side. Our calculator does tape, calipers and the BMI formula, shows how far apart they land, and ranks the result against CDC body scans.
Body fat calculatorHow accurate are body fat scales?
Less accurate than skinfolds or lab methods, according to this study. Bathroom scales and hand-held devices send a small electrical current through your body and estimate fat from the resistance; the foot-to-foot scale and the hand-held device both missed the ±10% cut-off, and the hand-held device was off by 4.9 points on average, with the largest individual error of all methods (21.9%). The lab analyzer that uses four electrodes on a hand and foot did much better (1.6 points). Hydration also moves impedance readings: the authors note that dehydration can push estimated body fat up and overhydration can push it down.
A scale is still useful for trends if you weigh in under the same conditions every time. Just don’t compare its number with a reading from a different method or device.
How to get a reliable reading
The study used a simple pre-test routine that you can copy for any method: no large meal in the 2 hours before, no vigorous exercise in the 12 hours before, no alcohol in the 48 hours before, no more than two glasses of water, finished at least 2 hours before, and an empty bladder. For skinfolds, each site was measured twice, with a third measurement if the first two differed by more than 2 mm.
Beyond that: use the same method, the same device and the same time of day; take each measurement more than once; and judge changes over weeks, not day to day.
What your number means
Once you have a number, compare it with what US adults your age actually measure on CDC’s DXA scans, and with the provisional healthy ranges for your age, on our body fat percentage chart. For why BMI and body fat can disagree, see body fat vs BMI.
FAQ
How do I calculate my body fat percentage at home?
Use skinfold calipers (most accurate at home if you’re practiced), a tape measure with the Navy equations, or the Deurenberg BMI formula. Our body fat calculator runs all three.
What is the most accurate way to measure body fat?
A DXA scan is the most widely accepted method. In a study using DXA as the reference, underwater weighing (1.0 point), skinfolds (1.4), the Bod Pod (1.6) and a 4-electrode lab analyzer (1.6) came closest.
How accurate are body fat scales?
In that study, a foot-to-foot scale and a hand-held device were not equivalent to DXA; the hand-held device was off by 4.9 points on average. Use a scale for trends under identical conditions rather than as an exact number.
Why do different methods give different numbers?
They measure different things (density, electrical resistance, skinfold thickness, circumferences) and convert them with different equations, so readings can differ by several points. Errors also grow at higher body fat.
How much body fat should I have?
Provisional healthy ranges matched to BMI are 8–19.9% for men and 21–32.9% for women aged 20–39, a little higher at older ages. See the body fat percentage chart.
Sources
- Burns RD, Fu Y, Constantino N. Measurement agreement in percent body fat estimates among laboratory and field assessments in college students: use of equivalence testing. PLOS ONE 2019;14(3):e0214029.
- Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness: age- and sex-specific prediction formulas. Br J Nutr 1991;65:105–14.
- National Center for Health Statistics. NHANES Dual-Energy X-ray Absorptiometry, Whole Body (documentation).
- A comparison of multiple body composition measurement methods to the Department of Defense’s Physical Fitness and Body Fat Program procedures. International Journal of Exercise Science: Conference Proceedings 14(1), abstract 40.
This guide is for information and is not medical advice.