Why body fat beats the bathroom scale
Weight and BMI tell you how much you are; body-fat percentage tells you what that weight is made of. It's the difference that matters most for a muscular person mislabelled "overweight" by BMI, and for someone at a "normal" weight who's actually carrying very little muscle. Skinfold calipers are the classic home-and-clinic tool for estimating it: you pinch the fat just under the skin at set sites, feed the millimetres into a validated equation, and get a defensible estimate for a few dollars of equipment.
The methods the Chonkometer offers
All three of these convert your skinfolds into body density, then into fat percentage using the Siri equation (BF% = 495 ÷ density − 450) (Siri, 1961).
Durnin & Womersley (4 sites)
Biceps, triceps, subscapular and suprailiac. This method sums all four, takes a logarithm, and uses age- and sex-banded constants — which makes it forgiving and widely applicable across ages. A solid default if you're not sure which to pick. The equations were developed on 481 men and women aged 16 to 72 (Durnin & Womersley, 1974).
Jackson-Pollock (3 sites)
The fast one. For men it uses chest, abdomen and thigh; for women, triceps, suprailiac and thigh. Fewer pinches, quicker reading, and very widely used in gyms — accurate enough for tracking change over time. The equations come from studies of men aged 18 to 61 and women aged 18 to 55 (Jackson & Pollock, 1978; Jackson, Pollock & Ward, 1980).
Jackson-Pollock (7 sites)
Chest, midaxillary, triceps, subscapular, abdomen, suprailiac and thigh. More sites means more consistency and less reliance on any single measurement, at the cost of more pinching. The choice for the most careful home estimate. Same research base as the 3-site version (Jackson & Pollock, 1978; Jackson, Pollock & Ward, 1980).
Note on methods: we've deliberately left out the Parrillo caliper method for now, because its published site list varies between sources and we won't ship a formula we can't stand behind. Numbers here are only worth having if they're defensible.
Where to pinch: the skinfold sites
All sites are on the right side of the body, standing relaxed. The direction is the direction of the fold you lift, not of the caliper.
| Site | Where | Fold | Used by |
|---|---|---|---|
| Chest | Halfway between the front of the armpit and the nipple (men); one-third of the way from the front of the armpit to the nipple (women) | Diagonal | JP3 men, JP7 |
| Abdomen | 2 cm (¾ in) to the right of the belly button | Vertical | JP3 men, JP7 |
| Thigh | Front of the thigh, midway between the hip crease and the top of the kneecap | Vertical | JP3 men and women, JP7 |
| Triceps | Back of the upper arm, midway between the tip of the shoulder and the elbow | Vertical | JP3 women, JP7, D&W |
| Biceps | Front of the upper arm, over the belly of the muscle, at the same height as the triceps site | Vertical | D&W |
| Subscapular | 1–2 cm (½–¾ in) below the lower tip of the shoulder blade | Diagonal (about 45°, pointing down and out) | JP7, D&W |
| Suprailiac | Just above the top of the hip bone, in line with the front of the armpit, following the natural angle of the hip bone | Diagonal | JP3 women, JP7, D&W |
| Midaxillary | On the side of the torso, in line with the middle of the armpit, level with the bottom of the breastbone | Vertical | JP7 |
Site descriptions follow the American College of Sports Medicine (ACSM, 2025). Durnin & Womersley originally took the suprailiac fold slightly further back, just above the hip bone in line with the middle of the armpit. Either works for tracking, as long as you always use the same spot.
How to pinch properly
Skinfolds are technique-sensitive — this is where most of the error lives. Always measure the right side of the body. Pinch a firm fold of skin and fat (not muscle) about a centimetre from where the caliper jaws will sit, apply the caliper, wait one to two seconds, and read. Take each site two or three times and use the average, and try to measure at the same time of day in the same condition (hydration and recent exercise shift the readings). Consistency matters more than any single perfect pinch — the same person measuring you the same way each time gives the most trustworthy trend.
Can you measure yourself? Partly. The men's 3-site version (chest, abdomen, thigh) is doable on your own with a bit of practice and a mirror. The triceps, subscapular and suprailiac sites are awkward or impossible to reach properly yourself, so for the women's 3-site, the 7-site and Durnin & Womersley you'll get far more reliable numbers with a helper — ideally the same person every time.
From pinches to a percentage: a worked example
Here's what the calculator does behind the needle, using the Jackson-Pollock 3-site equation for a 35-year-old man.
- Pinch and add. Chest 12 mm, abdomen 22 mm, thigh 16 mm. Sum: 50 mm.
- Estimate body density. Density = 1.10938 − 0.0008267 × 50 + 0.0000016 × 50² − 0.0002574 × 35 ≈ 1.0630 g/cm³.
- Convert to body fat with Siri. 495 ÷ 1.0630 − 450 ≈ 15.6% body fat.
A 30-year-old woman with triceps, suprailiac and thigh adding up to the same 50 mm gets a different equation (women carry more essential fat, so the constants differ) and lands at about 20.8%. Same millimetres, different bodies, different answers — which is exactly why the calculator asks for your sex and age.
What your result means
The needle uses sex-specific bands, because a healthy woman carries more essential fat than a healthy man:
| Reading | Men | Women |
|---|---|---|
| Very low (essential-fat zone) | under 5% | under 12% |
| Healthy range | 5–16% | 12–22% |
| A little above | 16–19% | 22–26% |
| Raised | 19–24% | 26–32% |
| High | 24% and above | 32% and above |
These match the bands in the calculator. For the finer-grained chart (athletic, excellent, good and so on) and what to do with your number, see what is a healthy body fat percentage? If your BMI says "overweight" but your reading lands in the healthy range, the extra weight is most likely muscle — why BMI misleads explains why. A "normal" BMI with a raised reading is the opposite case: skinny fat.
What the number can and can't do
Skinfold equations estimate whole-body fat from a handful of surface pinches, so they carry a real margin of error: typically a few percentage points either way compared with lab methods. Even DEXA, the most accessible lab method, can differ by a few points from the four-compartment model researchers treat as the reference (Van der Ploeg et al., 2003). What calipers do very well is track change in one person over time, as long as the same method, sites and measurer are used.
Two limits are worth knowing. The equations were built on people of roughly average to moderately high body fat; at very high body fat, folds become hard to lift cleanly and the estimate gets less reliable. In that case your waist measurement is the more useful number. And calipers only see fat under the skin, not the visceral fat around your organs — another reason to pair this with your waist.
Wondering how calipers compare with smart scales, tape-measure formulas and DEXA? See how to measure body fat.
Choosing calipers
Any caliper works for tracking if you use the same one every time. Clinical metal calipers apply a constant, calibrated jaw pressure and read more consistently, but cost considerably more. Budget plastic calipers are fine for watching your own trend; check that the jaws open wide enough for your thickest site, and expect slightly more variation between readings. Whichever you choose, technique and consistency matter more than the price tag.
When the number is worth a doctor
A genuinely high body-fat estimate — especially with a large waist — is a reasonable prompt for a conversation with a professional who can measure properly and look at the rest of your health. And a very low estimate isn't a prize: dropping below the essential-fat range carries its own risks, particularly for hormonal health. The Chonkometer flags both ends kindly and leaves the medicine to the medics.
Frequently asked questions
Which skinfold method should I choose?
How accurate are skinfold calipers?
Do I measure the left or right side of the body?
Is very low body fat healthy?
Where are the skinfold sites for the 3-site test?
Can I measure my own skinfolds?
Why does my caliper reading differ from my smart scale?
How often should I measure?
References
American College of Sports Medicine. (2025). ACSM's guidelines for exercise testing and prescription (12th ed.). Wolters Kluwer.
Durnin, J. V. G. A., & Womersley, J. (1974). Body fat assessed from total body density and its estimation from skinfold thickness: Measurements on 481 men and women aged from 16 to 72 years. British Journal of Nutrition, 32(1), 77–97.
Jackson, A. S., & Pollock, M. L. (1978). Generalized equations for predicting body density of men. British Journal of Nutrition, 40(3), 497–504.
Jackson, A. S., Pollock, M. L., & Ward, A. (1980). Generalized equations for predicting body density of women. Medicine and Science in Sports and Exercise, 12(3), 175–181.
Siri, W. E. (1961). Body composition from fluid spaces and density: Analysis of methods. In J. Brožek & A. Henschel (Eds.), Techniques for measuring body composition (pp. 223–244). National Academy of Sciences.
Van der Ploeg, G. E., Withers, R. T., & Laforgia, J. (2003). Percent body fat via DEXA: Comparison with a four-compartment model. Journal of Applied Physiology, 94(2), 499–506.