Body Metrics
Pelaris shows three body ratios: BMI, waist-to-hip and waist-to-height. Each is a simple division with decades of public-health research behind it, and each is a screening or descriptive measure rather than a verdict on you. Where the inputs come from a body scan, they are AI visual estimates read from photos. Here is what each one is for, and where each one stops.
They are often lumped together as “body stats”. They are not interchangeable, and knowing what each one is built to answer is most of the value.
weight in kilograms ÷ height in metres, squared
What it tells you. Where your mass sits relative to your height, on the scale public health uses worldwide.
What it cannot. Anything about what that mass is made of. Height and weight are the only two inputs, so muscle and fat are indistinguishable to it.
waist circumference ÷ hip circumference
What it tells you. Where you carry mass rather than how much of it there is. It is a shape measure, and shape changes as training changes.
What it cannot. Tell you anything on its own from a single reading. Tape placement moves the number, so consistency between measurements is what makes the trend readable.
waist circumference ÷ height
What it tells you. The same waist measurement, scaled to your frame, which is what earned it a place on the card ahead of waist alone.
What it cannot. Deliver a verdict on you personally. It performs well as a population screening signal, and that is a different job from assessing one individual.
Body mass index is your weight in kilograms divided by the square of your height in metres. Pelaris shows it exactly as the World Health Organization defines it, and the WHO fact sheet sets the adult cut-offs plainly: overweight is a BMI greater than or equal to 25, and obesity is a BMI greater than or equal to 30. Those are the same numbers public health uses worldwide, which is precisely why they are worth showing rather than inventing our own scale.
Then comes the caveat that matters most to the people who use Pelaris. BMI has two inputs, height and weight, and no third input that could tell muscle from fat. A rugby forward, a powerlifter or a well-built hybrid athlete can sit in the overweight band while carrying very little body fat, and the formula has no mechanism to notice. That is not a failure of your training and it is not a quirk to explain away. It is what a two-variable formula can and cannot do, stated up front.
Why we show the number without a label. Pelaris renders your BMI value and points you at the definition behind it. It does not stamp a word on you. The scale is public, the maths is two lines long, and what it means for you, with your build and your training history, is yours to read and your doctor’s to advise on.
Waist-to-hip ratio is waist circumference divided by hip circumference, and it is read against the World Health Organization’s expert consultation on waist measurements, held in Geneva in December 2008 and published as the global reference for what those measurements genuinely tell us. It is a shape measure. It describes where mass sits rather than how much of it there is, which is why it can move meaningfully during a training block even when the scales barely shift.
Waist-to-height ratio is the same waist measurement scaled to your frame, and it earns its place on the card on the strength of the evidence behind it. Ashwell, Gunn and Gibson pooled 31 studies in a 2012 systematic review and meta-analysis published in Obesity Reviews, and found waist-to-height a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors. That is a strong finding, and it is a finding about screening across populations.
The distinction is worth holding onto. A metric that performs well as a population screening signal is doing a different job from one that assesses an individual, and Pelaris does not blur the two. We show you the ratio, we show you which way it is moving, and we leave any judgement about what that means for your health to the professional qualified to make it.
There are two routes into these ratios. You can enter your measurements directly, taken with a tape, which is the most literal input available. Or you can take a body scan, two photos from your phone, and let the AI read your proportions from the images.
Scan-derived values are AI visual estimates, and we say so rather than presenting them as measurements. They read visible shape and proportion, not internal composition. The thing that makes them genuinely useful is consistency: same light, same distance, the same two angles every time. Any single scan carries some margin, and the trend underneath a series of consistent scans is trustworthy in a way no individual reading is. For the honest, method-by-method version of that comparison, we wrote up how AI photo analysis stacks up against DEXA, bioimpedance and 3D scanners.
Which brings us to the framing that governs this whole page. These are trend lines, not verdicts. What a coach watching you over months is good at is noticing the pattern in the noise, and that is exactly the job these numbers are here to do.
Three sources carry everything above. Each was checked by hand against a live page. The full shelf of sixteen sits on the science hub.
We show BMI exactly as the World Health Organization defines it, using the same adult cut-offs public health uses worldwide.
World Health Organization. "Obesity and overweight" (fact sheet). Geneva: World Health Organization, last updated 8 December 2025.
View the sourceYour waist-to-hip ratio is read against the World Health Organization's expert consultation, the global reference for what waist measurements genuinely tell us.
World Health Organization. Waist Circumference and Waist-Hip Ratio: Report of a WHO Expert Consultation, Geneva, 8-11 December 2008. Geneva: World Health Organization, 2011. ISBN 9789241501491, 39 pp.
View the sourceWaist-to-height ratio earns its place on your card, this review of 31 studies found it a sharper screening signal than BMI or waist alone.
Ashwell, M., Gunn, P. & Gibson, S. "Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: systematic review and meta-analysis." Obesity Reviews, 2012;13(3):275-286. doi:10.1111/j.1467-789X.2011.00952.x
View the sourceNot on its own, and this is the single most important thing to know about it. BMI is calculated from height and weight alone, so it cannot distinguish muscle from fat. A lean, heavily muscled athlete can land in the overweight band while carrying very little body fat, which is not a flaw in their training but a known limit of a two-variable formula. Read BMI alongside your waist ratios and the direction all three are moving, rather than as a standalone verdict.
They answer different questions. BMI relates your total mass to your height using the World Health Organization definition, which sets the adult cut-offs at a BMI greater than or equal to 25 for overweight and greater than or equal to 30 for obesity. Waist-to-hip describes where you carry mass rather than how much, and is read against the WHO expert consultation on waist measurements. Waist-to-height scales your waist to your frame, and a 2012 systematic review of 31 studies by Ashwell, Gunn and Gibson found it a better screening tool than waist circumference or BMI for adult cardiometabolic risk factors. Together they describe shape and trend. None of them describes health.
Pelaris shows your ratio and its trend rather than issuing a pass or a fail, and that is deliberate. The research behind the metric positions waist-to-height as a screening tool across populations, which is a genuinely different job from telling one particular person where they should sit. If you want a threshold applied to you, with your history and your build in the room, that is a conversation for your doctor rather than an app.
Scan-derived values are AI visual estimates read from photos, not measurements taken with a tape. They read visible shape and proportion rather than internal composition. What makes them trustworthy is consistency: same light, same distance, the same two angles each time, and the trend underneath becomes reliable even though any single scan carries some margin. You can also enter tape measurements yourself if you would rather the numbers came from a tape.
Because the number and its definition are useful, and stamping a word on somebody is not. Pelaris shows the raw BMI value and links the World Health Organization fact sheet that defines the adult cut-offs, so you can see exactly what the scale is and where you sit on it. What that means for you, in your body, with your training history, is yours to interpret and your doctor’s to advise on.
No. Every metric on this page is a screening or descriptive measure developed for populations, and Pelaris uses them to show you shape and trend as you train. Pelaris gives AI-assisted general training information, not medical advice. It does not diagnose, treat or screen for anything, and it is not a substitute for a doctor or a qualified health professional.
Two photos or a tape measure, and Pelaris turns your proportions into a line you can follow across a training block.
Start Training →Pelaris gives AI-assisted general training information. It is not medical advice, it does not diagnose anything, and it is not a substitute for a doctor or a qualified health professional.