Revision #1359

'''Body-fat calculation''' is how Loopa estimates your body-fat percentage from your measurements. Out of the box it works automatically and tells you which formula produced each number. With Loopa Premium you can choose the formula yourself and keep every estimate on it. Body-fat formulas are estimates. Different methods can produce different results from the same measurements, so follow one trend over time.

''General information, not medical advice. Discuss your body composition and what it means for your health with your doctor.''

== Where to find it ==

On iPhone, open the '''You''' tab, then '''Preferences''', and look under '''Body fat''' for the '''Body Fat Calculation''' row. It shows your setting and your current number, for example "Automatic · Navy 24.3%".

* If choosing a method is part of your plan, the caption reads "Automatic (U.S. Navy first) unless you choose a method. Readings you log yourself or from a scale are never changed." Tap the row to open [[The Body Fat screen]], then '''Change method'''.
* If it is not, the caption reads "Automatic (U.S. Navy first): Loopa uses the first method your measurements allow and names it beside every estimate. Choosing the method is part of Loopa Premium." The row wears a Premium badge and tapping it opens the Loopa Premium offer.

You can also reach the Body Fat screen from [[The Measurements screen]], from the body-fat card in [[Trends on iOS]], and from '''You''', '''Explore''', '''Body fat'''. The method picker lists every method, what each one needs, and what each would estimate from the measurements you already have, side by side. [[The Body Fat screen]] describes every control on it.

Your setting is saved to your account, so every device and every screen uses it: Today, your measurements, your charts, your reports and exports, and the coach.

== Automatic (U.S. Navy first) ==

Automatic is the standard setting. Loopa uses the '''U.S. Navy''' method when your neck measurement (and, for women, your hips) is on file. Without it, Loopa uses '''Relative Fat Mass''' from your waist, and without a waist measurement it uses the '''BMI-based''' method from your weight. Every estimate is labelled with the method that produced it, for example '''Navy estimate''' or '''RFM estimate''', so you always know where a number came from. When your chart holds estimates from more than one method, Loopa says so.

== The methods ==

| Method | What it uses | Who it was developed for |
|---|---|---|
| '''U.S. Navy''' (tried first in Automatic) | Height, waist and neck. Women also add hips. | Adults; developed on U.S. Navy personnel |
| '''Relative Fat Mass (RFM)''' | Height and waist | Adults aged 20 to 85 |
| '''BMI-based''' | Weight, height, age and sex | Adults up to 83 |

* The '''U.S. Navy''' method comes from research at the Naval Health Research Center and is the circumference method the U.S. Department of Defense adopted.{{cite web|title=Development of the DoD Body Composition Estimation Equations (Technical Document 99-2B)|url=https://apps.dtic.mil/sti/pdfs/ADA370158.pdf|website=Naval Health Research Center|access-date=2026-10-04}} Men measure the waist at the navel; women measure the waist at its narrowest point and the hips at their widest.
* '''Relative Fat Mass''' was developed from a large U.S. health survey and checked against DXA body scans.{{cite web|title=Relative fat mass (RFM) as a new estimator of whole-body fat percentage|url=https://pubmed.ncbi.nlm.nih.gov/30030479/|website=Scientific Reports|access-date=2026-10-04}}
* The '''BMI-based''' method (the Deurenberg formula) needs no tape measure. Because BMI cannot tell muscle from fat, it reads high for muscular bodies.{{cite web|title=Body mass index as a measure of body fatness: age- and sex-specific prediction formulas|url=https://pubmed.ncbi.nlm.nih.gov/2043597/|website=British Journal of Nutrition|access-date=2026-10-04}}

== Choosing a method ==

With Loopa Premium you can pick '''U.S. Navy''', '''Relative Fat Mass''' or '''BMI-based''' and keep every estimate on that one formula. If the method you chose is missing something, Loopa tells you exactly what to add, for example ''"Add a neck measurement to calculate body fat using the U.S. Navy method."'' A chosen method stays your method: the number you see always comes from it. Each method also has an age range it was developed for, and outside it Loopa shows no estimate for that method. You can go back to '''Automatic''' at any time.

Your sex comes from your health profile. If it is not set, add it there to see an estimate.

== Estimates and your own readings ==

Wherever body fat appears, Loopa labels it: '''Navy estimate''', '''RFM estimate''', '''BMI-based estimate''', or '''Measured''' for a number you entered, synced from a smart scale, or took from a scan. Changing the method changes only Loopa's estimates. Your own readings are never replaced.

Fat mass and lean mass are worked out from the body-fat percentage shown and your latest weigh-in.

== Changing the method ==

When you choose a method, Loopa recalculates your whole history from the measurements you logged on each day, using that method. A day without the measurements it needs is simply left out of the chart, so the line stays on one formula. If the switch moves your current estimate, Loopa tells you once. Your measurements and any body-fat target you set stay exactly as they were.

Under Loopa 1.5, automatic estimates and choosing the method are both part of Loopa Premium. Dismissing the "Your estimate changed" notice works on every plan. If your Premium membership ends, your choice of method stays saved, ready to apply again when you return to Premium. Logging tape measurements, which the U.S. Navy and RFM methods use, is part of Loopa Premium; the BMI-based method works from your weigh-ins.

== See also ==

* [[The Body Fat screen]]
* [[The Measurements screen]]
* [[Body measurements]]
* [[BMI and body composition]]
* [[your-scale-isnt-a-fat-scanner|Your scale isn't a fat scanner]]
* [[Weight tracking]]

[[Category:Loopa]]
[[Category:Tracking]]