There is actually a neat formula called the navy formula to estimate body fat and lean mass from four variables for men and five for women. For men: waist circumference, weight, neck circumference, height. Women should also add hip circumference.
There are calculators online, but for anyone interested I expressed the formula into this Shortcut for Apple devices. You enter your height (do this once, in the blank text field above height)
Then, measure your neck circumference and remember it. This won’t change much. I measure mine maybe once every 1-2 months or if my other measures changed significantly.
Then, whenever you would normally weigh yourself, measure your waist too. Then enter weight + waist + neck circumference. The Shortcut will then calculate the following and log to Apple Health:
* Weight
* Waist circumference
* BMI
* Estimated lean mass
* Estimated body fat
The last two obviously have an error, but it will be a fairly consistent error so the trend over time will be clear.
For instance recently I gained a bit of weight. But my waist circumference went down a little. So my weight increase is actually muscle, and I’m going in the right direction.
Shortcut is here: https://www.icloud.com/shortcuts/1be74cce1ac24c03821feb7e36d...
Don’t have a woman’s version unfortunately, made this for my own use. If there’s interest I could probably create a version that includes hip circumference