BMI Calculator

There was a BMI calculator of mine on the Play Store from 2013. It still had users, and it had aged badly. So I rewrote the whole thing in Kotlin and Compose and kept the listing.

The popular BMI apps are full of ads and stop at a single number, which is close to the least useful thing you can tell someone about their body. This one has no ads, no tracking and no account, and it keeps going: waist-to-height ratio, Body Roundness Index, a body fat estimate, and your healthy range in kilos rather than a category name.

Children are where it gets serious. Adult BMI categories say nothing about a nine-year-old, so once a birthday is set the app switches to growth percentiles, from the reference tables paediatricians actually use. A parent may read that their child sits in the 97th percentile and act on it, so every calculation is checked against those tables automatically rather than by me spot-checking a few. Nobody will ever notice that. I would not ship the feature without it.

My favourite piece is the goal system. It draws a smoothed trend through your weigh-ins and tells you the pace you are actually on, including when that pace is too fast to be healthy. Plenty of apps will congratulate you for losing four kilos in a week.

Then there is the typing, the step people give up on after a fortnight, so the app now reads your weight off a Bluetooth scale. Working out how mine actually talks took a while: the published description and the hardware disagree in places, and a mistake is silent. The scale connects, greets the phone, then never says another word. No error, nothing in a log. It pairs with one scale, yours, and ignores everything else in range, because those signals are unencrypted and your neighbour's weight has no business in your history.

Weight moves through Health Connect too, Android's own health store, so a weigh-in from Garmin or Samsung Health turns up here and yours goes back out. All on the phone, never through a server of mine. Scales also send a raw body composition reading, and every manufacturer has a secret formula for turning that into a body fat percentage. I store the raw figure and leave it there. The number the app does show comes from a published method, and it says which one.