BMI Calculator

There was a BMI calculator of mine on the Play Store from 2013, still with users and badly aged, so I rewrote the whole thing in Kotlin and Compose and kept the listing. The popular ones are full of ads and stop at a single number, close to the least useful thing you can tell someone about their body. This one keeps going, with no ads, no tracking and no account: waist-to-height ratio, Body Roundness Index, a body fat estimate, 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 reads your weight off a Bluetooth scale. Working out how mine talks took a while: the description and the hardware disagree, and a mistake is silent, so the scale connects, greets the phone, then never says another word. 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. Weigh-ins also sync through Health Connect, all on the phone and never through a server of mine, and the raw body composition reading the scale sends stays raw: the body fat number the app shows comes from a published method, and it says which one.