Questions

Usually rounding, or the units you entered. BMI is weight in kilograms divided by height in metres squared, so a centimetre of height or a decimal place of weight moves the result. If the gap is larger than that, check whether the other app is using a different formula: some use Trefethen's exponent 2.5 version, which gives a different number on purpose. This app shows both, and labels which is which.

It is accurate at what it measures, which is weight against height, and that is all it measures. It cannot tell muscle from fat, it does not know where you carry weight, and it was designed to describe populations rather than individuals. For one person it is a screening number, a reason to look closer, not a verdict.

Both, because they are measuring different things. BMI only knows your weight and height. A body fat estimate from your waist knows something about where that weight sits. When the two disagree, the waist based number is usually the more informative one, and it is worth mentioning to a doctor rather than picking whichever you prefer.

Poorly. Muscle is denser than fat, so a lean, heavily trained person often lands in the overweight band on BMI alone. That is a known limitation, not a fault in your training. Use the waist based methods instead, which is why the app offers them.

Less well. Height falls and body composition changes with age, and several health services treat a slightly higher BMI as acceptable after about 65. The app flags when you are in that group rather than applying the same band to everyone.

Yes, and it uses the right method. For ages 5 to 19 the app reads BMI for age against the WHO 2007 growth reference, using the child's exact age in months and their sex, and returns a percentile with the growth curves drawn. Adult cut points do not apply to a growing child and the app never uses them.

Every calculator is free, on the web and in the app, with no account and no sign up. Tracking over time, your history and the charts are free too. The only paid part is Plus, which adds cloud backup and sync between your devices and removes ads. Plus is a single lifetime purchase, not a subscription: you buy it once and it is yours.

It stays on your phone. The app works with no account at all, and nothing is sent anywhere unless you turn on cloud sync, which is part of Plus and requires signing in. The privacy policy sets out exactly what is stored in that case.

Not yet. The app is on Google Play only. If that changes it will be said here first.