Many free health apps require you to create an account before you can use them. They ask for your email, sometimes your name, and then upload your height and weight to a server. For a simple calculation, this is unnecessary. You do not need a cloud account to know your BMI or your BMR. The data can stay where it belongs: on your phone.
On-device computation means the math runs locally. Your height, weight, and any other inputs stay in the app's storage. Nothing leaves your phone unless you explicitly choose to sync. There is no account to manage, no password to remember, and no server to breach. The result is a simpler experience for a simple task.
What 'on-device' actually means
When an app is on-device, the calculations happen on the phone itself. The app reads your inputs, applies the formulas, and displays the result. Cloud sync is optional, not required. There is no account to create, no login screen, and no remote database holding your data. Because nothing is stored remotely, there is nothing to breach. If your phone is lost or stolen, the data is still there. If the company shuts down, your data is still there. The only way your data leaves your phone is if you choose to share it.
BMI and BMR are just formulas
BMI and BMR are not diagnoses. They are estimates based on well-known formulas, and they serve as general indicators rather than medical advice. BMI is a quick category indicator, not a diagnosis. BMR is an estimate of resting calories. Waist-to-height ratio is another simple proxy for health risk.
- BMI = weight divided by height squared. It gives a rough category — underweight, normal, overweight, or obese — but it does not account for muscle mass, age, or body composition.
- BMR = an estimate of resting calories, calculated from height, weight, age, and sex. It tells you roughly how many calories your body needs at rest, not how many you need to maintain your weight.
- Waist-to-height ratio = another simple proxy for health risk, calculated from waist circumference and height. It is easy to measure and does not require a scale.
These are general indicators. They are not medical advice. If you have a medical condition, consult a professional. The formulas are straightforward, and the results are useful for tracking trends over time.
What to look for in a private health app
- No account required. You should be able to open the app and use it without creating an account, logging in, or providing an email address.
- Works offline. The app should function without an internet connection. You should not need Wi-Fi or cellular data to calculate your BMI or BMR.
- No third-party tracking SDKs. The app should not include tracking SDKs from companies like Google, Facebook, or Apple that collect usage data. Check the app's privacy policy or the developer's website for this information.
- Clear about what it stores. The app should state clearly what data it stores, where it stores it, and whether it stores anything at all. If it stores data, it should be on your device, not on a server.
A concrete example
BMI Mate is a no-account BMI calculator for iOS. It uses a colour dial that turns green, yellow, or red depending on your result. You can switch between metric and imperial units instantly. Everything runs on device. The free tier gives you instant BMI, both unit systems, the colour dial, and an ad-free experience. A subscription adds weight-goal tracking, a BMR estimate, waist-to-height ratio, theme palettes, and a trend graph. BMI Mate does not include third-party tracking SDKs. It does not claim to diagnose anything. It does what it says it does.