Support

Yes. D-Fine is completely free to download and use. There are no in-app purchases, no subscriptions, and no advertising.

No, D-Fine works on iPhone alone. But Apple Watch makes a real difference: the watch passively tracks your time in daylight through its built-in sensor, so the app can calculate your sun exposure automatically without you having to remember anything. Without a watch, the app relies on the workouts you log in Apple Health for outdoor activity, plus what you tell it manually.

Almost everything stays on your device. The app reads health and location data through Apple's standard frameworks (HealthKit, CoreLocation, WeatherKit) only after you grant permission. Nothing leaves your device unless you explicitly turn on Anonymous data sharing in onboarding or in Settings — and even then, what we upload is fully anonymous and contains no identifying information. For the complete breakdown, see our Privacy Policy.

To compute how much vitamin D your skin can actually produce, the app needs to know the UV index where you are. UV depends on your latitude, the time of day, the season, and current cloud cover. Location is used only for this — it is never linked to your identity in any upload.

The app uses health data in two ways. It reads your time in daylight and your outdoor workouts (including their GPS routes) to estimate sun exposure. With your separate permission, it also reads your date of birth, biological sex, body mass, and height — these are converted into age, gender, and a BMI band, which influence the vitamin D synthesis calculation. Raw values are never stored or transmitted. The app can also write vitamin D supplement entries back to Apple Health if you log them.

D-Fine uses a science-based model that takes into account UV index, sun angle, cloud cover, skin type (Fitzpatrick scale), clothing coverage, and SPF use. The result is an estimate, not a medical measurement. Vitamin D synthesis varies considerably between individuals even under the same conditions. If you have specific concerns about your vitamin D levels, please consult a healthcare professional and consider a blood test.

Yes. Tap Add Vitamin D on the Home screen to log a supplement with its IU value. You can also use the slider that ranges up to 10,000 IU. Logged supplements are stored locally and also written to Apple Health so they show up across your other health apps.

D-Fine syncs through your iCloud account (private database only — nothing public, nothing shared). Make sure:

1. Both devices are signed in to the same iCloud account
2. iCloud Drive is enabled on both
3. D-Fine has permission to use iCloud (Settings → Apple ID → iCloud → D-Fine)

If the issue persists, force-quit and reopen the app on both devices. If it still doesn't work, write to us.

A full CSV export is planned for a future version of D-Fine. In the meantime, all your data is stored in your private iCloud account, so it is never locked into the app — uninstalling and reinstalling preserves it, and you can request a copy at any time by contacting us.

Open Settings → Share Anonymized Data and toggle it off. The app stops uploading immediately and purges any queued data. You can turn it back on at any time. We never upload anything before you have explicitly given consent.

Local data is removed when you uninstall the app. Data in your private iCloud account is removed according to your iCloud settings — typically by deleting the app data from Settings → Apple ID → iCloud → Manage Storage. Anonymous upload data is not linked to your identity and therefore cannot be selectively deleted, but it cannot be linked back to you either.