Sun ExposureSun Exposure Pro
Vitamin D Estimates
An hour-by-hour estimate of the vitamin D your skin makes from sunlight, based on the light your Apple Watch actually measured.
How the estimate works
Sun Exposure combines three things for every hour you were outside:
- The UV Index where you were, from Apple Weather.
- Your Time in Daylight and how bright it was. Full sun counts fully, while shade counts less or not at all.
- Your skin. Your Fitzpatrick skin type and how much skin you had exposed, from fully covered to a swimsuit, both set in Settings › Skin & Health.
It's an estimate of what your skin makes from sunlight. It isn't a measurement of your vitamin D levels, and it doesn't include food or supplements.
Why it's sometimes zero
Your skin makes little to no vitamin D when the UV Index is below 3. That's common early and late in the day, in winter, and far from the equator. An hour outside on a bright winter morning can count fully toward your sun goal and still add nothing to your vitamin D.
Auto-log to Apple Health
Turn on Auto-Log to Health in Settings › Skin & Health, and Sun Exposure saves its estimate to Apple Health throughout the day, under Nutrition › Vitamin D. Nothing to start or stop.
Your iPhone and Apple Watch both run the logger, so before saving, Sun Exposure checks what it has already saved today on either device and adds only the difference. Vitamin D logged by other apps, such as a supplement tracker, doesn't change what Sun Exposure logs.
Sun Sessions save a vitamin D estimate to Apple Health too, when you have Pro and location turned on.
Ask Siri
Say “How much vitamin D have I gotten in Sun Exposure?” or use the Check Vitamin D shortcut.
What you need
- Sun Exposure Pro.
- Your skin type, set in the app or read from Apple Health.
- Location, for the UV Index. Approximate location is enough.
- An Apple Watch for all-day estimates, or Sun Sessions on your iPhone.
Questions about vitamin D
Why is my vitamin D estimate zero?
Your skin makes little to no vitamin D when the UV Index is below 3, which is common early and late in the day, in winter, and far from the equator. Time in shade or dim light doesn't count either.
Also check that your skin type is set, that your skin coverage isn't Full Coverage, and that Sun Exposure can use your location to get the UV Index. Vitamin D estimates are part of Sun Exposure Pro.
How does vitamin D auto-logging work?
Turn on Auto-Log to Health in Settings > Skin & Health. It needs Sun Exposure Pro and your skin type. Sun Exposure then saves the vitamin D it estimates from your Apple Watch's Time in Daylight to Apple Health throughout the day, under Nutrition > Vitamin D.
Before saving, it checks how much vitamin D Sun Exposure has already saved today on either device and only adds the difference, so your iPhone and Apple Watch don't log the same sun twice. Vitamin D that other apps log, such as supplements, doesn't change what Sun Exposure logs. It uses the day's UV Index, which Sun Exposure gets when you open the app or a widget refreshes.
Which skin type should I choose?
Sun Exposure uses the Fitzpatrick scale, from Type I (always burns, never tans) to Type VI (never burns, deeply pigmented). Choose the type that best matches how your skin reacts to the sun without protection. If you've set a skin type in the Health app, Sun Exposure starts with that one.
Your skin type affects both Burn Time and vitamin D estimates.
Start Your Sun-Smart Journey Today
Free on the App Store for iPhone and Apple Watch.
Download on the App StoreiPhone with iOS 18 or later. Automatic tracking needs an Apple Watch with watchOS 10 or later.
Disclaimer: Sun Exposure gives estimates, not medical advice. Consult your doctor to determine the right amount of sun exposure for you. Always use sunscreen, hats, and shade as needed.