SunDose
SPF Reminders
SPF & Sunscreen Reminder App
The problem with sunscreen is rarely the bottle. It is how much goes on, and whether anything reminds you to top it up while you are still outside.
The short answer
A sunscreen reminder app prompts you to reapply during sun exposure. The American Academy of Dermatology recommends reapplying at least every two hours, and after swimming or sweating. SunDose runs that prompt off your active session timer rather than a fixed clock alarm, so it arrives while you are actually in the sun.
Most people under-apply, by a lot
The SPF number on a bottle is measured under laboratory conditions at 2 mg of product per square centimetre of skin — about a shot glass for a full adult body. The Skin Cancer Foundation notes that people typically apply only 25–50% of that amount.
This is a dosage problem, not a product problem. Reaching for a higher SPF while applying the same thin layer changes less than most people assume.
Why a session-based reminder beats an alarm
A repeating two-hour phone alarm fires whether or not you are outdoors, which is exactly how people learn to dismiss it. Within a week it is noise.
SunDose starts the reapply clock when your session starts. The prompt lands during exposure, at a moment when it is actionable — and stops when the session does.
Reapply after water and sweat
Swimming and sweating both shorten how long a layer lasts, which is why the AAD calls those out separately from the two-hour rule.
Applying before you go out, rather than once you are already warm, also matters: it gives the layer time to be even and complete before exposure begins.
What else changes your reapplication needs
Photosensitising products and medications shorten the margin further. Retinoids, exfoliating acids, isotretinoin, tetracycline antibiotics such as doxycycline and some NSAIDs all make skin react to UV faster.
Flagging those in SunDose shortens the suggested session and, with it, the exposure the reminders are pacing. If you are unsure whether something you take is photosensitising, a pharmacist can tell you.
Sunscreen timing at a glance
| Situation | What the guidance says |
|---|---|
| Before going out | Apply generously and evenly, before exposure begins. |
| During exposure | Reapply at least every two hours (AAD). |
| After swimming | Reapply, regardless of how long has passed. |
| After heavy sweating | Reapply, regardless of how long has passed. |
| Amount for a full body | About one ounce — roughly a shot glass (2 mg/cm²). |
Frequently asked questions
- How often should you reapply sunscreen?
- The American Academy of Dermatology recommends at least every two hours during sun exposure, and immediately after swimming or heavy sweating. SunDose runs this prompt from your session timer so it arrives while you are still outside.
- How much sunscreen should I use?
- Sunscreen is tested at 2 mg per square centimetre, which works out to roughly one ounce — about a shot glass — for a full adult body. The Skin Cancer Foundation notes most people apply only a quarter to a half of that.
- Does a higher SPF mean I can reapply less often?
- No. A higher SPF does not extend the reapplication interval; the two-hour guidance and the after-swimming rule are unchanged. Applying enough, and topping up on schedule, matters more than the number on the bottle.
- Is SunDose only for tanning?
- No. The timer and reapply reminders apply to any deliberate time in the sun — gardening, running, a day at the beach or a long walk. The app also works purely as a UV index and exposure tracker if you never start a session.
Read more
Sources
- The Skin Cancer FoundationHow Much Sunscreen Is Enough?
- American Academy of DermatologyHow to apply sunscreen
- US Food and Drug AdministrationThe Sun and Your Medicine
- World Health OrganizationRadiation: The ultraviolet (UV) index