Sunscreen, on a schedule.
A smart sunscreen timer that pulls live UV for your spot and tells you exactly when to reapply — calibrated to the dermatologist two-hour standard.
Install as an app — full-screen, works offline
Sunscreen reapplication timer
Using the dermatologist 2-hour default — use your location for a precise reading.
Based on the dermatologist 2-hour standard, adjusted for your conditions. Not medical advice.
How the smart sunscreen timer works
Every dermatology body — the American Academy of Dermatology, the Skin Cancer Foundation, the British Association of Dermatologists — agrees on the same baseline rule: reapply sunscreen every two hours of sun exposure. The smart sunscreen timer starts from that 120-minute baseline, then adjusts in two directions based on conditions you actually face.
When you allow location access, the timer pulls the live UV index from currentuvindex.com (sourced from NOAA). High UV shortens the window; low UV slightly extends it. Your activity matters too — swimming, sweating, and beach reflection all cut sunscreen's effective lifetime.
What the timer factors in
Skin type (Fitzpatrick I–VI). Fair skin reaches a burning dose of UV faster, so the timer adds a 15% urgency adjustment for Types I and II. Darker skin types use the standard 2-hour baseline because natural melanin extends safe exposure — but protection still wears off at the same rate. Full Fitzpatrick guide.
SPF strength. Saved with your preferences so the timer remembers, but the reapply window does not scale with SPF. That's a common myth — higher SPF doesn't last longer. SPF 30 vs SPF 50 explained.
Activity. Indoor near a window extends the window to roughly 3 hours. Beach or pool deck cuts it to 100 minutes because reflected UV from sand and water amplifies your dose. Sports or heavy sweating drops to 80 minutes. Swimming uses 60 minutes by default — match your bottle's water resistance label. Swimming guide.
UV index. Live, point-in-time, from your exact coordinates. UV 6–7 (High) shortens the window by 17%. UV 8–10 (Very High) cuts it by a third. UV 11+ (Extreme) halves it. If you'd rather not share location, use the manual slider — the calculation is identical. UV index explained.
Why not just trust the SPF number?
Existing online sunscreen calculators get this wrong. They multiply SPF by some assumed "minimum erythemal dose" to spit out a reapply time. That math has no basis in real dermatology — SPF measures UVB blocking at a single moment, not duration. Two hours has been the standard since the 1980s, and that's what this timer uses.
Quick guides
Specific situations, real answers:
- How Often to Reapply Sunscreen
- Sunscreen While Swimming
- SPF 30 vs SPF 50 Reapplication
- Sunscreen for Kids: Timing
- Fitzpatrick Skin Type Guide
- Reapply Sunscreen on Makeup
- UV Index Explained
- Sunscreen at the Beach
- Sunscreen While Hiking
- Cloudy Day Sunscreen
- Sunscreen and Sweating
- Sunscreen for Tattoos
- Sunscreen Expiration Guide
- Water Resistant vs Waterproof Sunscreen
- Sunscreen Application Amount Guide
Frequently asked questions
How often should I reapply sunscreen?+
Dermatologists recommend every two hours of sun exposure as a baseline. The smart sunscreen timer tightens that window automatically when UV is high or you're swimming, sweating, or at the beach — and stretches it slightly only for low UV or indoor light exposure.
Does SPF determine how long sunscreen lasts?+
No. SPF measures how much UVB radiation gets blocked at a given moment, not how many hours of protection you get. Even SPF 100 breaks down on your skin at roughly the same rate as SPF 30. Reapply every two hours regardless of SPF strength.
How accurate is this UV-based timer?+
The live UV index is pulled from currentuvindex.com, which sources NOAA data. The reapply calculation uses dermatology-standard intervals (2-hour baseline, shortened for high UV / heavy activity / water). It's a guide, not a medical instrument — when in doubt, reapply early.
Do I need to reapply sunscreen indoors?+
Probably not — unless you're sitting near a window for prolonged periods. Window glass blocks UVB but lets UVA through, which contributes to skin aging. For most indoor activities, the timer's 'Indoor near window' setting (which extends the window to about 3 hours) is appropriate.
Is this timer suitable for children?+
Yes. For kids, drop the activity setting to Sports or sweating if they're playing actively — it tightens the reapply window to 80–90 minutes, which matches pediatric guidance. Babies under six months should be kept out of direct sun rather than relying on sunscreen.
What if I deny the location permission?+
The timer falls back to a manual UV slider (0–12). You can read your local UV index from any weather app or the iPhone weather widget and enter it directly. The calculation is identical.
Why does the calculation use Fitzpatrick skin type?+
Fair skin (Fitzpatrick I–II) reaches a burning dose faster than darker skin. The timer adds about 15% urgency for those types. Darker skin types (V–VI) still use the standard 2-hour baseline — natural melanin extends safe exposure but doesn't change the rate sunscreen wears off.
Does the timer keep running if I switch tabs?+
Yes. The countdown is based on a target end time rather than a tick counter, so it stays accurate when the tab is in the background. If you allow browser notifications, you'll also get an alert at the 10-minute warning and at zero — even if this tab isn't focused.