How the timer calculates your reapply window
Updated
Most online sunscreen calculators are black boxes. This one isn't. Below is the exact formula, every multiplier we use, the sources we draw on, and three worked examples so you can spot-check the number the tool gave you.
The 2-hour baseline
Every major dermatology body uses the same starting number: 120 minutes. The American Academy of Dermatology, the Skin Cancer Foundation, the British Association of Dermatologists, the Australian Cancer Council — all converge on "reapply every two hours of sun exposure" as the default. That figure is not a random round number. It tracks how long a properly applied layer stays evenly distributed on real skin before sweating, friction against clothing, and the skin's natural oils begin to thin it.
We start every calculation from 120 minutes, then adjust in two directions based on what you're actually doing and what the sun is actually doing.
Activity multiplier
Activity matters because it determines how fast the layer comes off — through sweat, water, sand, towels, or simple friction against clothing.
| Activity | Multiplier |
|---|---|
| Indoor near window | ×1.5 |
| Light outdoor | ×1.0 |
| Beach / pool deck | ×0.83 |
| Sports / sweating | ×0.67 |
| Swimming | ×0.5 |
The numbers are calibrated against the FDA's official water-resistant labeling (40 / 80 minutes), the Skin Cancer Foundation's beach guidance (≤100 minute reapply), and the AAD's sport-specific advice (every 80 minutes when actively sweating).
UV index multiplier
The WHO UV index is the global standard for ambient UV intensity. We map its five categories to multipliers that tighten the reapply window as UV climbs.
| UV index | WHO category | Multiplier |
|---|---|---|
| 0–2 | Low | ×1.5 |
| 3–5 | Moderate | ×1.0 |
| 6–7 | High | ×0.83 |
| 8–10 | Very High | ×0.67 |
| 11+ | Extreme | ×0.5 |
Live UV readings come from currentuvindex.com, which sources NOAA satellite data. If you'd rather not share location, the manual slider feeds the same multiplier table.
Skin type adjustment
The Fitzpatrick scale, developed at Harvard Medical School in 1975, classifies skin by how it responds to UV exposure. Very fair skin reaches a "minimal erythemal dose" — the point of visible reddening — much faster than darker skin, so the reapply window for those types is tightened by 15%.
| Type | Description | Multiplier |
|---|---|---|
| I | Very fair — always burns, never tans | ×0.83 |
| II | Fair — usually burns, lightly tans | ×0.83 |
| III | Medium — sometimes burns, tans gradually | ×1.0 |
| IV | Olive — rarely burns, tans easily | ×1.0 |
| V | Brown — very rarely burns | ×1.0 |
| VI | Deeply pigmented — almost never burns | ×1.0 |
Critically: darker skin still needs sunscreen. The Fitzpatrick scale only tells us how fast a visible burn forms, not how much UV damage is occurring. Melanoma in deeply pigmented skin is often diagnosed later and at more advanced stages.
Floor, cap, rounding
After the multipliers, we apply three small post-processing steps:
- Round to the nearest 5 minutes — produces a clean number that fits a real timer instead of "67 minutes".
- Floor at 30 minutes — even under the worst possible combination (extreme UV + swimming + fair skin) we don't recommend a reapply window shorter than half an hour. Below that, you should be seeking shade rather than relying on sunscreen.
- Cap at 180 minutes — even in low UV indoors, we don't suggest stretching past three hours. UVA is always present.
2-hour forecast peak
The currentuvindex.com API returns not just the current UV reading but a forecast for the next several hours. We look ahead 2 hours and take the highest expected UV in that window — then use that for the calculation, not just the right-now value.
Why: if you're at UV 5 right now but it's going to climb to UV 8 in 45 minutes, the safer reapply schedule is the one tuned to UV 8. The timer surfaces both numbers on screen ("Currently 5.2 · peak in ~45 min") so you can see what we did.
Worked examples
Three real-world combinations, traced step by step:
Example 1 · Beach day, fair skin
Inputs: Beach activity, UV index 8 (Very High), Fitzpatrick II.
120 × 0.83 × 0.67 × 0.83 = 55.4 min
→ rounded to 55 min
A Mediterranean beach in July with Type II skin: reapply just under an hour.
Example 2 · Walk to the park
Inputs: Light outdoor, UV index 5 (Moderate), Fitzpatrick III.
120 × 1.0 × 1.0 × 1.0 = 120 min
A late-spring afternoon walk: the textbook two-hour interval.
Example 3 · Swimming under extreme UV
Inputs: Swimming, UV index 11+ (Extreme), Fitzpatrick III.
120 × 0.5 × 0.5 × 1.0 = 30 min
→ hits the 30-minute floor
Pool day at the equator under direct midday sun: reapply every 30 minutes — or seek shade and skip the calculation entirely.
What we deliberately don't do
A few decisions worth being explicit about:
- SPF strength is not a time multiplier. Many online sunscreen calculators multiply SPF by an assumed "minimum erythemal dose" to spit out a reapply window. There is no dermatological basis for this — SPF measures UVB blocking at the moment of application, not duration. Reapply intervals are the same for SPF 30 and SPF 50. More on this.
- We don't trust "waterproof" claims. The FDA banned that term in 2011 for a reason. The Swimming activity multiplier is set to the lower bound of FDA's water-resistant testing (40 minutes), not the upper bound.
- We don't extend the window for cloudy skies. Light cloud cover blocks only ~10% of UV; the live UV index already reflects what's getting through. More.
- We don't store your location. Coordinates are passed once to the UV API and discarded. No analytics fingerprinting beyond standard GA4 page-view collection. Privacy details.