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Enter UV index, Fitzpatrick skin type, and continuous outdoor minutes to estimate unprotected burn time and a minimum labeled SPF (15 / 30 / 50 / 50+). Educational model—real protection depends on application thickness, reapplication, UVA broad spectrum, and individual factors.
Last updated: June 5, 2026
Labeled SPF assumes 2 mg/cm²—most users apply far less. Pair sunscreen with shade, UPF clothing, and wide-brim hats.
Rough burn time (no sunscreen)
~11.1 min
Time ÷ burn time (ratio)
16.22×
Educational minimum labeled SPF
SPF 30
Raw need ≈ 17 before label snap · floor SPF 15
Sunscreen does not fully “block” UV. Seek shade midday, wear UPF clothing and a wide-brim hat, and use sunglasses. Check product expiry and store away from heat.
Low UV, Type VI, 30 min
SPF 15
Burn ~266.7 min
Hike: UV 7, Type III, 2 h
SPF 15
3.82× ratio
Default: UV 9, Type I, 3 h
SPF 30
Burn ~11.1 min
Pool: UV 11, Type II, 4 h + swim
SPF 30
Raw need 25
Extreme: UV 12, Type I, 5 h
SPF 50
36.14× ratio
Simplified burn-time ratio—not a substitute for dermatology advice or in-vivo SPF testing.
| Step | Formula | Example |
|---|---|---|
| Baseline burn time at UV 10 | base_min = Fitzpatrick table (e.g., Type I → 10 min) | Type I at reference UV 10 ≈ 10 min to minimal erythema |
| Scale for forecast UV | burn_min = base_min × (10 ÷ UV_index), clamped 6–360 | Type I, UV 9 → 10 × (10/9) ≈ 11.1 min |
| Time-to-burn ratio | ratio = planned_minutes ÷ burn_min | 180 min ÷ 11.1 min ≈ 16.2× |
| Raw SPF need | raw = ceil(ratio); if swim/sweat × 1.35; floor 15 | ceil(16.2) = 17 → snap to labeled SPF 30 |
| Label snap | ≤15 → SPF 15; >15 → 30; >30 → 50; >50 → 50+ | Consumer shelves rarely show SPF 17—round up to next tier |
| Type | Description | Min burn |
|---|---|---|
| I | Very fair; always burns, rarely tans | ~10 min |
| II | Fair; burns easily, tans minimally | ~15 min |
| III | Medium; sometimes burns, gradual tan | ~22 min |
| IV | Olive/light brown; rarely burns | ~33 min |
| V | Brown; very rarely burns | ~50 min |
| VI | Dark brown/black; highly melanized | ~80 min |
| Index | Level | Guidance |
|---|---|---|
| 0–2 | Low | Minimal protection for most; sunglasses on bright snow |
| 3–5 | Moderate | SPF 15+ if outdoors >30 min; shade midday |
| 6–7 | High | SPF 30+, hat, reapply every 2 h |
| 8–10 | Very high | SPF 30–50, reduce midday exposure |
| 11+ | Extreme | SPF 50+, seek shade 10 a.m.–4 p.m. |
| Label | UVB blocked | Note |
|---|---|---|
| SPF 15 | ~93% | Minimum floor in this calculator; AAD routine outdoor use |
| SPF 30 | ~97% | Common recommendation for extended exposure |
| SPF 50 | ~98% | Diminishing returns vs 30 if under-applied |
| SPF 50+ | ~98%+ | FDA caps label claims; real-world gap often from thin application |
| Activity | Interval |
|---|---|
| Routine dry outdoor | Every 2 hours |
| Swimming / heavy sweat | Every 40–80 min |
| After toweling dry | Reapply immediately |
| Under-application | Consider higher SPF tier |
Result: SPF 30 for a 3-hour beach block—still reapply every 2 hours (or 40–80 min if swimming), seek shade midday, and wear a hat.
| Site | Amount |
|---|---|
| Full adult body | ~1 oz (shot glass) |
| Face & neck | ~¼ teaspoon |
| Timing | 15 min before sun |
| Expired product | Replace yearly |
| Factor | Effect |
|---|---|
| Fresh snow | Up to ~80% UV reflected |
| Water / sand | ~10–25% reflected |
| Altitude | ~4% more UV per 300 m |
| Clouds | Partial UV passes |
Educational disclaimer: This SPF calculator uses a simplified burn-time ratio model. It does not replace dermatology advice, account for all photosensitizing drugs, or guarantee protection from skin cancer. Perform regular skin checks and consult a clinician for changing moles.
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