Calculate recommended SPF, safe sun time, and sunscreen amount based on UV index, Fitzpatrick skin type, coverage area, and reapplications.
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Sunscreen SPF Formulas
The calculator runs three modes. Each uses a separate formula.
1. Safe sun time (protected exposure)
Time = (BurnBase / UV) * SPF * 0.5
- Time – minutes before erythema (skin redness)
- BurnBase – minutes to burn at UV 1 by skin type: I=67, II=100, III=200, IV=300, V=400, VI=500
- UV – current UV Index
- SPF – the SPF rating on the bottle
- 0.5 – real-world factor; most people apply about half the lab-test dose
2. Amount needed (grams of sunscreen)
Grams = 2 mg/cm² * Area(cm²) * Applications / 1000
- 2 mg/cm² – FDA test density
- Area – skin surface covered (full adult body ≈ 15,000 cm²)
- Applications – one every 2 hours of exposure
3. Recommended SPF is a lookup based on UV Index and Fitzpatrick skin type, with fair skin bumped up one tier.
Assumptions: broad-spectrum sunscreen, applied 15–20 minutes before exposure, reapplied after swimming or sweating. Math estimates do not replace reapplication every 2 hours.
Reference Tables
UV Index to SPF guidance:
| UV Index | Risk | Minimum SPF | Action |
|---|---|---|---|
| 0–2 | Low | SPF 15 | Daily moisturizer with SPF |
| 3–5 | Moderate | SPF 30 | Apply before going out |
| 6–7 | High | SPF 30+ | Hat, shade midday |
| 8–10 | Very High | SPF 50+ | Avoid sun 10am–4pm |
| 11+ | Extreme | SPF 50+ | Cover skin, reapply every 90 min |
Sunscreen needed per application (2 mg/cm² standard):
| Area | Grams | Teaspoons |
|---|---|---|
| Face & neck | 1.5 g | ⅓ tsp |
| Both arms | 6 g | 1¼ tsp |
| Both legs | 12 g | 2½ tsp |
| Chest, back, shoulders | 10 g | 2 tsp |
| Full adult body | 30 g (1 oz) | 6 tsp / 1 shot glass |
FAQ
Does SPF 50 last twice as long as SPF 25?
No. SPF 50 blocks about 98% of UVB; SPF 25 blocks about 96%. The protection time difference is small, and both need reapplying every 2 hours.
Why does the calculator halve my safe time?
SPF lab tests use 2 mg/cm² of product. Most people apply 0.5–1 mg/cm². Real-world SPF is roughly 30–50% of the label, so the formula multiplies by 0.5.
Example: SPF 30, UV 7, skin type III.
Unprotected: 200 / 7 = 28.6 min. Protected real-world: 28.6 × 30 × 0.5 = 429 min (~7 hr). Still reapply every 2 hours after sweating, swimming, or toweling off.
Do darker skin tones need sunscreen?
Yes. Skin types V and VI burn less but still develop UV damage, hyperpigmentation, and skin cancer risk. SPF 30 is the floor at UV 3 or higher.
Is "broad-spectrum" the same as high SPF?
No. SPF only measures UVB. Broad-spectrum means the product also blocks UVA, which causes aging and contributes to cancer. Always pick both.