Sunscreen SPF Calculator

How does the calculator choose an SPF?

By fixed rules, giving only 15, 30 or 50. It returns 50 when the UV index bracket is 11+ or the skin type is I or II. Otherwise it returns 30 for UV 8 to 10, skin type III, or water and beach activities, and 30 again for any UV bracket from 3 up. Only the remaining low-UV cases with skin types IV to VI on lower-exposure activities return 15.

Where do the “minutes to burn” figures come from?

The tool starts from built-in base times by Fitzpatrick type: 10, 15, 25, 45, 60 and 90 minutes for types I to VI. It multiplies by a UV factor (1.0, 0.9, 0.75, 0.65 or 0.5, from low to extreme) and an activity factor (1.5 indoor, 1.0 casual outdoor, 0.8 active, 0.7 water or beach). These are rough heuristics inside the tool, not clinical measurements of your skin.

How is the protected-time estimate worked out?

Minutes to burn × SPF × 0.4, rounded, and then capped at 120 minutes. The 0.4 reduces the plain multiplication to reflect that real-world protection falls short of the number on the bottle. The cap reflects the two-hour reapplication interval, so the tool never reports more than 2 hours per application.

Does the planned time in the sun change the SPF?

No. The time you enter is echoed in the summary and, when you plan to reapply, used to count applications at 2-hour intervals (180 minutes gives 2). It does not alter the recommended SPF or the minutes-to-burn figure. An empty or zero value falls back to 60 minutes.

Why is the UV index a set of brackets and not a number?

The input is a select with five choices, 0–2, 3–5, 6–7, 8–10 and 11+, and each is represented by one value inside the code (2, 5, 7, 10 and 11). The calculator does not look up your location or the live forecast, so read the day’s index from a weather app first.

What does SPF 15, 30 or 50 mean in the output?

The labels describe the share of UVB filtered if the product is applied as tested: about 93 per cent for SPF 15, 97 per cent for SPF 30 and 98 per cent for SPF 50, which is 1 − 1/SPF. The calculator does not measure UVA protection or tell you whether your product is broad-spectrum.

Does a darker skin type mean no sunscreen is needed?

The tool still returns a recommendation for types V and VI. People of every skin type can sustain UV damage, even though they burn less readily. This is general information, not medical advice, and a dermatologist can advise on skin checks and personal risk.