Select your skin type (Fitzpatrick scale), the current UV index, and the SPF of the sunscreen applied to estimate sunburn time with no protection and how long that sunscreen should protect the skin before it needs to be reapplied.
Reapply at least every two hours regardless of the figure above, and immediately after swimming, towel-drying, or heavy sweating: water resistance and rubbing reduce the sunscreen film well before the theoretical protection time is over.
These are general order-of-magnitude estimates based on the theoretical SPF protection factor and published Fitzpatrick reference burn times, for information only. This tool does not replace dermatological advice, particularly for young children, photosensitising medication, a personal or family history of skin cancer, or any skin showing unusual moles or lesions: consult a doctor or dermatologist in those cases.
How UK sunscreen labelling categorises SPF
UK retailers follow the same labelling categories set out in the EU Cosmetic Products Regulation, which the UK kept after leaving the EU: low protection covers SPF 6 and SPF 10, medium protection covers SPF 15, SPF 20, and SPF 25, high protection covers SPF 30 and SPF 50, and very high protection covers SPF 50+. A product cannot claim a specific number above 50, since testing variability beyond that point is considered too wide to state precisely, which is why the label simply reads "50+".
British Association of Dermatologists guidance
The British Association of Dermatologists and Cancer Research UK recommend at least SPF 30 with high UVA protection (shown as a UVA circle logo on UK products) for everyday summer use, applied generously about 20 minutes before going outside and reapplied straight after swimming or sweating. Their guidance is explicit that no sunscreen blocks 100 percent of UV radiation, and that SPF alone should not be the only sun-safety measure: shade and clothing matter just as much.
UV index through a British summer
The UV index across the UK rarely reaches the extreme levels seen closer to the equator, but it still climbs to 6 or 7 in southern England on a clear day in June and July, and can reach 8 on the south coast during a heatwave. Cloud cover reduces the index but does not remove UV risk entirely, since a large share of UV radiation still passes through light cloud, which is a common source of unexpected sunburn on overcast British days.
Why this differs from a general UV index guide
A general UV index guide reports the World Health Organization risk band for the current index and a minimum recommended SPF, without factoring in a sunscreen already applied. This calculator instead starts from a specific sunscreen with a known SPF and estimates a protection window in minutes for that exact combination of skin type, UV index, and product, which is a more specific and more personal figure.