SPF Calculator — Find Your Sunscreen

How the SPF Calculator Works

This SPF calculator combines two key variables to give you a personalised recommendation: your Fitzpatrick skin type and the local UV index. The Fitzpatrick scale classifies skin into six types based on how it responds to sun exposure. Type I (very fair) burns almost immediately and never tans, while Type VI (very dark) rarely burns and has the most natural melanin protection.

The UV index is a standardised scale that measures the intensity of ultraviolet radiation reaching the Earth's surface. A UV index of 1-2 is low (minimal risk), 3-5 is moderate, 6-7 is high, 8-10 is very high, and 11+ is extreme. The higher the UV index, the more protective SPF you need, and the more frequently you need to reapply. Check your local weather app or national meteorological service for today's UV index.

The calculator also accounts for your activity. Swimming and water sports wash off sunscreen faster — even water-resistant formulations are tested for only 40 or 80 minutes of water exposure. Outdoor sports that cause heavy sweating have a similar effect. The activity type adjusts your reapplication interval accordingly.

Understanding SPF Numbers

SPF 15 filters approximately 93% of UVB rays, SPF 30 filters 97%, and SPF 50 filters 98%. While the differences seem small in percentage terms, they matter significantly in practice: an SPF 30 allows twice as much UVB through compared to SPF 50. For most people in moderate to high UV conditions, dermatologists recommend SPF 30 as a minimum and SPF 50 or above when UV index is 6 or higher.

SPF only measures UVB protection — the rays that cause sunburn. For complete sun protection, look for a sunscreen labelled "broad-spectrum," which means it also protects against UVA rays. UVA rays penetrate deeper into the skin and are the primary driver of photoaging and skin cancer risk. In Australia and New Zealand, all sunscreens with SPF 15 or above must pass broad-spectrum tests. In the EU and UK, look for the UVA circle logo.

Sunscreen Tips for Maximum Protection

Apply sunscreen 15-20 minutes before going outdoors to allow it to bond with your skin. Most people apply far too little — the recommended amount for an adult is 2mg per square centimetre of skin, which works out to roughly one teaspoon for the face and neck, and about one shot glass (35ml) for the full body. Using less than this significantly reduces the effective SPF you get.

Even with high-SPF sunscreen, other protective measures improve overall sun safety: seek shade during peak UV hours (10am-4pm), wear a wide-brimmed hat, UV-protective sunglasses, and UPF-rated clothing when possible. Sunscreen is your last line of defence, not the only line. Reapplying is just as important as the initial application — set a timer based on this calculator's recommendation and stick to it.