The highest SPF rating commonly available in sunscreen is 110. However, experts weighing in at WebMD and The New York Times generally agree that products with SPF ratings above 30 offer only marginally superior benefits when it comes to sun protection.
SPF is calculated based on the difference between the time it takes for someone to get sunburned when unprotected and the time it takes for someone to get sunburned while using the product being tested. SPF indicates by what multiplier a product extends the amount of time someone can go without being burned. For example, if someone who usually takes 10 minutes to burn applies SPF 30 sunscreen, it should take about 5 hours for him to burn. While high-SPF sunscreens theoretically offer hours of sun protection, things like swimming and sweating cause these products to rub off long before their theoretical limits are a concern.