SPF stands for sun protection factor. That's how much longer you can stay in the sun before your skin burns compared to no sunscreen. The number you see is the 'factor' you multiply times the amount of minutes your skin usually takes to burn. If your skin naturally burns in 5 minutes, then SPF of 30 means you have 30 times the number of minutes you burn naturally. So in this case - you'll be protected for 150 minutes (2.5 hours).
Minutes to burn without sunscreen x SPF number = maximum sun exposure time
Is SPF 45 or 60 better than SPF 30?
Since sunscreen ingredients wear off, sweat off, or come off by swimming, you should actually reapply every 2 hours no matter what the SPF number is. So, there really isn't much benefit to an SPF of more than 30 since you need to reapply before then anyhow. Likewise, an SPF 45 or 60 has more of the sunscreen ingredients which can feel very heavy and thick.
How do I properly apply sunscreen?
The sunscreen must be absorbed into the top layers of your skin to be effective. Apply 15 to 30 minutes before you step out into the sun. Remember to use a full ounce (shotglass) to cover your body, and a different sunscreen formulated for your face.
What is Natural Sunscreen?
A natural sunscreen creates a physical barrier between your skin and the sun, and contains zinc oxide and titanium dioxide. It can feel heavy and dry on the skin, which is something to consider for those hot days.