There are several misconceptions about SPF. SPF, or Sun Protection Factor, is a measure of UVB protection in a particular sunscreen. People always want to know what the right SPF is, and they’re often mistaken about the difference between SPF 15 and SPF 30, for example. Both provide the same amount of protection, blocking 95 percent or more of sunlight, but one lasts longer than the other.

Let’s take a step back and talk about how soon skin burns and what an SPF number actually conveys. Most Caucasian skin will start to burn after eight to 10 minutes of exposure to noontime sun. SPF typically tells us how often we need to reapply sunscreen.

Related: Here’s What Really Happens to Your Skin When You Get a Sunburn

How To Determine The Correct SPF

To determine how often you should reapply sunscreen, multiply the SPF number by 10 to get the number of minutes between reapplying. 

For example, if you’re using SPF 15, you should reapply every 120 to 150 minutes, or two to two-and-a-half hours. That means you should reapply frequently. However, if you’re using SPF 30, you should reapply every 240 to 300 minutes, or four to five hours.

SPF 15 and SPF 30 are equally effective – for the first two hours or so. SPF 30 will function for a longer period of time. Contrary to popular belief, using a lower SPF will not help you get a darker tan. It just means you have to reapply sooner if you want to protect yourself from the sun’s UVB rays.

What We Recommend

We recommend using sunscreen with an SPF of at least 30. SPF 50 is even better because it provides you with longer-lasting protection, especially with young children. SPF 15 is still an option, but only if you’re willing to reapply every two hours.

Related: How to Choose the Best Sunscreen for Your Skin

That said, if you’re swimming or sweating a lot, you’ll probably need to reapply sooner. If the heat of the sun becomes more noticeable, or you feel your skin burn, sting, or become more sensitive to the fabric of your clothing, reapply sunscreen right away. These are signs that your skin is beginning to burn. Make sure you keep a close eye on the kids and reapply sooner to be safe.