The SPF rating is decided in a lab. Your day happens in the real world, where sunscreen gets a workout.

In the lab, a precise amount of sunscreen is applied to still skin to measure its UV protection. Out in the real world, that sunscreen is getting a workout. You’re wiping your face, pulling on a t-shirt, chasing a toddler, or jumping in the surf. Every one of these small actions wears your sunscreen down.

This brings us to the simple answer for how often to reapply sunscreen: every two hours.
The reason for this rule is physical. Over a couple of hours, life will have rubbed, sweated, or wiped enough of the product off your skin that you need a top-up to ensure proper coverage.

Getting the application right from the start.

The lab tests also assume you're using enough in the first place: about a teaspoon for your face and neck, and a full shot glass for your body. Most people apply less than that. If your first coat is too thin, you're not getting the full SPF on the label, which makes the need to reapply sunscreen even more important.

This applies to everyday life, too.

The two-hour rule is a practical safety net for the days you might not even think about it.

• Driving: UV rays penetrate glass. If you’re in the car for a long stretch, you’re getting exposure.
• Cloudy days: Up to 80% of UV radiation can get through clouds. The need to reapply sunscreen doesn't go away just because the sun isn't out.
• At the office: Your morning application will see you through if you’re inside all day. But if you’re heading out for a lunch break or a walk, it’s time for a top-up.

The goal is to build a simple habit. Use a generous amount in the morning, reapply when you need to, and get on with your day.