As I understand it, you can't combine multiple suncreens to combine their SPF values. This makes sense as they all work on the same concepts and your skin can't simply "hold more" because it's two different brands.
But how do you measure combined sunscreen and clothing? Or for that matter, combined layers of clothing?
My instinct is that you would multiply them:
Clothing: UPF 3, passes 33% of UV. C=0.33 Sunscreen: SPF 5, passes 20% of UV. S=0.20 Total = C * S = 0.066, or SPF 15
Can anyone confirm this?
Bonus question: What about tan + sunscreen? Would they add, multiply, or just be the maximum of the two? I'm guessing it's not the last because black skin with a natural SPF of around 13 still benefits significantly from an SPF 15 sunscreen.