Sunscreen Maths: Can You Add up SPFs?

By | June 1 2011 | 40 Comments

sunscreen maths

Since we’re taking a look at all things sun-care this week, lets address – again! – one of the greatest beauty myths there is.

Namely, that you can add up your protection factors to achieve an overall level of SPF.

The myth goes something a little like this. “So, if my foundation has SFP15 and my moisturiser has SPF10 then BINGO! I must be getting a total of SPF25!”

Well, no. It just don’t work like that, sister.

Let me draw you a simple, yet very illustrative analogy. If you go to the pub and order two pints of 4% strength beer and pour them into a larger container, what do you have? Unfortunately, by the very power of pouring and mixing divine intervention has not occurred and it has not suddenly transformed into a beer of 8% strength. No, what you have now is merely two pints volume at – you guessed it – 4% strength.

And it is exactly the same with sunscreen – you’re only protected to the highest level you applied (i.e. SPF15 in your foundation), not the aggregate of the lot.

The end.

Anti-ageing, Face, Tanning, Skin , , , ,

40 Replies to "Sunscreen Maths: Can You Add up SPFs?"

  • Aifs says:

    Hehe love the beer analogy and it’s spot on. You can’t get clearer than giving us an alcoholic reference! :)

  • Baglady says:

    Wow, is there people out there who really believed that?

  • papillon says:

    Kirstie, it’s amazing that you used that analogy. I remember a flatmate of mine in college (way back in the day!) came home and announced she was drunk as she’s had 9% alcohol. She went on the explain how she’d had two bottles of cider each at 4.5%

    I spent hours trying to explain what volume meant re percentages but she never got it. Glazed look over the eyes.

  • Lis says:

    Great analogy! love it!

  • Lisa says:

    Em, I do not think you are really wrong here but surely the water in the sun cream either evaporates off your skin or your skin absorbs it, while the particles that provide protection against sun stay on your skin?
    As in layering on foundation ten times results in lots of coverage but not a wet face?

  • Kirstie says:

    Lisa – well foundation isn’t a great analogy because that’s the visible plane of light and that’s not how UV works. So no – and I have checked this with a couple of experts, by the way!

  • BerG says:

    Well put Kirstie, an example all Irish’s can relate to. I like to put the actual sunscreen (usually the highest SP factor) on first.

  • BerG says:

    Kirstie: On the tanning theme, could you do a post on Tan Maximisers if you get a chance. Just wonder are they actually a gimmic.

  • Ola says:

    Very well put girls. The only gripe I have with this analogy is that if you mixed 6% beer with 4% beer, it won’t be two pints of 6% beer ;) but I suppose it doesn’t matter does it? We’re talking SPF here…

Leave a Reply

Content © and partners