[thisguy]

I don't know if this question is applicable to this forum, but I have faith in the intellectual aptitude of this sights subscribers. I'm attempting to make my own sun block. My question is, whether or not the spf value of my end product increases by the individual spf rating of each substance or whether the spf of my end product is only as high as the highest individual spf. Maybe it is neither. Thank you for your consideration of the matter.  (Example) substance A has spf rating "4" , substance B has spf rating "10". Combine in equal parts and what do you get? "10"? "14"?.

[Arkcon]

No, they won't sum together.  The concentration of ingredients that protect against the sun, relative to other ingredients that  carry it or soften your skin have been metered to give a certain protection number.  Perhaps mixing equal quantities of two products would give you the average protection, except ...

Well, see, I once mixed some sunblock level 2 oil with some sunblock 8 lotion, figuring I'd get some protection between 4 and 6.  I mixed them as well as I could, in my bottle, but after using it and spending a day in the sun -- I sunburned ... in patches.  I guess they didn't mix.  I looked worse than sunburnt, because everyone had to ask, what happened to my skin, and I had to explain ... and they all said, "Damn you Arkcon, you and your chemistry and random mixing.  Gotta mix everything.  Can't just buy the right SPF sunblock."  So, unless you know a bit more about formulation engineering, I'd suggest you not start mixing.

[fledarmus]

Without knowing the individual components of your sunblock, it is almost impossible to predict. One issue, as Arkcon pointed out, is that there are components of sunblock which are present only to keep the UV-absorbing particles spread out in the mixture. These are not necessarily compatible between sunblocks. If you have a sunblock which contains zinc oxide particles, for example, they will have a surfactant which keeps them from forming clumps and spreads them so that a thin film will have the proper number of particles evenly spread to reduce the UV the amount you want. Disturbing that surfactant by adding a second, non-compatible surfactant, may simply clump up your zinc oxide, and your film will contain some patches with a very dense collection of zinc oxide particles, and other patches that have none at all.

If you are going to try to make your own sunblock, I would recommend that you start with the individual components rather than trying to mix already complex mixtures.