Superfatting… What’s that?

Superfatting title

Superfatting, it’s probably a word you’ve seen in many articles and on many blogs, but what does it actually mean?

If you’ve already got to grips with SAP values (and if you haven’t you can read more about it here) you’ll know that each oil has its own sap value and that it takes an exact amount of sodium hydroxide to turn a specific oil into soap, that’s saponification. So when creating a formula you calculate how much sodium hydroxide is needed to turn the weight of each oil into a soap and add them together.

So not only is soap making a science, it’s a mathematical formula too.


Weight of oil x SAP value = weight of sodium hydroxide need for saponification.

  • Olive Oil has a SAP value of 0.134, so if your formula has 500g of olive oil
  • 500g x 0.134 = 67g of sodium hydroxide
  • Coconut (refined) has a SAP value of 0.190, your formula has 300g of coconut oil
  • 300g x 0.190 = 57g of sodium hydroxide
  • Palm oil has a SAP value of 0.141, you are using 200g of palm oil
  • 200g x 0.141 = 28g of sodium hydroxide
  • 67g + 57g + 28g = 152g of sodium hydroxide


For your 1kg of combined oils you would need 152g of sodium hydroxide to turn all of the oils into soap.

Superfatting is either adding extra oil into the formula or discounting some of the lye used, so that there will be oil within the soap that has not saponified, that is oil that has not turned into soap

So why?

Basically if every last drop of oil is combined with lye and turns into soap there’s not much oil left to make your skin feel good is there. Superfatting allows small molecules of oil to become trapped between the soap molecules and these are transferred to your skin when washing. Now luckily whilst the lye turns the oils into soap it also produces glycerine, which is a humectant, attracting moisture to the skin and forming a protective barrier to prevent your skin losing moisture. Essentially it is what makes natural soap so moisturising.

But the main reason to superfat is to eliminate the risk of your soap being caustic. If every drop of oil finds every drop of lye (the caustic alkali) then the soap should have no caustic alkali left. So what happens if you had a gram or 2 more lye or a few grams of oil didn’t make it into the final pot? You’ll have a nasty caustic bar of itchy scratchy soap that’s what.

SAP values are averages, I mean really, really, good averages but oils will change very slightly from batch to batch. And however brilliant our equipment is, unless you have lab quality scales you can never be totally accurate on the weights and small variations may happen. Even a small amount of extra lye would make your soap unusable. A formula that does not allow for superfatting would never receive a CPSR (cosmetic product safety report), so you wouldn’t be able to sell it either.

So now on to how?

The easy way is to use a soap calculator, there are loads available on the internet, but if you try 3 or 4 different sites you may be surprised to see they all give you different calculations.

If you regularly use a soap calculator you’ll be used to it doing the calculations for you, but it’s really quite easy. Personally I prefer to work it out myself, just to be safe.

But the next question really is how much to add or discount. In order to superfat you don’t actually have to add any more oils, I find it much easier to discount some of the sodium hydroxide instead. There is no hard and fast rule as to how much, many people work on 5% some less although I wouldn’t recommend it. We work on around 13%, it sounds high but we perfected our formula over 15 years ago and it works, so why change it.

As always you’ll need to experiment, you don’t want your soap to become too soft and a lot will depend on the oils you are using as they all lend different attributes to your soap.

So this is how I do it.

Oil Weight (g) x Sap Value =
Olive Oil 500 x 0.134 67
Coconut Oil 300 x 0.190 57
Palm Oil 200 x 0.141 28.2
Total 152
Sodium Hydroxide (Caustic Soda)       (Total – 15%) 129
Total Weight of Oils 1000
Total Sodium Hydroxide 129
x 33% (= Water needed) 373

Hopefully this will give you the confidence to give it a try, and as always if you have any questions just ask. If we don’t know the answer we’ll do our best to find out.

If you’re looking for some ideas on what oils to use, why not read our previous blog article on “What oils to use in cold process soap” by clicking right HERE

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