How Does Soap Calc Calculate Recipes?

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Jul 22, 2019
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San Francisco
I’ve seen many good articles, some referred to me here, on how SoapCalc calculates its figures for hardness, conditioning, etc, but how does it calculate its recipes?

It seems that anything more than a 2 layer soap taxes my abilities of concentration to the breaking point, given my already faulty organizing. It could be a sign of early Altzheimers or an indication of an exercise (Soap making) that could ward off the above. But here’s the basic—-this is a math question related to soap making and Soap Calc in particular.

I set out to make a 3 layer, 5 oil soap with pencil lines in indigo. I’d measured and melted the solid oils intending to divide them by 3 after melting. The olive oil had been divided on paper earlier in the week so that 2/3 of the recipe amount could be infused (1/3 each) with natural colorants: spirulina and parsley powder. On soaping day I measured and melted the solid oils (palm, coco, shea butter) together, and set out the two colored olive oils (filtered and measured to the appropriate amount) and a third, plain olive oil whose colorant, French green clay would be added to its lye solution. All I needed to do was to divide the Castor Oil by three and add those three equal portions to each of the olive oils... Divide by 3. I forgot that crucial part.

Back to the beginning. The total weight of the oils for my recipe (a 10” mold) is 32.4oz. I’d specified on the formulating page that the Castor Oil would be at 7% (understanding that more than 10% would make a rubbery soap). So...

In my distracted mind, I screwed up and added the full recipe amount for castor oil to two of the three olive oil containers——1.13oz each. The third got the correct amount, or .376oz (1/3 of the recipe amount or 1.13oz)

After the usual stomping about, hollering, and cussing passed, I finished and poured the soap. This morning, after unmolding a nice brick of soap I went back to recipe to see how much Castor I’d actually over wrought...

I’d measured in grams as my new scale allows that, reading two decimals past the point. But not speaking that language, I looked to the ounce column and immediately realized something was off.

At 1.13 oz in a batch where the total weight of all the oils is 32.4, the Castor oil is roughly 1/32 of the total oils—-Am I wrong? Come on math minds, let me know.

1/32 on conversion charts, online, is 3% more or less. So, good news, if more than 10% of a recipe in Castor Oil will make a rubbery soap, I’m safe: At the most I have 61/3% of Castor Oil in my soap (1.13+1.13+.376= 2.636oz). But, if I specified 7% Castor Oil on the formulating page, why is Soap Calc only specifying 3% (or 1.13oz) of Castor Oil? At worst, this is the first time out of four that I’ve made this recipe with anything close to the 7% castor oil I’d specified, but I don’t understand how Soap Calc is coming up with its numbers.

If I specify that a given oil is to be 7% of the total, why is SoapCalc calculating a recipe wherein that oil is but 3%?

Feel free to math me, just do in in plain English.

If it is necessary: Lye is at a 33% solution and I have always left super fat at 5%.
Last edited:


Well-Known Member
Oct 20, 2018
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Lancashire UK
What's the full recipe in g?

Sounds easier if the castor had been put in with the solid oils, I don't see the benefit of two sets of dividing (OO and Castor).


Well-Known Member
Feb 20, 2013
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"...If I specify that a given oil is to be 7% of the total, why is SoapCalc calculating a recipe wherein that oil is but 3%?..."

Honestly, at this point I have no clue. The numbers are obscured by the verbiage. Can you pare your story to the essentials and also post a screenshot of the soapcalc recipe that pertains to your basic question?