How to Make Soap to Use Up Old Oils - Recipe Formulation Technique and an Example Recipe

Have you ever wondered how to use up those old bottles of random oils in your soap making collection or kitchen pantry at home?

This video and written post will explain how to calculate a soap recipe just for your old oils, and use them up!

Waste not, want not 🙂

The soap recipe I demonstrated in the video was made with coconut oil, rice bran oil, canola oil, castor and sunflower oils, however you are likely to have a different oils at your disposal.

If you’re not sure which oils and/or combinations are best for soap making, watch this video where I discuss all the different oil types for soap making and how to choose them, including a live demonstration of how I evaluate my recipes using SoapCalc:

• VIDEO > Choosing Oils for Soap Making - Oil types and fatty acids, balancing recipes and Soapcalc explained 

Once you have decided which oils you would like to use up for your soap recipe, these are the steps to follow:

Step 1: Choose your mould and calculate the total oil amount needed

• Take a look at the oils you have to use, and decide which mould you would like to use.

• Work out the specific amount of oils needed for your mould in order to calculate a soap recipe to fit.

• I use the 70% rule to calculate my oil amount for my moulds. Approximately 70% of your total mould volume is a good amount of oil to use for your soap recipe. This leaves room for the water, sodium hydroxide and other additional ingredients (eg. essential oils/fragrance, which are optional). If you are unsure how to calculate this, I explain it fully in this video:

  • VIDEO > Calculate Soap Recipes to Fit Any Size Mould

• Write down your total oil amount needed for your soap recipe.

Step 2: Weigh each of your oils and record their amounts

• Choose a suitable container to weigh your oils into. I like to weigh my oils directly into the jug or pot I will use to make the soap.

• Starting with the oils you most want to use (you may have already decided approximate proportions for your recipe, or have annoying little amounts of oils you want to completely use up), weigh each oil into your container, one by one. Make sure to zero/tare your scale in-between each oil and write down the exact weight of each oil added before you move on to the next oil.

• As you weigh each oil into your container, keep a running total of the amounts you’re adding. Use this running total to give you an idea of when you are approaching your limit (total oils needed for your recipe - 70% of mould volume), and to decide how much of the final oil type to use to bring you to the desired total amount.

Step 3: Calculate your soap recipe in a soap calculator

• Use a soap calculator to calculate your soap recipe. Here is a list of my recommended soap calculators (I use SoapCalc out of habit).

• If you are unsure how to decide your superfat or water level for your recipe, please view these resources that cover those topics in more detail.

  • VIDEO > How to Calculate Your Own Soap Recipes Using SoapCalc

  • VIDEO > Calculating Water in Soap Recipes and Clarifying 'Lye Concentration'

  • VIDEO > Superfat Explained - Understanding 'Superfat' and 'Lye Discounting' in Handmade Soap Making

• Record your completed soap recipe in your notebook (or print the recipe).

• You will already have the specific oil amounts recorded, the soap calculator will tell you the exact water and sodium hydroxide amounts needed.

Step 4: (Optional) Calculate citric acid and adjust sodium hydroxide amount

• Citric acid is a useful ingredient in soap making. When dissolved into the water used for a sodium hydroxide (bar soap) lye solution, citric acid becomes sodium citrate, which helps to reduce soap scum in hard water areas and reduces spoilage and DOS (dreaded orange spots) in finished soap bars. This is especially useful when making soap with old oils that might be going rancid or are beyond their expiry date, however it is completely optional.

• If you do not have, or do not want to use citric acid, just leave your sodium hydroxide amount as given by the soap calculator and proceed to making your soap.

• If you would like to use citric acid, first calculate the amount you would like to use for your recipe:

  • I like to use citric acid at a rate of 1-2% of the total oil amount for my soap recipe (not including essential oils or fragrance, just the total of the main soaping oils).

  • In my video example recipe: Total oils 1500g x 2% (1500 x 0.02) = 30g citric acid

• Now calculate the amount of extra sodium hydroxide needed.

  • Each 1 gram of citric acid consumes 0.624g of sodium hydroxide, so we need to calculate how much extra sodium hydroxide is needed to avoid an unnecessarily high superfat in our finished soap.

  • My recipe example: I used 30g citric acid (2% of oils). 30g x 0.624 = 18.72g of extra sodium hydroxide needed.

  • Add the extra sodium hydroxide amount needed to the amount given by the soap calculator. I round the new total amount down to the nearest whole gram of sodium hydroxide.

  • Record your adjusted sodium hydroxide amount in your soap recipe/notebook.

Step 5: Make your soap

• This is the fun part :)

• Watch my video demonstration to see the adventure I had in making my batch of general purpose cleaning soap using up my old oils. Make sure you use a stick blender when mixing your soap batter to avoid the problem I encountered!

• If you are new to soap making, check out the many videos and playlists I have arranged on my soap channel home page to become more familiar with the process of making soap. You can also read this written page on where to start with soap making, and view this essential soap making safety video.

• To learn how to test and check your soap for correct pH after making (to ensure it was made safely), these videos have all the details on that topic:

  • VIDEO > Handmade Soap Testing - How to test the pH of your handmade cold process or hot processed soap

  • VIDEO > The Soap Zap Test - An old fashioned but reliable way to pH test your handmade soap bars before use

My example recipe from the video:

I thought I’d share a photo of the soap recipe from my notebook to show how I write out my soap recipes.

I have been making soap since 2014, and am on to my third full notebook! That’s a lot of soap :)

In the video I used slightly different essential oils than written here. I used 60g in total, a combination of mostly eucalyptus with small amounts of cedar and patchouli.

Other useful links:

• My coconut and rice bran oil laundry soap recipe

• How to make your own laundry powder with handmade soap base

Thank you for reading. Have fun making your own soap recipes!

If you found this video and post helpful, please consider supporting my work via Buy Me a Coffee ☕ Thank you!

See you next time,
Elly 🌸

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