Making soap with used cooking oil


New soaps sitting in a dark, dust-free container just waiting for the 6 week waiting period to be over.

Usually, I make my soap with brand new vegetable oil of some description; it is reliable and it doesn’t smell too bad. This time I decided to up my up-cycling game and use the oil from our deep fryer. The deep fryer was bought by my partner who decided he likes to deep fry stuff so much he had to have a specialised tool for the job. I protested the purchase, but was over-ruled.

It uses a lot of oil to fill (about 3 litres) which has to be changed regularly and that used oil has to go somewhere, so I decided to try making soap from it. The oil my partner uses in his automated heart attack machine is rice bran oil, which is apparently quite good for your skin, even after having cooked half a tonne of chips and a few fish pieces.


Old rice bran oil, just waiting to become soap

I did some reading about people doing the same thing and researched some recipes. I found that a lot of people make soap from used cooking oil and many people make rice bran oil soap, although usually not as the largest proportion of oils. In the end I resorted to using an online soap calculator to make my recipe for me, these things are great, they do the complicated maths for you and spit out a printable recipe at the end.

This is the recipe I ended up with for this soap;

Rice bran and oatmeal soap

1500g used rice bran oil (filtered)

500g coconut oil

700g water

283g caustic soda



Weigh out ingredients carefully

Add caustic soda to water in a heat proof container (I use a glass coffee pot). Use only glass, plastic or stainless steel (anything else will melt). Stir well in a highly ventilated area.

Place oils in a stainless steel pot large enough to hold the entire soap mixture. Heat until oils are at 30-35 degrees C.

Place caustic solution in a cold water bath if required until it is at the same temperature as the oils.


Slowly pour caustic solution into oils, mixing with a long plastic spoon. Stir until combined. Continue to stir until solution reaches trace, or do what I do; get bored with the whole thing and blend it with a stick blender until trace. Add any other bits and bobs you have at this point.


oil and caustic solution combined


After a little bit of stirring


At trace. You can see the faint line left by the soap mix as it is dribbled back into the pot.

Trace is reached when dribbles on the surface of the mix leave a ‘trace’ of a mark. A bit like when whipped cream can be formed into peaks (but not as extreme). If you can see a faint line left on the surface of the mix when you dribble some back into the pot, you have a trace.

Pour soap into molds of some kind and cover to keep dust, animals and small children out.


Wash up the huge amounts of pots and pans you have somehow made dirty with this project.

After a day or two your soap can be unmolded and set in a dust free, ventilated area to cure for six weeks. Then it is usable.


The soap in its mold


A close up of the new soap. The white on the surface is soda ash, which doesn’t effect the soap at all, but is considered undesirable as far as looks go.


I will be making more soap from my used cooking oil in the future (if only to avoid wasting it). It feels like another step in the right direction to be using litres of purchased oils twice rather than using and throwing away. I have about 10 litres of used oil in various bottles in my already overcrowded craft room, Maybe a soap making spree would free up some shelf space for me?


3 thoughts on “Making soap with used cooking oil

  1. Pingback: Less waste lifestyle review | Chronicles of a humpy dweller

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