How to Calculate the Volume of a Soap Mould

There are so many mould options available when you make soap and looking for suitable moulds can become a bit of an addiction. Trust me I’ve been there!

There are all sorts of mould shapes and sizes, and you don’t even have to buy conventional soap moulds. 

When you are starting out you might find that trips to the supermarket take on a whole new purpose. When I was new to soap making, I was often side tracked in supermarkets with looking for suitable packaging and containers that would make good soap moulds – there are so many when you start looking – such as pringles cartons and yogurt pots to name a few. 

If you are like most soap makers, you will probably start out using silicone loaf moulds. Or if you are like me when I started making soap, I made my first batch of soap using an orange juice carton (empty of course). 

Once you have selected a suitable container to use as a soap mould you will then need to work out your recipe size. In other words, how much soap will fill your chosen mould? You can of course choose to make a recipe from a book, but it may not fill your mould to the desired level. 

Calculating your recipe size is really quite easy. And trust me maths is not my strong point. 

Please bear in mind that your calculation is never going to be exact and this is because soap recipes contain both oils and water which have different densities. So, although 1 litre of water and 1 litre of oil have the same volume, 1 litre of water is heavier than 1 litre of oil. 

According to official estimates, olive oil has a density of 0.92. Therefore, 1 litre of olive oil weighs 0.92 kg, or 920 grams. 

You can do a simple calculation to work out the volume of any container as follows:

Volume = Length x Width x Height

For example, you have a nice carton that measures 18cm x 13cm x 5cm. And you calculate the volume as 1,170 grams. This is the total volume for your recipe – that is water and oils. 

But you will also need to find out how much oil this recipe will contain. To do this you need to minus 40% off the volume to allow for the water element in your soap recipe. This leaves you with 702 grams for oils. I would recommend using 700 grams as the total oil weight into a soap calculator. 

Then all that remains is your decision on the lovely ingredients combination that you want to use to create your soap.   

There is also a handy online calculator which will do the volume calculation for you. Click here to use the volume calculator. 

Top tip: When making soap it’s always a good idea to have a few smaller containers to hand like small cupcake cases or mini yogurt pots. These are handy to fill with the remainder of your soap batch if you don’t want to fill your mould to the brim. 

Click here to read my blog post on how to choose a soap mould. 

If you would like to join me on one of my soap making courses please click here.