Learn to Formulate & Design Custom Soap Molds, Plus How to Calculate How Much Soap a Mold Holds

Updated: Nov 13, 2019



How to Calculate a Custom Soap Mold Size

Designing and creating a custom soap mold is actually really easy. It takes a little bit of math, a few pieces of wood, and some nails. That's it! Of course you can also use other materials like acrylic and silicone, but how do we determine the size? How do we know how long or wide it should be? And once we are finished, how do we know how much oil to use and how much soap it will make? What if we find a new mold or something like a like a wooden wine box or wooden drawer organzier (cover image) that would make the perfect mold? How would we know how much soap it needs to fill it or how to formulate our recipe?


These are all excellent questions, so let's find out!

Mold Size First, you want to figure out what size/shape you need your mold to be, then you can worry about the volume and total recipe weight. What size bars do you want? Do you want them to be square or rectangle? Tall and skinny or short and thick? Do you want a flat slab mold, a rectangular loaf mold, or a combination of a slab/loaf mold? Do you want single layer or multiple layers? By answering these questions, you can begin to plan and design your custom soap mold!

After you have an idea about the type of soap mold and bar shape you would like to have, then you need to take the size of your desired soap bar and find out how many bars of soap you want your mold to produce and how they will fit into the mold. A loaf mold measures the bar dimensions when it is standing up on a flat surface, while a slab mold measures bars that are laying down on a flat surface. With slab molds, you can't have a quantity of bars that is unable to be evenly divided. For example, you can create a slab mold that will hold 15 bars which would give you 3 rows of 5 bars, but you can't create a slab mold that would have 11 bars due to spacing issues and it would need to be raised to 12 or lowered to 9/10. You also want these measurements to be your interior mold size, not your exterior size. For example, if you are using 1 inch thick wood, you need to account for that, as it adds an additional inch to the total mold size.

I always try to visualize my mold and then draw a sketch of what I want my mold to look like and then a little sketch of my soap bar. We are essentially trying to figure out how many little soap bars can fit into one square or rectangle. I always add 1/4 inch to the ends of my loaf molds for end loaf pieces too.


How to Calculate the Total Oil and Soap Weight Needed for a Soap Mold

Example 1: If you want your final bars to a 3-inch square bar that is 1 inch thick (LxWxH or 1x3x3), and you want to have a loaf mold that holds 20 bars, you would find the interior mold size by multiplying the width by 20 (and then optional adding 1/2 inch for ends). For this example, your mold would be 20.5 inches long (1×20+ optional 0.5 for both ends) by 3 inches wide by 3 inches high.



How to design and size a loaf soap mold

Example 2: If you wanted to make a square flat slab mold that holds 9 bars, you would multiply the width/length of a flat bar by 3 because we will need 3 rows of 3 bars. Our example would fit 3 bars in each row and would need to be 9 inches long by 9 inches wide by at least 1 inch tall (add however much additional height you want).



How to design and size a slab soap mold

Mold Volume Next, we need to determine the volume of our mold. This part is easy, we simply multiply the length by width by height. Our first example from above has a volume of 184.5 cubic inches (20.5×3×3). Our second example has a volume of 81 cubic inches (9×9×1). If you decide to make your molds higher or shorter or design purposes, do NOT include this in the volume. It is likely that you will make your slab mold higher than 1 inch, but including this in your measurements will increase the size of the bar.


Mold Weight After we have determined the size and volume of our mold, we need to find out how much soap it will hold. This will allow us to properly formulate the recipe with the correct amount of oils and other ingredients needed to fill the mold without a surplus or not enough soap. Once you figure out the size and volume of your mold, then you can figure out the total weight that it will hold. The formula for this is Volume= Mass÷Density. Every material used has a different density. Water has a different density than oil. Olive oil has a different density than coconut oil. Thus, we need to determine the density of our soap recipe when in its pouring phase.

Put simply, Mold Volume ==> Mass = Total Estimated Mold Weight (Easily done using a volume to mass conversion calculator)


Because every recipe is different and has a different chemical composition, the density will not be the same. I like to pick a standard density and apply it to all of my calculations which makes things much easier. I like to use the density of olive oil, not water like many others suggest. I do this because our total recipe is often comprised mostly of oils, commonly with 50-70% soft oils.

To find the mass or the total mold weight, I personally cheat on this part and use a volume/weight calculator (because I don't want to convert everything to kilograms and meters, although you certainly can calculate this by hand using the density of olive oil at 920 kg/cubic meters). When I use the mass/volume calculator, I select olive oil as the density in my calculator and use it to convert the total old volume to mass. Be sure to select the proper units of measurements. You will then have the total weight that your soap mold will hold.


You can find the calculator for converting volume to mass here.

Example One: Our first example created a loaf mold that holds 20 square bars that are 3 inches tall, 3 inches wide, and 1 inch thick with a total mold volume of 184.5 cubic inches. Now that we know our volume, we can use the volume to mass calculator to determine the approximate weight of soap that the mold will hold. We set the density of our soap calculator to olive oil and then we can convert the mold volume to mass. I used ounces/inches, but you can use any of the units of measurement. This would give us a total mold weight of approximately 102.24 ounces. Thus, we know that to fill our soap mold, we will use a recipe that totals approximately 102.4 ounces and can calculate our recipe accordingly.



How to Calculate the Total Oil and Weight of A Soap Mold

Example Two:

For our second example, we created a flat slab mold that holds 9 bars of soap. The mold is 9 inches long by 9 inches wide by 1 inch tall. Let's pretend that we added an extra two inches to the height to help keep the soap in and provide a little more room for playing. We wouldn't change the volume of the portion that holds the soap though because we don't want to increase the bar size. Our total soap mold volume would remain the same at 81 cubic inches. We then need to convert the mold volume to mass using the calculator and olive oil as the density. Our total mold weight would be approximately 45 ounces. We could then calculate our total recipe requirements with the knowledge and understanding that our mold will hold approximately 45 ounces of soap.



How to Calculate the Total Oil and Weight of A Soap Mold

Now that you know how much estimated soap your mold holds, you can determine the amount of ingredients you need to use. Most recipes are calculated based on the total oil weight and the percentages of each oil, so I find that is the best starting place.



Alternate Formula for Calculating Soap Mold Weight

Many people will also use the popular formula [Total Oil Requirement=Volume×0.40] to estimate the total recipe oil weight needed to fill the mold. Although this is a great starting place for beginners, and I recommend it for the absolute beginner, I don't recommend it for experienced soap makers. This equation does not take into consideration variances in water concentration, fragrance oils, additives, PCSF, and more.

If most soap making websites recommend the equation “Total Oil Weight= Volume X 0.40,” then why do we make our process a little more complicated? Let’s look at a quick example:

Say we are using a mold that is 10x3x3 (inches) or has a volume of 90 inches cubed. If we used the 40% equation (90x. 4),), our recipe should use 36 ounces of oil to fill the mold. Does this equation always work to figure out the precise measurements needed?

What happens if we made a traditional castile soap with a 5% superfat, made with only olive oil, lye, and water using a 1:1 lye: water solution? If we used a soap calculator, we would see that this recipe requires 4.63 ounces of lye, 4.63 ounces of water, and 36 ounces of olive oil, for a total batch weight of 45.26 ounces. Now, what if wanted to use the same recipe with a higher water concentration, like a 3:1 lye: water solution, and include 8% of a strawberry fragrance oil? This recipe would require 13.89 ounces of water, 4.63 ounces of NaOH, 36 ounces of olive oil, and 2.88 ounces of fragrance oil, or a total batch weight of 57.41 ounces.

Our first batch was made with a lower water concentration and without any additives and had a total batch weight of 45.26 ounces. Our second batch was made with a higher water concentration and fragrance oil and it weighed 57.41 ounces. Both recipes used 36 ounces of oil, but both recipes had very different total batch weights. If we elected to formulate our recipe using the Vx.40=Oil Weight equation, we would never have a consistent batch weight and could end up creating too much soap, or not enough soap. If instead, we changed the mold volume to estimated mass, our mold example would have a total estimated batch weight of 47.86 ounces. We could then adjust the amount of oil, additives, and water, so that we never have too much or too little soap. This may not be something of importance when making soap as a complete beginner, but when you gain experience and need more precise measurements, this can certainly help.


Final Thoughts

Something to also consider is to account for shrinkage. Most soaps shrink by approximately 10%, more or less depending on the recipe and the total water used. The more water that is in your bar, the more weight it will lose as it reaches a point of equilibrium with the environment around it. If you use a really high water concentration, this may be something to factor into your mold size because your final bar size may be 10% smaller.

So that's it! Easy peasy!