129.48 * .4 = weight of oil in oz. This is the APPROXIMATE weight of oil that you would need to use in your formula to fill you mold. In this case you would need 51.79 oz of oil by weight.
This question is pretty common. The simple answer is multiply the volume of the mold by .40 or .70 if you’re measuring in metric, but there are a number of factors that can influence the result. If you want to be more precise, here’s the process I use.
What you need to know:
- Lye concentration (lye %)
- Superfat (%)
- Volume of the mold
- Saponification value of your recipe
- Density of your lye concentration
The lye concentration and the superfat value are arbitrary numbers that you get to determine.
The volume of the mold you can figure out using a volume calculator on the Internet. Here’s a good one ==>
Volume Calculator. I won’t go into details here.
The saponification value is the number of grams of NaOH required to saponify 1 gram of oil. To get the saponification value for your recipe, you’ll need to run the recipe through a
soap calculator, like
soapmakingfriend.com. Set the superfat value and fragrance values to 0 and look at the results. You're interested in two numbers, the NaOH weight and the Oil weight. Divide the NaOH weight by the Oil weight. The result is the saponification value for your recipe. This will be unique for every recipe and is the reason you need to use a
soap calculator.
You can get the density of the lye solution from something like this ==>
The Complete Sodium Hydroxide Density-Concentration Table Calculator.
The basic process is to calculate the oil volume ratio based on 1 gram of oil and then apply that ratio to the volume of your mold. (I use .92 as the average density of oil, but you could use the actual oil densities for your recipe if you want to get really precise. For my purposes .92 is more than good enough.)
Here are the steps:
- Weight of oil = ( 1 + ( superfat% / 100 ))
- Volume of the oil = ( 1 + ( Superfat% / 100 )) / .92
- NaOH weight = saponification value from above
- Weight of the water = ( NaOH weight * ( 100 - lye% ) / lye% )
- Weight of the lye solution = ( NaOH weight + H2O weight )
- Volume of the lye solution = ( weight of the lye solution / density of lye solution )
- Total volume = Volume of oil + Volume of lye solution
- Ratio of weight to volume = weight of oil / total volume
- Amount of oil = Ratio of weight to volume * volume of mold
Here is an example:
I have a mold that is 9.5 long, 3⅛ wide, and I pour to 2⅛ high. This is 63 cubic inches or 1034 cubic centimeters.
I’m using 38% olive oil, 29% coconut oil and 33% lard for the oils with a 36% lye solution and 5% superfat.
To get the saponification value, I add the oils to
soapmakingfriend.com, using the percentage values, with zero % superfat and zero % fragrance. This gives me 15.12 grams of NaOH and 100 grams of oil. Saponification value = (15.12 / 100) = .1512
I look up the density of the lye solution at 40 degrees Celsius, from here ==>
The Complete Sodium Hydroxide Density-Concentration Table Calculator, and I get 1.3768
I plunk these numbers into a spreadsheet and this is what I get. (I’ve added the spreadsheet formulas and the equations for reference in this post)
| Row | | Spreadsheet Col "E" | Equation | Value |
| 3 | Volume (cc) | 1033.1 | 1033.1 | 1033.1 |
| 4 | lye conc. % | 36 | 36 | 36 |
| 5 | Superfat % | 5 | 5 | 5 |
| 6 | Saponification value | 0.1512 | 0.1512 | 0.1512 |
| 7 | Lye density (g/cc) | 1.3768 | 1.3768 | 1.3768 |
| 8 | | | | |
| 9 | weight of oil (g) | =(1+(E5/100)) | (1+(5 / 100)) | 1.0500 |
| 10 | volume of oil (cc) | =(1+(E5/100))/0.92 | (1 + (5 / 100)) / 0.92 | 1.1413 |
| 11 | | | | |
| 12 | weight of NaOH (g) | =E6 | 0.1512 | 0.1512 |
| 13 | weight of water (g) | =E12*(100-E4)/E4 | 0.1512 * (100-36) / 36 | 0.2688 |
| 14 | weight of lye solution (g) | =E12+E13 | 0.1512 + 0.2688 | 0.4200 |
| 15 | volume of lye solution (cc) | =E14/E7 | 0.42 / 1.3768 | 0.3051 |
| 16 | | | | |
| 17 | total volume per gram (cc) | =E10+E15 | 1.1413 + 0.3051 | 1.4464 |
| 18 | RATIO (grams/cc) | =E9/E17 | 1.05 / 1.4464 | 0.726 |
| 19 | | | | |
| 20 | total oil in grams | =E18*E3 | 0.726 * 1033.1 | 750.0 |
In this case I would need to use 750 grams of oil to bring the height of the pour up to 2⅛. I can now go back to
soapmakingfriend.com and adjust the recipe to the correct volume.
To convert the RATIO from metric to imperial (oz/ci) just multiply by .578. (in this case it would be .42)
I know I gave the dimensions of the mold in inches, and the example is in metric. As a Canadian who lived through the change to metric in the 70s, I sometimes mix things up.
Here's an app that I wrote while learning java. It does the math part. I make no warranty, implied or otherwise as to the accuracy of the app, but I've used it a couple of times and it worked for me. App ==>
https://soapsizer.web.app/ It would be nice to see
soapmakingfriend apply the same type of methodology.
