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How to Resize a Soap Recipe for Your Mold (U.S. Standard and Metric!)

One of the most common questions I receive is how to resize a soap recipe here on Modern Soapmaking to fit a different mold than what is used in a tutorial. To resize a soap recipe, we first need to know how much soap or oil your mold will hold by calculating your mold capacity.

How To Resize Your Recipe for Your Soap Mold
How To Resize Your Recipe for Your Soap Mold

There are two calculations that are widely accepted by soapmakers to determine the amount of oils needed for a recipe to fit a mold. These calculations are based on determining the volume of a soap mold and then finding the amount of the volume that belongs to the oils themselves.

The U.S. Standard (United Stated Soapmakers) Calculation:

Length of Soap Mold in Inches x Width of Soap Mold in Inches x Depth of Soap Pour in Inches x .40 or .38 = Amount of Oils Needed in Ounces

The Metric (International Soapmakers) Calculation:

Length of Soap Mold in Centimeters x Width of Soap Mold in Centimeters x Depth of Soap Pour in Centimeters x .70 or .65 = Amount of Oils in Grams

Now, these calculations are based on using close to full water (or full water if using .38 or .65 instead) in a soap recipe, so if you use a water discount, the final calculations won’t be as accurate for you. You can easily modify these calculations for your water discount by customizing your calculations.

Before you pour, know how much soap you really need!
Before you pour, know how much soap you really need!

Let’s first run through where I think the .40/.38 or .70/.65 comes from:

The .40/.38 or .70/.65 appears to come from calculating the individual volume amounts for each ingredient and determining the amount of the total that belongs to the oils. So, to start, we need to know the density of our oils, the lye, and our water. The density is nearly the same numerically as the specific gravity, which you can usually find on an ingredients MSDS (Material Safety Data Sheet) which your supplier should have on hand and available to you.

The density of most oils is 0.900 to 0.920 grams per cubic centimeter. For instance, a specific variety of olive oil I have on hand is 0.918 grams per cubic centimeter. The density of  lye is 2.13 grams per cubic centimeter, and the density of water is 1 gram per cubic centimeter.

To find out what our multiplier is (the .40 or .70) for our recipe, we need to find the split of volume per ingredient. Let’s say that the base formula we always use is 100% olive oil soap, with a 7% superfat, and 40% lye concentration.

You’ll need to pick a random oil weight – I’m using 1078 grams (roughly 38 ounces) which is usually around the amount of a Bramble Berry 10″ mold. You can use 16 ounces (454 grams) if you’d like – this number doesn’t matter, we just need to know what amount of the total volume of a supposed recipe is the oil itself.

If we throw our recipe into Soap Calc, we know that for a 7% superfat and 40% lye solution of 1078 grams of olive oil gives us the following water and lye amounts:

Water = 203.72 grams

Lye = 135.82 grams

So, we have these amounts of each ingredient, which we will divide by the density of the material to find the volume of each:

Olive Oil = 1078 grams / 0.918 grams per cubic centimeter = 1174.29 cubic centimeters

Water = 203.72 grams / 1 grams per cubic centimeter = 203.72 cubic centimeters

Lye = 135.82 grams / 2.13 grams per cubic centimeter = 63.77 cubic centimeters

If we add the individual volume of each ingredient (1174.29 + 203.72 + 63.77), we find that we have a total volume of 1441.78 cubic centimeters.

From here, we can determine the amount of the volume dedicated to the olive oil since we know the total volume of the formula and the amount of the olive oil alone.

1078 grams of Olive Oil / 1441.78 cubic centimeters = .748

So, a soapmaker using this formula and the metric calculation would use .748 instead of .70 to reach a more accurate calculation of the amount of olive oil they would need for a formula.

Need to go U.S. Standard? Follow the metric calculations and convert to U.S. standard:

1078 grams converts to 38.03 ounces, and 1441.78 cubic centimeters converts to 87.98 cubic inches.

38.03 ounces of Olive Oil / 87.98 cubic inches = .432

An U.S. standard measurement using soapmaker (aka a soapmaker in the United States ) would use .432 for their calculation instead of .40.

How accurate is the new calculation?

This is still slightly off since we rounded a lot throughout our math, and also because we do need a waste allowance (the smidgen of soap left in our soap pot after pouring) so I recommend rounding the hundredth place digit up by one. Doing so would give the metric soapmaker a 0.75 and the U.S standard soapmaker a 0.44 in the above calculation and formula.

How do I calculate for my specific formula?

You’ll need to know the density of each oil you use, and then replace the olive oil part of the calculation with each oil individually. You can even take this a step further and hand-calculate saponification values and specific volume of each oil during saponification, but I think that’s just a bit too much. (I believe this is how the Soapmaker software determines recipe resizing per mold, but I’m not 100% sure.)

For example, I used olive oil once again, but changed the lye solution strength and amount of example olive oil used to 500 grams, and these are the metric & U.S. standard calculation numbers, rounded up:

100% olive oil soap with 25% lye solution, 7% superfat = .66 metric calculation -OR- .38 U.S. standard calculation

100% olive oil soap with 33% lye solution, 7% superfat = .72 metric calculation -OR- .42 U.S. standard calculation

100% olive oil soap with 40% lye solution, 7% superfat = .75 metric calculation -OR- .44 U.S. standard calculation

If you don’t want to figure the calculations for your own recipe, using the above modified numbers will get you a bit closer than the default standard of .40 or .70 but it’s best to find the full calculation for your oil profile.

A recap on how to use these multipliers as an example:

Let’s say you are using a 100% olive oil recipe with a 33% lye solution and 7% superfat. From above, we know that our multiplier is .72 for metric or .42 for U.S. standard.

We measure the interior of our mold (that’s where the soap goes!) and find that our measurements are as follows:

Length of mold: 10 inches / 25.4 centimeters

Width of mold: 2.5 inches / 6.35 centimeters

Depth of pour: 3.5 inches / 8.89 centimeters

*A note on depth of pour: most soapmakers do not fill their soap mold to the brim. Your depth of pour is going to be determined by how tall you want the soap to be that fits in the mold. For instance, if a mold is 4 inches high and you only want the soap to be 3.5 inches high, your depth of pour is 3.5 inches – NOT 4 inches.

If you are a metric soapmaker, you would then multiply your L x W x D x .72 (your multiplier).

25.4 cm x 6.35 cm x 8.89 cm x .72 = 1032.385 grams of oil for this mold

If you are an U.S. standard soapmaker, you would then multiply your L x W x D x .42 (your multiplier).

10 in x 2.5 in x 3.5 in x .42 = 36.75 ounces of oil for this mold

Now, we can easily use a lye calculator such as Soap Calc to calculate our full recipe based on the ounces of oil needed for the mold (Step 2 of the SoapCalc form.)

Need to Know How to Resize a Soap Recipe for a Cylinder or Cavity?
Need to Know How to Resize a Soap Recipe for a Cylinder or Cavity?

But what if I’m not using a box shaped mold?

If you are using a cylindrical mold, such as a PVC pipe, you’ll need to find the volume differently.

Instead of L x W x D, you will be multiplying Pi x Radius x Radius x Depth of Pour.

So if you have a 3 inch PVC pipe and want to pour 10 inches deep, you would then do:

3.14 (pi) x 1.5 (radius) x 1.5 (radius) x 10 (depth)

And then you would multiply this by your multiplier (.40 or .70 – or your more exact multiplier you found above).

3.14 X 1.5 X 1.5 X 10 X .40 = 28.26 ounces of oils for this mold

If you are a metric soapmaker, you would simply use centimeters instead of inches and the metric multiplier for your recipe.

If you are using an irregular mold, such as single cavity shapes, you can always fill the mold cavity with water and weigh the water amount. Then use the water weight as the weight of your oils, but this is not hugely accurate and usually needs to be rounded up.

Did I lose ya? What multiplier do you find the most successful for your recipes? 

p.s. This method has never failed me, but I’d love to hear how it works for you. Leave a comment below! 🙂

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62 Responses

  1. I like to work backwards. I fill up the mold I’m using with water. I now know the approximate total of the raw soap and I make sure the water, lye and oil total up to that number. It isn’t exact since I am measuring and not weighing, but it works for me.

    1. Using water can give you a number less than it should be since almost all of your ingredients have a lower density than water, but it’s a quick and easy method, for sure!

  2. Wow, I think you just planned a Chemistry Lesson for my Sophomores 🙂 Seriously, all the math and chemistry. I think I will use this for one of my labs when we get to bases. SO COOL!

  3. Kenna I always love your tutorials!Thank you so much for the great content. With that said, this is all about me not you – I’m totally math phobic and horrible at it.
    I’m some what new to soap making (I have 15 batches under my belt) and I’m thinking about buying a bigger mold. I’m not sure that I have this correct. The mold is 6x8x12. My intent is to have 3x2x1″ bars when cut. So given your calculations I did 6x8x11x.40 = 211.2. Is that correct?

  4. I love this web site.
    I currently use a soap maker program which I love but the one big thing I have learned from you here is, where to find the specific gravity for oils. In my program the specific gravity has been set which I can change bit I didn’t know it was on the MSDS sheet and I didn’t understand how it affected the success of the finished soap. I am grateful to you for the explanation. I’m going to check the information for each of my oils and change it. I’m hoping to see that it improves my soap. Can’t wait for the next email and tutorial. Thanks

  5. This is so interesting, Kenna. I don’t make CP or HP soap yet, but I’m definitely saving this article for when I finally do take the plunge into the addiction. Thank you!

  6. I made my own wooden molds, and I needed a way to figure recipe size out in metric (cm/g), thank you so much!

    I used the x 0.70 with a little water discount and got exactly the soap loaf size I wanted!

    Just wanted to verify with you if possible, that by “full water” and using 0.65 you mean 38% water as most calculators have preset?

    Thank you again, love your blog!

  7. I have a soap recipe I use but would like triple the recipe ro make a larger batch I have know I no idea how to use a soap calculator I have tried and it is completely off I need someone who knows how to do it accuratly do it .
    Can u help

  8. It is a hardship for sure to be a soapmaker for 20 years and be SO BAD at math! I have relied on Soap/Lye Calculators and the kidness of my Mathemagician friends to help me with my recipe. SERIOUSLY bad at math. Like, no head for numbers at all. Fortunately, my other half is a genius and has been able to parse out all the formulas for me. We are about to build my wooden 20 lb molds (sizing up from 14 lb molds that I use currently) and we can do so because he is so brilliant at math! It sure isn’t because I am!

    Thanks for your fantastic tutorial! It makes no sense to me but he gets it and that’s what matters! 🙂

  9. Thanks for all this information. I really appreciate how comprehensive and detailed your information is…and I may have missed the answer to the following question…if so, I apologize.

    I have seen this .40 multiplier on other sites as well and understand how it works with CP. But is the multiplier for HP exactly the same? The reason I ask is because on the soapcalc tool it states, where showing the total amounts, “Soap weight before CP cure or HP cook.” Since we are cooking the soap in HP *before* it gets molded, shouldn’t we start out with slightly more volume than we would with CP?

  10. Thank you for sharing this. I didn’t realize there was a formula that could be so easily used. This will help with my future soaps! I very much appreciate the detailed calculations.

  11. Using full water of 38% has never worked for me as the batch would over fill the mold. I use 33% water or 30% lye concentration and it always works.

  12. I cannot get my head around this, i am not sure whether my maths are just no good but i am really struggling with working out how to modify the recipes i have seen to fit my mould. I have really tried to figure it all out im still having no luck, feeling abit down about it now.

    1. HI Zana, i am terrible at maths so what i did was input all the calcs stated above into an excel spreadsheet making sure that each of the formulas used totalled to what the figures above are. that way, i now only need to input 1 or 2 things into my spreadsheet and everything else automatically calculates for me. maybe give that a go and see (assuming you know the basics of excel & setting up formulas) 🙂

  13. I like that you talked about how you should adjust the calculations, if you usually use a water discount. There are some new molds I want to get, but I didn’t know how to make my recipe work with them. I can see how it would be smart to account for the water I don’t usually use, so I don’t make any mistakes.

  14. Hi Kenna
    Thanks for this article!
    I am just wondering what happens when you have additives like clay or colloidal oatmeal. Do you add them to the math as well?

    1. Hey there, Tais,
      Since we only recommend 1 tablespoon total additives per pound of oil, I don’t do any extra math to account for them. Also, the powders you mentioned soak up some of the liquid ingredients, so they don’t displace their total dry volume, making the difference even more minimal.
      Hope that helps!

      1. Helped heaps!!

        Thanks so much, Stephanie!! I just did a new mould here and I am really looking forward to using it! Knowing how much I need to fill it up will be very handy since is a “big” mould (only 11 bars hehehe, but I usually do only 4 per new formula)

        Have a lovely weekend!!

  15. FYI, the US doesn’t use imperial measurements but US standard measurements. As an example, a pint is 20 fluid ounces and a gallon is 160 fluid ounces.

  16. It’s a great tutorial.. I calculates for the 7in 3 inch diameter. pvc mould I have. But. While calculating using US metric *.4. And then converting ounces to grams… Results 373 grams
    Next I calculated in cms… International metric *.72. Surprisingly.. I get 153.73 grams

    Why so much difference… What am I doing wrong?? Please help

    1. Hey there, Akii,

      Just to clarify, we are talking U.S. standard measurements and metric. There isn’t a U.S. metric…metric is the same everywhere.

      I did the math with your mold dimensions, but, when I converted my U.S. standard ounces to grams, I was quite close to the figure I got when working out the weight in metric. (Being a bit off is expected with rounding.)

      3.14 (pi) x 1.5 in (radius) x 1.5 in (radius) x 7 in (depth) x .4 = 19.782
      Rounding up to account for waste, 19.79 ounces needed for this mold. That’s 561.03706 grams

      3.14 (pi) x 3.81 cm (radius) x 3.81 cm (radius) x 17.78 cm (depth) x .7 = 567.295575084 grams

      Can you see where you went wrong?

  17. hi all … I am very new to soap making, just gathering materials now and trying to figure out my first recipe and found your site. Planning to try with an olive oil 70% and coconut oil 30% recipe. One thing that is very confusing to me on the topic of this thread, is why soapcalc site doesn’t start with a “total yield” that I could enter, ie the volume of my mold? The mold I am buying says it has a 42oz capacity, if soapcalc would let me enter that number, then I could enter the percentages of various oils, it would calculate the ratios to get me to the 42oz for my mold. Seems like otherwise we have to back into that number with some custom calculations as in your thread above. I understand each oil is different weight and volume, but isn’t that what soapcalc could calculate those ratios for us? So, given the way it is today, instead of your calculations in this thread, why can’t I just change item 2-Weight of Oils, with a little trial and error looking for Soap Weight on the final recipe sheet to be the 42oz for my mold? If I put 42oz in for Weight of Oils, I end up with Soap Weight of 65oz which is too much for my mold because that also includes the lye water. So I leave the water% at 38, and reduce the Weight of Oils to, lets say 25oz, and recalculate and now Soap Weight is 38oz … getting closer. With a few more tries, I find that 27oz in Weight of Oils yields almost exactly the 42oz Soap Weight that I need for my mold. I know I’m brand new at this… am I completely missing something? This technique took me about 2 minutes and required no math for me to mess up 🙂
    This is the recipe I’m going to use based on 27oz oils and water% 38 … Water 10.26oz, lye 3.84oz, olive oil 18.9oz, coconut oil 8.10oz, fragrance .84oz.
    Thanks for any other insight …
    soap newbie … cjsalzano

    1. Hey there, cjsalzano,
      If your technique works for you, great! This is a quick and accurate method for folks who would rather do the math that work their way through trial and error. Everyone approaches problem solving a little differently.

      As for why SoapCalc doesn’t offer the feature you suggest, any extra calculations add complexity and expense to a website. I’m guessing it might be cost prohibitive. But, you’d have to check in with the owners to confirm that.

  18. Hi,

    I’m new to this whole soap making thing. I have read the information on this site and several others on how to resize and develop soap recipes. For my inquisitive nature though, they didn’t go quite deep enough into the details (I love getting down into the details) . I have done a lot of research on this and have written a somewhat geeky paper to document what I have learned The paper provides exact formulas for how to calculate the amount of each constituant in a soap recipe to fill any volume mold. It is a work in progress with some more details to confirm, but it is 99% complete. I thought it might be worthwhile to share it at this point for whatever feedback (positive or negative) others might have. It is an 8 page pdf file that can be found here: http://gregngigi.net/SoapEquations.pdf

    Greg

  19. Math is impossible for me and so is soap calc. I bought a 7 pound mold, measured it out now where do I put that number? I know the percent of each oil. Anything I put in soap calc never comes out 7 pounds.

  20. Just to clarify – when you calculate the volume of a mold, it calculates the amount of oils the mold holds. But soap batter isn’t just oils, it’s also water, lye, fragrance or essential oils, etc. So let’s say I have a mold that holds 63 ounces of oils but my recipe also needs 17.98 ounces of water, 8.97 ounces of lye, and 1.97 ounces of fragrance oil. Won’t that recipe make too much batter for the mold to hold?

    1. Yes, Bonnie, that technique would give you and over filled mold. That’s much of the point of this article and why we explain customizing recipe size to fit any mold. Our method takes lye solution (water and lye) into account. (Did you read the article completely?)

      Since we recommend limiting additives to 1 tablespoon per pound of oils, that extra volume is nominal

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