How to figure how much oil for mold.

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Soapmaker Man

Well-Known Member
Jul 17, 2007
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SW Missouri
Hi everyone.I wanted to post this as a sticky so all could learn how much oils are needed for your mold. Here is the way to figure it.

Lets use a log mold that is 12 inches long, 3-1/2 inches wide, and you want to pour to a depth of 2-1/2 inches.

You take length X width X height of pour, that number X .40. So;

12 X


42 ounces of oils needed for this mold.

Apply these same numbers for your mold size, weather a log, slab, or block. :wink:

IanT said:
hey paul, thanks for the helpful thread!!..

just curious, where does the .40 come from??

Something I learned by asking questions at another forum years ago. :) I am not the one who figured this out, can't take credit for it. I can 101% absolutely guarantee this to work! I have been using this calculation for all my TOG Molds since I started making them, and it always works perfectly! 8)

IanT said:
Im going to do some more research but i do trust you that it works! i just like to know the logistics behind everything (again just my personality :) ! Thanks!

Im going to have to try this...get a big mold and just do it up :)

Tell you what Ian, in front of everyone, if it doesn't work, I'll send you a new TOG Mold, FREE! How is that for confidence buddy? :p :lol:
hahaha works for me!! I trust your word though! never led me astray yet through the what...million and a half questions ive asked and youve answered so far!??
Basically, you simply multiply the volume of the mold by .40, regardless of the shape of the mold.

So, if the mold is cylindrical with the base radius of r and height of h, the formula would be: pi * r * r * h * .40, or about 1.257 * r * r * h.

If the mold is a half-sphere or a dome with radius r, the formula is: 2/3 * pi * r * r * r * .40, or about 0.829 * r * r * r.

Hopefully the math wasn't too confusing. Perhaps someone can explain it better than I can.

--Metal Substance

P.S. -- one way to figure out the volume of the mold is to fill it all the way with water, then pour the water into a measuring cup. Then, simply multiply the amount of water in the cup with .40 to get the amount of oil to use.
MetalSubstance said:
P.S. -- one way to figure out the volume of the mold is to fill it all the way with water, then pour the water into a measuring cup. Then, simply multiply the amount of water in the cup with .40 to get the amount of oil to use.

LoL That way seems the best. :lol:

p.s. The value of pi is 3.14, so the formula for a cylinder (round mold) is
3.14 x r x r x h
r = radius
h = height

Heck, I'm going the water method. :?
Im so horrible at math its not even funny....I had to study soooooo long and hard to pass those classes in school, came out o.k. thanks to the math labs and such but it was HARD :)
Thank you Paul. It was extremely helpful. I desperately needed to find out how much oil to use in the mold and then I stumbled upon your thread. Your calculations were very clear and helpful. I'm positive my soap turns out great.

Good to know that.
Thanks you all.
Last edited by a moderator:
Great information to have! BTW...I have one of Paul's latest molds and he is right on with his calculations! I love it so much...I rarely want to use the small, 4 cavity soap molds from the craft store now. They have pretty designs...but Paul's mold is so much easier to use! I've made half a dozen or so batches with it, so far, Paul! Lovin' every minute of it! :D
oil for molds

you are correct. depending on the size of your molds, i have found this to be accurate within an ounce or two. just upgraded from 4# molds to 7# molds. it was right on the money.

lather up!


Just one question .. when the M&P base is in blocks, how do you know how much to melt to get the "liquid amount"? I hope that makes sense.. for example, say my mould held 100ml of water, how much solid block soap do i need to weigh out in order to have 100ml of melted soap? One would assume around 100g, but i dont know...

That may have a really easy and "commonsense" answer, but times when i've used this method, ie measuring how much water the mould can hold, i've not been totally successful, had a bit too much most of time..

Thanks very much everyone :) would really appreciate an answer for this :)
Thank you for that Paul.
Here riding out tropical storm Fay.
Up listening to the wind battering on our windows.
I like to know the reasons behind things to. Is it because the amt of oils used in the process equals 40% of the total volume of the final outcome? That would be the only reason I can figure for the .40 there.

Hope someone can help me on that. I haven't done this stuff yet, so I am a complete noob. But, first batch is coming soon and I want to be sure I have all this right. I studies and studied the RTCP method last night and lye calculators this morning. :)

Thanks for any help!~

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