SoapMaking Friend Recipe Calculator INCORRECT Regarding Adapting Recipe Size to Cylinder Dimensions

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Since I make shave soap and use a 3" PVC Sch40 pipe for a mold, I needed to adapt my recipes to fit that mold size volume-wise.

Here's the interesting part:
1. Use the calculator for a cylinder dimension of: 1.5" radius and 23" height. NET RESULT IS: Total Batch Weight 1884.47 grams
That number is shy of the true amount by about 786 grams! Whoa Nellie! I kept looking at the number given me and in my mind I just knew that the 1884 number was off by quite a bit... actually by about 42% !! Time for the webmasters to "git 'er done" and correct this anomaly.

2. Correct way to compute is: Radius of 3" cylinder is 1.5", this number needs to be squared: 1.5 x 1.5= 2.25
Since the classic formula is Pi x r/2 for area, for a cylinder it is:  V=πr2h Volume = pi x r/2 x h (height)
So the 3" x 23" cylinder's volume works out to: 1.5 x 1.5 = 2.25 x pi (3.14) = 7.07 sq. inches x 23" height = 162.61 cubic inches.
(Now we go to Google to convert cu. inches to cubic centimeters (equivalent... more or less to grams of oil).
We'll round this off to 163 cu. inches for simplicity's sake.

163 cu. inches converted to cubic centimeters is: 2671.09 cu. cm. this is "roughly" equivalent to the amount of oils, etc. used to fill the cylinder with soap mixture.
Actually my recipes are set to 2670 grams on the "By Total Size" adjustment and they completely fill up the PVC pipe which is actually 23-3/4" in height with a 1/2 thick thick internal plug for the bottom of the mold.

Thoughts? Suggestions? Comments? I know there will be some MENSA scholar or M.I.T. grad out there who will take me to task on my above though processes, but hey it's not the first time I've been around the "proverbial block". At my age, Rhino Hide Skin is a blessing. I'm no Einstein that's for sure!
einstein (1).jpg


So... I wanted to point this out in the hope of getting that part of the calculator recipe resizer tweaked and corrected. I'm someone resized a batch and was a little baffled about the net result.




 

SoapDaddy70

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I do think there is something wrong with the cylinder conversion on Soap Making Friend but what you have listed is incorrect. In your first bullet point where you say the TOTAL BATCH WEIGHT is 1884.47g. That is actually not the total batch weight but the total amount of oils/fats that should be used for a cylinder of that size. Someone can correct me if I am wrong but from what I have learned - to find out the amount of OILS/FATS to use in a recipe for a cylinder mold - the calculation you have is correct as pi x r/2 x h (height). After that you take that number and multiply by .4 and that gives you total ounces and then you multiply by 28.35 to get total amount of oils in Grams. I actually came up with 1842 grams of oils for a 3 x 23inch cylinder. I plugged these numbers into Soap Making Friend and it actually gave me exactly half the number it should be.
 
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Not an Ivy League graduate, but I confirm that you've really hit something strange there.

First, the volume formula itself:
Since the classic formula is Pi x r/2 for area, for a cylinder it is:  V=πr2h Volume = pi x r/2 x h (height)
You somehow pasted this wrong, but went on working correctly with it. It's A = πr² = π×r×r = π/4×d×d for the area of a circle with radius r (or diameter d), and V = πr²h = π×r×r×h = π/4×d×d×h for a cylinder with height h. Mind the square power (not “divide by two”).

Good news: soapmakingfriend does respect these dependencies. Doubling the radius will quadruple the volume/weight, and doubling the height will double it.

BUT the absolute values of these numbers are indeed utterly off. Shortly switching over to the rectangular mould, the calculator recommends for a mould of 10×10×10 cm³ = 1 L volume a “Total Batch Weight” of 986.87 g (recipe: 100% almond oil, 2:1 lye, but we'll come to this later). A cylindrical mould with the same 1 L volume has at r = 10 cm a height of h = V/(π r²) = 1 L/(π × 100 cm²) = 3.183 cm. But when we enter these values, the batch weight shrinks down to 493.42 g. That's exactly half of the actual batch size. No other explanation than a stupid formula error.

Next, another issue that isn't just a mathematical copy-paste mistake: Batch size does not depend on lye concentration. SMFriend is just assuming some average recipe. Then it computes the amount of oils for it, and scales the recipe (amount of lye/water/…) so that it fits this number. You can change the lye concentration/ratio/water amount, the “total batch size” varies accordingly, but not the amount of oils!
To be fair, estimating the density (specific gravity) of a soap batter is a nontrivial thing to do, and depends heavily on a lot of factors I wouldn't be fond of maintaining in an intermediate-level hobbyist soap calculator either. But AT LEAST the amount of water is easy to track. I mean, the script knows about X grams of oil and Y grams of water, which, roughly at the same density, need roughly the same volume for some weight. So I'd expect that the amount of oils goes down when I lower lye concentration – but it just doesn't. They have the numbers, the math doesn't need to be complicated, not even particularly precise, but it should better be considered at all. There is indeed some advanced logic behind the calculation, since the amount of oils (hence, batch size) does depend on the saponification value of the oils.

Another (unrelated) bug in the SMFriend GUI that just now popped into my eye is that the value of the Saturated:Unsaturated ratio is given in the wrong order (unsaturated first, saturated second).

ETA: @SoapDaddy70 was a few seconds quicker than me 😜
 
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Thanks to both of you - @SoapDaddy70 & @ResolvableOwl !

I figured I missed something or wrong digits or whatever. Having ADD can be little frustrating sometimes. Used to drive my Algebra and Geometry teachers nuts because they knew that I knew the formulas; it was the mathematics aka simple arithmetic that I would FUBAR. LOL!

@ResolvableOwl Your reasoning about the batch size, etc., is one of the reasons I went to Google to figure out the cu. inches to cubic centimeters to oil to blah blah blah. My shave soaps turn out excellent using the "By Total Size" computation, after conversions are made to grams from cubic inches to cubic centimeters etc.

Hopefully, this will encourage the folks at the top of the food chain here to rectify this error.
 
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Yup, I've totally had that problem - their cylinder calculations made 2.5x as much soap batter as I needed. First time, I thought it was me. Second time, I did my own math to confirm that yes, the calc was generating 2.5x the amount of batter needed.

I love so much about SMF but this, and the glitch for lye adjustments (when using vinegar or CA with master-batched lye) drive me batty. It should not be hard to fix these!
 

Tara_H

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It's got a lot of issues really, which is a shame! The design is nice and modern and some of the features (if they worked) would be quite innovative, but it feels like it's been abandoned before being quite finished :(
 
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I went and posted a copy of the text along with a couple of the comments of my thread here on their forum page... whether of not it gets looked at and/or resolved is another question.

We, as soap makers, need to ensure that the results we're getting from these "time saving" calculators are actually accurate.
I have noticed that using a simple recipe will net different results depending on which website's calculator one uses.
 
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First is Snip & Sketch capture of the test I did using the calculator's cylinder input of; 1.5" radius (half of 3" cylinder width) and 23" height.
Test of Soap Calc Using correct Cylinder Sizing.png


This second Snip & Sketch capture is of the actual recipe I use based on "By Total Size" adjustment. I had to do several Google search when trying to get the right volume / weight for my 3" x 23" cylinder mold. No... I didn't get obsessive / compulsive about it. Even though the recipe amount is for a 23" tall cylinder my mold is actually 24" tall and with the acetate sheets they stick out about another 1/2". The extra I use the fill any air pocket voids that may occur.
Actual Cylinder Volume from My Recipe Stash.png



I am now off to check out other soap calculators to see what differences that there may be, if any, or even if they offer a cylindrical calculation.
 

gloopygloop

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We, as soap makers, need to ensure that the results we're getting from these "time saving" calculators are actually accurate.
I have noticed that using a simple recipe will net different results depending on which website's calculator one uses.
[/QUOTE]

This has always been an annoyance of mine and I dont understand why this is.
 
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I've worked long enough with SAP tables and hand calculations, to appreciate the ease of soap calculator websites.

SoapCalc, a “first-gen” online calculator, brought a pretty much exhaustive oil & FA database – but that's about it. I was somewhat convinced of it since it appeared that all SAP tables out there wrote off their numbers from them anyway.

I quickly switched to LyeCalc or SMFriend since the FA profile is updated interactively while tuning the amounts of oils. Nowadays, I'm using calculators EXCLUSIVELY to compose the oil blend: adjust amounts of oils until they add up to 100 g, then dial superfat and note the effective SAP (g NaOH per 100 g oils). I'm comfortable doing anything beyond this (batch size, lye concentration, dual-lye/KOH, additives like citric/acetic acid) by hand. I know too much really intimidating math to not be frightened by a few straightforward additions and multiplications – nobody should, FWIW.

That said, I don't have a slab mould that I'd need to fill up spot-on, but most of my batches are at 100 g oils anyway 🤭.

Tbh, I used SMFriend's mould size calculator the first time yesterday, to check what @Professor Bernardo has observed.
 
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Easy-peasy. What I do is weigh my mold then tare the scale. I then fill the mold with water and weigh it, then multiply the weight by 90% to get a batch weight. I then tweak my recipe for that weight, make some soap and adjust as necessary.
I agree that figuring out the mold capacity isn't hard. What annoys me is that the calculator offers to do that for people, but doesn't actually do it correctly. It would be better if they didn't offer.
 
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In defense of SoapMaking Friend Recipe Calculator I will state that overall it has been an invaluable tool for me personally. This cylinder volume issue is really the only glitch that I have come across.
Easy-peasy. What I do is weigh my mold then tare the scale. I then fill the mold with water and weigh it, then multiply the weight by 90% to get a batch weight. I then tweak my recipe for that weight, make some soap and adjust as necessary.
Easier said than done with a 3" ID PVC pipe x 24" long. Rather cumbersome to deal with. Besides I like the mental challenge... keeps me sharp to ward off all those millenials and their texting minions!
 

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I agree that figuring out the mold capacity isn't hard. What annoys me is that the calculator offers to do that for people, but doesn't actually do it correctly. It would be better if they didn't offer.

I agree. I’m learning how to calculate Lye Solution by hand. It’s not that difficult, just takes to get the hang of it.
 
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In defense of SoapMaking Friend Recipe Calculator I will state that overall it has been an invaluable tool for me personally. This cylinder volume issue is really the only glitch that I have come across.
It is also my favorite calculator, and the only one I use.

But be forewarned, there is at least one (technically two) other glitches: the lye adjustments for vinegar and citric acid don't work if you are using master-batched lye.

Interesting that your calculation result ended in more soap batter than needed. My result was that I had half of what I needed.
Very interesting, esp since I soap with 40% lye solution, making my batches smaller in volume compared to most others using the same amount of oils. Perhaps they tried to fix the issue at some point in between our respective attempts, and over-corrected in the other direction.
 
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My grief with soaping in general is that my mental acuity isn't sharp enough for me to rely on it for accurate mathematical calculations. Right now I know I'm on the edge of a migraine simply because I'm not using simple English to write this post.
Too often lately, if I try to make even the most basic calculation, such as the quantity of each oil, according to the percentage needed by the recipe, I can't do it.
As an example: 50% lard in a recipe that calls for 4000g of total oils would be 2000g of lard. For some bizarre reason, while doodling on a piece of paper, before using the lye Calc, I was writing down 3000g for the lard. And to my addled brain it looked correct.
I've been a business manager since I was 16 yo, and running small mom and pop businesses based solely on my math skills. I was taking calculus, or trying to, by my Senior yr of high school, so my math has never been an issue for me. Until just these past few years. I really need to be able to trust that these calculators in order to satisfy my need to soap! 😭🧼
Sorry. Rant over. I apologize for hijacking the thread, just wanted to point out that these calcs aren't just convenient for some, but a necessity quite possibly quite a few.
 

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But be forewarned, there is at least one (technically two) other glitches
There's a bunch, sadly... Maybe not as egregious as these, but things like; the batch functionality doesn't seem to work at all as intended, if you copy a recipe with additives and then try to remove them, they'll keep coming back, and at least one other thing which doesn't come to mind right this second... I wanted to love it, but it just kept betraying my trust 😫
Since moving to SM3 I'm so much happier and haven't looked back with regret even once
 
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