One of the most frustrating aspects of making soap that I encountered as a beginner was trying to figure out how much oil to use to make soap fit in a mold. I knew my soap mold held 3-4 pounds of soap, but what did that translate to in terms of amount of oils? My first batch didn’t have enough, and I wound up with squat-looking soap. My second batch had way too much, and I wound up making a weird looking extra bar (that really could have been two extra bars). It turns out I just wasn’t looking in the right place.

Soap Making Essentials is bar none one of the most comprehensive and helpful websites for beginners learning to make soap. Did you catch that terrible pun? I’m sorry. It turns out they had the answer to my problem on their Homemade Soap Recipes page. You can figure out how much oil to use with a very simple mathematical formula:

**Length of mold X Width of mold X Height of mold X 0.40 = Ounces of oil needed**

I just can’t believe this simple mathematical formula is not in every basic soap book. The only one I have found that even mentions is is Anne-Marie Faiola’s *Soap Crafting: Step-by-Step Techniques for Making 31 Unique Cold-Process Soaps*.

I probably should have paid more attention in math class, or I might have been able to figure out the formula myself, but I’m glad I stumbled on it anyhow. Thank you again, Soap Making Essentials, for the wonderful resource.

So what should you do if you have a round mold? Time to go back to Geometry 101.

The area for a cylinder (which is basically what a round mold is, even if it’s a short cynlinder), is Πr²*height of the mold. If it’s been a while for you, remember that Π is roughly equal to 3.14 and r means the radius, which is half of the diameter—the measurement of of the mold from side to side. So, if you have a cylinder mold that is 4 inches in diameter, then it has a radius of 2. Two squared (the little ² by the r) means multiply 2 times itself, which happens to be 4. Multiply Π and the radius of 2², or 4, and then multiply the product of that calculation by the height of the mold. Let’s pretend the mold is 10 inches. Multiply 4 x 3.14 (Π) x 10, which would equal 125.6 ounces. But wait! Remember we have to account for the water, so multiply one more time by 0.4. 125.6 x 0.4 = 50.24, which means your cylinder mold will hold a recipe using about 50 ounces of oils.

**Height of mold X Radius of mold squared X 3.14 X 0.40 = Ounces of oil needed**

Note: Be careful that you do square the radius (multiply the number by itself) rather than just double it. It will not always be double. For example, if you have mold with a radius of 3 inches, then squared, it would be 9 rather than 6.

If you have an mold with an odd shape, then it is best to calculate how much oil to use by using a water-volume calculation. You should fill your mold with water and then pour the water out into a measuring cup with volume markers. Then, multiply the number of ounces in the cup by 1.8 to calculate the total number of cubic inches in the mold. Then multiply that number by 0.4. For example, if your irregular mold holds 6 ounces of water, multiply 6 x 1.8, which is 10.8. Then multiply 10.8 x 0.4, which is 4.32. You would use 4.32 ounces of oils to make soap with this mold.

**Amount of water in ounces X 1.8 X 0.40 = Ounces of oil needed**

A final word of advice: I use grams in all my measurements because I find them more accurate, but these formulas work for ounces. I am sure there are similar formulas for grams, but I have not found a good calculation for metric volume. My advice would be to create a recipe using ounces, and then use the metric amounts given in SoapCalc, which always delivers the amounts in ounces, pounds, and grams for easy conversion. Alternatively, you could calculate your amount of oils in ounces and multiply by 28.35 (the number of grams in an ounce) or use an online conversion tool, such as this one, to calculate how many grams your mold will hold.