Soap Mold Volume Calculator
Figure out how many ounces of oil your mold holds so your recipe fills it without overflowing.
Pour height is how deep you'll actually pour the batter, not the mold's full wall height. Take this oil total to the Lye Calculator to work out the matching lye and water.
How it works
Before you can run a recipe through a lye calculator, you need to know how much oil your mold actually holds. Soap makers use a simple rule of thumb for this: multiply the mold's volume in cubic inches by 0.4 and you get a close estimate of how many ounces of oil will fill it. That 0.4 figure accounts for the fact that soap batter is denser than water but still lighter than the solid oils it's made from once it saponifies and cures.
Worked example: a standard 10 inch silicone loaf mold measures roughly 10 inches long, 3.5 inches wide, and you plan to pour 2.5 inches deep (not necessarily the mold's full height, since most loaf molds have some empty wall space above the batter line). That gives you 10 × 3.5 × 2.5 = 87.5 cubic inches. Multiply by 0.4 and you land on 35 oz of oils, which converts to about 992 g. That 992 g becomes the oil total you plug into the lye calculator to get the matching lye and water for the batch.
Why pour height matters more than mold height
A lot of beginners measure the full inside height of the mold and wonder why their recipe overflows. The mold's wall height is not the same as how deep you'll actually pour. Most loaf molds have extra headroom above the fill line so you can add texture or a swirl on top without the batter spilling over the edge. Measure the depth you actually intend to pour to, usually an inch or two below the rim, and use that number instead of the mold's total height.
FAQ
Does this work for round or oddly shaped molds?
The 0.4 rule works for any mold shape as long as you can estimate its volume in cubic inches. For a round mold, use the volume of a cylinder (radius squared × π × height) instead of length × width × height, then multiply by 0.4 the same way.
Why 0.4 and not a simpler number like 0.35 or 0.5?
Soap makers arrived at 0.4 through years of trial and error across typical cold-process recipes. It's an estimate, not a law of physics, so expect your actual yield to land within a few ounces of this number rather than exactly on it.
Should I fill the mold to the very top?
No. Leave some headroom, both because cold-process batter rises slightly as it goes through gel phase and because a slightly underfilled mold is much easier to unmold cleanly than an overfilled one.
My mold has dividers or cavities. Does this still apply?
Yes, just calculate the volume of a single cavity rather than the whole tray, then multiply that per-cavity oil amount by however many cavities you plan to fill.
Once you know your oil total, head to the lye calculator to get the matching lye and water, or read how to adapt and resize a soap recipe and how to unmold and cut cold process soap once your batch is ready.