Just tried this calculation over the weekend - I had a new large mold and wasn't sure how much soap to make. Anyhow, it was kind of close, but did not make enough soap to fill my log mold to the desired height. I even added 4 oz of oils just for good measure (it's a 22 inch log mold). I think the difference is in the amount of water (or lye concentration) you use. I was using a fairly low water amount (33% lye concentration). I know people frequently use more water than that, so I'll have to take that into account next time. Is 38% pretty much the standard (I saw someone else on this post mention it)? I've used all different amounts, depending on the recipe. To the original poster - thanks for posting this. It was helpful to get a general idea of the amount.