How much oil does my soap mold hold?
A soap mold is sized in volume, but a recipe is written in oil weight — and the two are not the same number, because the batter that fills the mold is oils plus lye plus water. Getting from one to the other takes two facts: how dense fresh batter is (about 0.95 g per mL — a touch lighter than water) and what share of that batter is oil, which depends on how much water your recipe uses. At a 33% lye concentration, oils are about 70% of the batter. So a 10 × 3.5 × 2.5 in loaf mold holds 87.5 in³ (1,434 mL), which is 1,362 g of batter, which means weighing out 956 g (33.7 oz) of oils.
Every soaping site tells you to multiply cubic inches by 0.4 to get ounces of oils. Nobody says why. It is simply the two facts above multiplied together: 1 in³ = 16.39 mL; × 0.95 g/mL of batter = 15.6 g; × 70% oil at 33% concentration = 10.9 g = 0.39 oz. The rule rounds that up to 0.40, which is why molds sized by the rule run a few percent full — fine if you leave a little headroom, messy if you pour to the brim.
Why the factor changes with your lye concentration
More water in the recipe means more of the mold is taken up by water, so less oil fits. A recipe at a 28% lye solution carries noticeably more water than one at 40%; use the same 0.4 for both and the wetter batch overflows while the drier one comes up short. This calculator reads the concentration you enter (it remembers the one you use in the recipe builder) and derives the factor:
| Lye concentration | Oil share of batter | oz of oils per in³ | g of oils per mL |
|---|---|---|---|
| 28% | 66.7% | 0.366 | 0.633 |
| 30% | 68.2% | 0.374 | 0.648 |
| 33% (default) | 70.2% | 0.386 | 0.667 |
| 35% | 71.4% | 0.392 | 0.679 |
| 40% | 74.1% | 0.407 | 0.704 |
Batter density 0.95 g/mL; typical NaOH 0.14 g per g of oils at 5% superfat. Computed by this site's engine. The spread from 28% to 40% is about 11% — the difference between a clean pour and batter on the counter.
The most accurate method: weigh the water
Geometry works for boxes and cylinders. For anything else — a rounded silicone loaf, a tray of shaped cavities, a mold with tapered walls — line it with a bag, fill to where you would pour, and weigh the water. One gram of water is one milliliter, so the scale reads the volume directly. Pick Water fill above and type the grams in. The forum shortcut “fluid ounces × 1.8 = cubic inches” is the same idea in US units (the exact figure is 1.805).
Common soap mold sizes and how much oil they take
| Mold | Volume | Oils (g) | Oils (oz) | Batter (g) |
|---|---|---|---|---|
| 10 × 3.5 × 2.5 in loaf (the common “2.5 lb” silicone loaf) | 1,434 mL · 87.5 in³ | 956 | 33.7 | 1,362 |
| 12 × 3.5 × 2.5 in loaf | 1,721 mL · 105.0 in³ | 1,148 | 40.5 | 1,635 |
| 9 × 3.5 × 3 in tall loaf | 1,549 mL · 94.5 in³ | 1,033 | 36.4 | 1,471 |
| 9 × 12 × 1.25 in slab | 2,212 mL · 135.0 in³ | 1,476 | 52.1 | 2,102 |
| 3 in PVC round, 10 in tall | 1,158 mL · 70.7 in³ | 773 | 27.3 | 1,100 |
| 12-cavity tray, 100 mL each | 1,200 mL · 73.2 in³ | 800 | 28.2 | 1,140 |
| 25 × 8 × 7 cm loaf | 1,400 mL · 85.4 in³ | 934 | 32.9 | 1,330 |
Filled to the stated height, 33% lye concentration. Check your own mold's inside dimensions — “10-inch loaf” describes the outside of a lot of different molds.
A mold sold as “holds 2.5 lb of soap” means about 2.5 lb of finished batter, which at 33% concentration is roughly 1.8 lb (796 g) of oils. Running a 2.5 lb oil recipe into that mold puts 481 g of batter on the counter.
How many bars will the loaf make?
Bars come off the loaf at the thickness you cut, so bar count is loaf length divided by bar thickness, rounded down — a 10 in loaf cut at 1 in is 10 bars; at 1.25 in it is 8. Each bar weighs the batter weight divided by the count before cure, and loses roughly 10–15% of that as water over four to six weeks. Enter a bar thickness above and the count appears with the result.
Pour height, not wall height
Measure the inside of the mold, and the height you will actually fill to. Most people leave a quarter inch or more of headroom, and soap that is going to be textured on top needs more. The Fill field does the same thing as a percentage if you would rather keep the full wall height in the dimensions. Either way, a little under beats a little over: a batch that is 5% short of the top still makes perfect bars; one that is 5% over makes a mess you have to clean up with gloves on.