Too much batter and your soap overflows the mold. Too little and you get thin bars with wasted space. Getting the batch size right starts with understanding how mold volume translates to oil weight — and the fill factor that accounts for the rest of the recipe.
How mold volume calculation works
Soap batter is denser than water, but the exact density varies with your oil blend. A common approximation used by most mold calculators is 0.40 — meaning 40% of the mold volume (in ml) equals the oil weight you need in grams. This is sometimes called the fill factor or soap batter multiplier.
Oil weight (g) = Mold volume (ml) × Fill % × 0.40
Example: a 1500ml loaf mold filled to 90% → 1500 × 0.90 × 0.40 = 540g of oils
FormulaAlchemy's built-in mold calculator handles this automatically. Enter your mold dimensions and fill percentage, and it returns the exact oil weight — which is then pre-filled into the lye calculator so you can build your recipe around it.
Calculating volume for different mold shapes
Rectangular / loaf molds
The most common soap mold type. Volume is simple: Length × Width × Height. Measure the inside dimensions, not the outside. Fill to 80–90% to leave headroom for expansion during gel phase.
| Mold size (L×W×H cm) | Volume (ml) | Oil weight at 90% fill (g) | Approx. bars (115g each) |
|---|---|---|---|
| 20 × 7 × 7 | 980 | 353 | ~3 |
| 25 × 9 × 7 | 1,575 | 567 | ~5 |
| 30 × 10 × 8 | 2,400 | 864 | ~7 |
| 40 × 10 × 9 | 3,600 | 1,296 | ~11 |
| 50 × 12 × 9 | 5,400 | 1,944 | ~17 |
Cylindrical molds (PVC pipe, Pringles tins)
Volume = π × radius² × height. For a 5cm diameter tube that is 30cm tall: π × 2.5² × 30 = 589ml. At 90% fill and the 0.40 multiplier, that is about 212g of oils — roughly 2 bars.
Individual / cavity molds
For silicone molds with multiple cavities, calculate the volume of one cavity and multiply by the number of cavities. Use the mold calculator to find the total oil weight for all cavities combined.
Getting the batch size right
The water displacement method
For oddly shaped molds, forget the formula. Fill your mold with water to your desired fill level and pour into a measuring jug. That volume in ml, multiplied by 0.40, gives you the oil weight in grams. Simple, fast, and accurate for any mold shape.
Why 80–90% fill?
Soap batter can expand during the gel phase — particularly in insulated molds or high-water recipes. Filling to 90% rather than 100% prevents overflow. First-time batches in a new mold: fill to 80% until you know how that mold and recipe behave together.
Scaling your recipe
Once the mold calculator gives you your oil weight, that number becomes your batch size. The lye calculator then uses that total to compute lye and water amounts. You can adjust the oil blend percentages freely — the mold-driven oil weight stays fixed and everything else scales with it.
| Fill factor (multiplier) | When to use |
|---|---|
| 0.40 | Standard — most oil blends, 33% lye concentration |
| 0.38 | High-water recipes or very fluid batters that will rise more |
| 0.42 | Stiff batters, high-coconut recipes, or hot process soap |
For a brand-new mold, make a small test batch — 300–400g of oils — to confirm how the mold behaves during gel phase and how many bars you get at your preferred slice thickness. Once you know the mold, you can confidently scale up.
Calculate your mold size and batch in one place
FormulaAlchemy's mold calculator returns the exact oil weight for your mold dimensions, then pre-fills it into the lye calculator. Free, no account needed.