A recipe is a weight; a mold is a volume
Every cold process soap recipe is written as oil weights, because saponification is a chemical reaction and the lye must be matched to the oils by weight. Every mold, on the other hand, is a container with a volume. Bridging the two is the calculation soap makers do most often and get wrong most often, and the consequence is either a loaf two inches short or lye batter running down the outside of the mold.
The bridge is a single conversion factor: 0.40 ounces of oils per cubic inch of mold. Multiply your mold's volume by it and you have the oil weight to enter into a lye calculator. Everything else on this page follows: the water and lye that get added to those oils, the total batter weight that actually fills the mold, how many bars the loaf cuts into, and what each weighs.
Measure the mold inside, and measure to the fill line rather than the top of the wall. Soap batter that goes through gel phase expands, rises and can crack or overflow, so a quarter inch of freeboard is a minimum and more is sensible if you are piping texture on top.
Where 0.40 comes from, and when to move it
The rule is not arbitrary, and checking why it works tells you when it stops working. A full-water recipe adds water at about 33% of the oil weight and lye at roughly 14%, so the batter that goes into the mold weighs about 1 + 0.33 + 0.14 = 1.47 times the oils. If oils are 0.40 oz per cubic inch, the batter is 0.40 × 1.47 = 0.588 oz per cubic inch.
Convert that to metric and the reason becomes obvious: 0.588 oz is 16.67 g, and a cubic inch is 16.387 cm³, so the implied batter density is 16.67 ÷ 16.387 = 1.02 g/cm³ — essentially water. Soap batter is mostly water and oil at roughly water's density, so a rule that fills a volume at about 1 g/cm³ is the right rule.
That derivation tells you exactly when to adjust it. A water discount removes water from the batter, so the same oils make less batter and fill less mold — the oils figure per cubic inch has to go up. A recipe run at 25% water instead of 33% gives a batter multiplier of 1.39 rather than 1.47, so to fill the same volume you need 1.47 ÷ 1.39 = 1.058 times as much oil, or about 0.42 oz per cubic inch. Going the other way, a very high-water recipe fills more mold per ounce of oil, which is where the sometimes-quoted 0.36 comes from.
The calculator shows the implied batter density from whatever combination of rule, water and lye you enter, precisely so you can check this. If that number wanders far from 1.0 g/cm³, one of the three inputs does not describe the recipe you actually intend to make.
The lye percentage is the one figure here you should not take from a general rule. It depends on which oils are in your blend, because each oil has its own saponification value, and on your superfat. Run the actual recipe through the lye and saponification calculator and bring the real number back here.
Worked example: a standard 8 × 3.5 in loaf mold
You have a silicone loaf mold measuring 8 in long and 3.5 in wide inside, and you intend to fill it to 2.5 in deep. You run a full-water recipe: water at 33% of oils, lye at 14%. You want 1 in bars, and you have an existing recipe built on 40 oz of oils that you would like to reuse.
- Mold volume. 8 × 3.5 × 2.5 = 70 in³, which is 70 × 16.387064 = 1,147 mL.
- Oils. 70 × 0.40 = 28 oz, or 28 × 28.349523 = 793.8 g. This is the number that goes into a lye calculator.
- Water. 28 × 0.33 = 9.24 oz.
- Lye. 28 × 0.14 = 3.92 oz.
- Total batter. 28 + 9.24 + 3.92 = 41.16 oz. Check the density: 41.16 oz is 1,166.7 g in 1,147 mL, so 1.017 g/cm³ — right where soap batter should be.
- Bars. floor(8 ÷ 1) = 8 bars.
- Weight per bar. 41.16 ÷ 8 = 5.145 oz uncured.
- Scaling factor. 28 ÷ 40 = 0.70, so multiply every ingredient in the 40 oz recipe — each oil, the water, the lye, the fragrance — by 0.7.
The per-bar figure is uncured weight. Cold process soap loses water through a four to six week cure, so a bar that comes out of the mold at 5.145 oz will label at less. If you sell by weight, cure a test batch and weigh it rather than working from this number.
Reading the result and resizing safely
The oils figure is the only number a lye calculator wants, so it is the output to write down. Everything else is a check or a consequence.
The scaling factor is how you reuse a recipe you trust. Multiply every ingredient by it — every oil, the water, the lye and the fragrance — and the chemistry is unchanged because all the ratios are preserved. What you must not do is scale the oils and leave the lye, or round some ingredients and not others. A recipe scaled by 0.7 with the lye left at its original value is a lye-heavy soap, which is caustic rather than merely disappointing.
Round the scaling factor as little as your scale allows, and always recheck the lye against the scaled oils. A digital scale reading to 0.1 g removes most of this risk; a kitchen scale reading to 5 g does not, which is why small batches need finer equipment rather than more care.
The batter density check is the quiet safeguard on the whole page. If it reads near 1.0 g/cm³, your three inputs are mutually consistent and the mold will fill. If it reads 1.3 you have almost certainly entered a water percentage that belongs to a different recipe, and the mold will overflow.
Finally, treat the fill depth as a design decision rather than a measurement. A 2.5 in deep pour in a 3.5 in wide mold gives a bar roughly 3.5 × 2.5 × 1 in, which is a chunky bar. Dropping the pour to 2 in gives a slimmer bar and 20% less soap, and changes nothing else about the recipe.
Batch size for common mold sizes
| Mold | Volume | Oils (oz) | Oils (g) | Batter (oz) |
|---|---|---|---|---|
| 8 × 3.5 × 2.5 in loaf | 70 in³ | 28.00 | 793.8 | 41.16 |
| 10 × 3.5 × 3 in loaf | 105 in³ | 42.00 | 1,190.7 | 61.74 |
| 12 × 3.5 × 3.5 in loaf | 147 in³ | 58.80 | 1,667.0 | 86.44 |
| 14 × 12 × 2.5 in slab | 420 in³ | 168.00 | 4,762.7 | 246.96 |
| 18 × 12 × 3 in slab | 648 in³ | 259.20 | 7,348.2 | 381.02 |
| 12 cavities × 3 in³ | 36 in³ | 14.40 | 408.2 | 21.17 |
Every row is volume × 0.40 for the oils and oils × 1.47 for the batter. Change the water or lye percentage in the calculator and the batter column moves; the oils column does not.
Scale the lye with everything else, every time
Sodium hydroxide is caustic and the amount in a recipe is matched to the oils, not to the mold. When you resize, multiply every ingredient by the same factor — each individual oil, the water, the lye and any additives. Scaling the oils by 0.7 and leaving the lye at its original weight leaves the batch with roughly 43% more lye than its oils can saponify, which produces a bar that burns skin rather than one that simply does not lather well. Safest practice is to re-enter the scaled oil weights into a lye calculator and take a fresh lye figure from it, rather than multiplying the old one and trusting the arithmetic.
Assumptions this calculation makes
- Straight sides. Length × width × depth assumes a box. Tapered silicone molds hold less than their rim dimensions suggest; measure the width at the fill line, or fill the mold with water and weigh it.
- Cavity volume measured, not guessed. Fill one cavity with water and weigh or measure it. A US fluid ounce is 1.8047 in³ and a millilitre is 0.061024 in³.
- Cold process, not melt and pour. Melt-and-pour base has no separate water and lye, so fill the mold by the base's own weight per volume instead.
- No allowance for what stays in the pot. A little batter always clings to the bowl and the stick blender. On a small batch that is worth an extra couple of percent.
- Uncured weights throughout. Every weight here is at the pour. Cure loses water, so finished bars weigh less, and how much less depends on your water content and cure length.
- Additives are extra. Fragrance, clays, purees and milks all add volume the oils rule does not account for. Fragrance at 5% of oils adds about 1.4 oz to the default batch.
Where mold sizing fits in the soap workflow
Soap making runs in a fixed order and this calculation sits at the front of it. Size the batch to the mold here, take the oils figure to the lye and saponification calculator to get the exact sodium hydroxide for your specific oil blend and superfat, and only then weigh anything. Doing it the other way round — building a recipe first and then hunting for a mold that fits — is how most makers end up with half-filled loaves.
If you are making your own mold rather than buying one, the silicone mold material calculator works out the rubber for it, and it uses the same displacement idea in reverse: mold box minus master rather than mold volume times a density rule. Fragrance is costed the same way across this whole category — the candle fragrance load calculator handles the percentage-of-what question that trips people up in soap as well, since soap fragrance is quoted as a percentage of oils exactly as candle fragrance is quoted as a percentage of wax.
The 0.40 rule itself is craft folklore rather than a published standard, but it is folklore with physics behind it, and the batter density readout on this page is there so you can verify it against your own recipe rather than taking it on faith. If your own poured batches consistently overfill or underfill, measure the batter your standard recipe actually produces, divide by your mold volume, and use that as your personal figure instead.