Soap Mold Batch Calculator
Oils to fill your soap mould, from its size and your recipe. It does not calculate lye.
Result
Oils to weigh out: 40.8 ozEstimate
Total oils and fats, before lye and waterAn estimate by volume. Trace, additives and the mould’s own shape change the fill a little, so keep a spare cavity handy.
- Batter to expect
- 62 oz (3.9 lb, 1,757.1 g)Estimate
- Mould volume to fill
- 105 in³ (58.2 fl oz)
- Oils per unit of mould volume
- 0.388 oz/in³ (0.672 g/cm³)Estimate
https://calcgallery.com/crafts/soap-mold-batch-calculator/
Results are arithmetic on the figures you enter and are estimates. Products, materials and animals vary, so follow the maker's instructions and label, make a test piece where it matters, and ask a vet about feeding. Nothing here is safety or veterinary advice. Read the full disclaimer
How to use the Soap Mold Batch Calculator
- Choose a loaf or slab, a round cavity, or a capacity you measured with water.
- Enter the inside size and the height the soap will fill to, and how many cavities.
- Enter your recipe's lye solution as a share of the oil weight, from your lye calculator.
- Read the oils to weigh out, then put that oil weight into your lye calculator for the lye and water.
How it is calculated
mould = length × width × height, π × (diameter ÷ 2)² × height, or the capacityoils per unit volume = 1 ÷ (1 ÷ oil density + r ÷ lye solution density)oils = mould × cavities × oils per unit volume × (1 + extra)batter = oils × (1 + r)Here r is your recipe’s lye solution, the lye plus the water, as a share of the oil weight. A mould full of batter holds the oils and the solution side by side, so a pound of oil brings its own volume and r pounds of solution with theirs. Solving for the oil weight that fills the mould gives the figure above.
The figures behind it
The oil density is olive oil’s: Codex standard CXS 33-1981 gives a relative density of 0.910 to 0.916 at 20 °C, and the page takes the top of the range. Coconut oil, lard and butters differ a little. The lye solution is taken at 28% sodium hydroxide by weight, a common full-water strength, whose specific gravity is 1.3105 in OxyChem’s Caustic Soda Handbook. The default r of 52% is the worked recipe on Classic Bells: 10 ounces of oils with 1.46 ounces of lye and 3.74 ounces of water. Change any of them to match your own.
Estimates, not measurements
Volumes do not add exactly when lye and oils react, air is stirred in, and additives such as clay or exfoliants take up space. The answer is close enough to plan a batch, and a little over is easy to pour into a spare cavity. For moulds you make yourself from silicone, the silicone mold calculator works out the rubber.
With your numbers
- Oils per volume = 1 ÷ (1 ÷ 0.916 + 52% ÷ 1.3105) = 0.388 oz/in³
- Oils = 105 in³ × 0.388 oz/in³ = 40.8 oz
Quick reference
An estimate by volume. Trace, additives and the mould’s own shape change the fill a little, so keep a spare cavity handy.
| Example | Oils to weigh out |
|---|---|
| 10 × 3.5 × 3 in loaf | 40.8 oz |
| 10 × 3.5 × 3 in loaf, 5% over | 42.8 oz |
| 3 in round × 2 in, 6 cavities | 32.9 oz |
| 40 fl oz mould | 28 oz |
| 9 × 9 × 2 in slab | 62.9 oz |
| 25 × 9 × 7 cm loaf | 1,058.1 g |
| 25 × 9 × 7 cm loaf, 45% solution | 1,097.5 g |
| 1.2 L mould | 806.2 g |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
4 sources
NIST Chemistry WebBook, water at 4 °C and 0.101325 MPa: 999.9749 kg/m³ (IAPWS-95), https://webbook.nist.gov/cgi/fluid.cgi?ID=C7732185, rounded to the conventional 1000 kg/m³ of NIST SP 811 (2008) Appendix B, centimeter of water, conventional = 98.0665 Pa.
Used for
- The conventional density of water, 1000 kg/m³. Pure water at 4 °C, its densest, is 999.97 kg/m³.
Codex Alimentarius, Standard for Olive Oils and Olive Pomace Oils, CXS 33-1981 (revised 1989, 2003, 2015, 2017 and 2024), Appendix I, 2.1, p. 10: "Relative density (20°C/water at 20°C): 0.910-0.916"; the same range is in the superseded 2015 revision, Appendix "Other quality and composition factors", section 3, p. 8, https://www.fao.org/input/download/standards/88/CXS_033e_2015.pdf
Used for
- Relative density of soap oils, taken as olive oil at 20 °C against water at 20 °C: the top of the Codex range 0.910 to 0.916. Coconut oil, lard and butters differ a little.
OxyChem, Caustic Soda Handbook, Table 1 "Density and NaOH Content of Membrane Grade Caustic Soda Solutions at 60°F", row 28.0 wt% NaOH: specific gravity 1.3105. https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-soda-handbook.pdf
Used for
- Specific gravity of a 28% by weight sodium hydroxide solution, the "full water" strength the 0.40 rule assumes: 1.3105. Stronger solutions are denser (30%: 1.3317; 33%: 1.3634, same table).
classicbells.com, "Estimate batter to fill a mold": 10 wt oz of 1/3 coconut, 1/3 olive, 1/3 lard at 5% superfat and 28% lye concentration, "Soapcalc NaOH = 1.46 wt oz", "Soapcalc water = 3.74 wt oz". https://classicbells.com/soap/moldBatchSize.asp
Used for
- Lye solution (lye plus water) per unit weight of oils in one worked recipe: 5.20 oz per 10 oz of oils. A starting figure only; every recipe has its own.
Common questions about the Soap Mold Batch Calculator
- How much oil do I need to fill my soap mould?
- Work out the mould's volume to the soap height and multiply by the oils per unit of volume, which depends on your recipe. For a recipe with 52% lye solution to oils, that is about 0.39 ounces of oil per cubic inch, or 0.67 grams per cubic centimetre. A 10 by 3.5 by 3 inch loaf is 105 cubic inches, so about 41 ounces of oils.
- Where does the 0.40 rule come from?
- It is a shortcut for the same arithmetic: the batter is oils plus lye water, and each has its own density. With olive oil at a specific gravity of 0.916, a 28% lye solution at 1.3105 and 52% solution to oils, the oils come to 0.388 ounces per cubic inch. Rounding up to 0.40 leaves a little batter over. Classic Bells works the same derivation to 0.37 with its own figures, so the rule is an estimate, not a constant.
- Does this calculate the lye?
- No, and on purpose. The amount of sodium hydroxide depends on the saponification values of your particular oils and the superfat you choose, and getting it wrong makes a caustic soap. Put the oil weight from this page into a lye calculator such as SoapCalc, which works out the lye and water for your recipe.
- What if I use less water in my recipe?
- Enter your own lye solution share. Less water means less solution per ounce of oil, so more oil fits the same mould. A stronger solution is also denser; you can change its specific gravity under More options.
- How do I measure an odd-shaped mould?
- Fill it with water to the height the soap will reach, then measure the water in fluid ounces or millilitres and choose Capacity I measured. That works for any cavity shape.