Maker and DIY Calculators
Soap mold volume.
Soap recipes are in grams of oil, but molds are in centimetres or inches. Enter the mold size to get the batter weight that fills it and the oil weight that makes that batter.
Oil weight = mold volume (ml) x fill x batter density x oil share. A 1,000 ml mold filled to the top at 1.0 g/ml needs about 1,000 g of batter, of which about 700 g is oil (the rest is lye and water). Put those oils into the lye calculator for exact weights.
Mold and recipe
Batch size
Fill the form and press Calculate.
Worked example
A loaf mold measures 22 x 9 cm inside and you pour to 6 cm: 22 x 9 x 6 = 1,188 ml. Filled to 95% at 1.0 g/ml that is 1,128.6 g of batter. At 5% superfat, a 0.14 average SAP and a 33.3% lye concentration, lye is 0.133 g and water 0.266 g per gram of oil, so oils are 1 / (1 + 0.133 + 0.266) = 71.5% of the batter, about 807 g of oil. The rest is roughly 107 g of lye and 215 g of water.
How it works
Batter is mostly oil, lye and water, and its density is close to 1 g/ml, so the mass that fits a mold is close to its volume in millilitres (1 cm³ = 1 ml). Oils make up 1 / (1 + lye per oil + water per oil) of that mass, and lye per oil is the average SAP x (1 - superfat). That ratio sets how many grams of oil to start with. Add fragrance, clay or other additives and the batter grows a little, so leave headroom.
This is a sizing tool, not a recipe. The lye and water shown here use an average SAP value. Once you pick actual oils, run them through the soap lye calculator, which uses a value for each oil, and weigh the lye from that, never from this page. Cut-soap bars shrink as they cure and lose roughly a fifth of their weight as water evaporates over four to six weeks.
Sources and basis (checked 2026-10-05): geometry of the mold (volume = length x width x height); the saponification arithmetic is the one used by the lye calculator, with oil, lye and water proportions from the same formulas published in soap-making references such as SoapCalc. Batter density near 1 g/ml is a working assumption you can change.
FAQ
How much oil do I need for a 1 kg or 2 lb soap mold?
Soap molds are usually described by the oil weight they take. As a rule of thumb the oils are about 70% of the total batter, so a mold that holds 1,000 g of batter takes about 700 g of oil.
Why use millilitres for a mold?
Soap batter is close to 1 g per ml, so the volume in millilitres is nearly the batter weight in grams. Measure your mold by pouring in water and weighing it: 1 g of water is 1 ml.
What if my mold is an odd shape?
Fill it with water, pour the water into a measuring jug, and type the number into the volume field. That gives you the exact volume without any geometry.
Should I fill the mold completely?
Leave 3 to 5% room for fragrance, additives and soap that rises while it gels. The tool defaults to 95%.
Can I weigh lye from this page?
No. Use the lye calculator on your actual oils. This page uses an average SAP value, so its lye weight is only an estimate for planning.
Safety and disclaimer
Sizing estimate only. Lye is a corrosive chemical, so work with goggles and gloves, add lye to water and never weigh lye from this page. Calculate the real lye weight for your oils with a lye calculator and check it with a second source. Not medical advice.