How much melt and pour soap do I need for my mold?
Measure the mold by volume, then convert to weight with the base's density. The volume part is easy: fill a cavity with water and weigh it (a gram of water is a milliliter). It's also the only reliable way for a shaped cavity. The density is the hard part, because no base maker we could find publishes one. The safety data sheets for Stephenson's Crystal bases and Bramble Berry's bases leave it out, and one supplier's sheet lists a bulk figure (0.72 g/cm³) that would make solid soap float. The one direct number comes from a supplier's FAQ: Nature's Garden's 31 grams per fluid ounce of mold, which is 1.048 g/mL. That's this tool's default, and you can change it.
12 × 4 fl oz = 48 fl oz = 1,420 mL. At 1.048 g/mL that's 1,488 g of base (52.5 oz) to fill every cavity level. Weigh out about 5% more (1,562 g) for what stays in the jug. At 2% fragrance the batch takes 29.8 g of fragrance oil.
Why "1 ounce per fluid ounce" comes up short
The rule of thumb on the forums is that melt and pour is "1 : 1 to water", so a 4 fl oz cavity takes 4 oz of soap. It doesn't, and not only because soap is denser than water: a fluid ounce of water already weighs 1.043 ounces. The rule mixes a volume ounce with a weight ounce.
| Base density | Weight oz per fl oz of mold | "1 oz per fl oz" comes up short by |
|---|---|---|
| 1.000 g/mL (water) | 1.043 oz | 4.1% |
| 1.048 g/mL (default) | 1.093 oz | 8.5% |
| 1.080 g/mL | 1.127 oz | 11.2% |
| 1.120 g/mL | 1.168 oz | 14.4% |
1 US fl oz = 29.574 mL; 1 oz = 28.350 g. Weight per fl oz is the density × 29.574 ÷ 28.350; the shortfall is the share of that weight the 1 oz rule leaves out.
The 12-cavity tray above takes 1,488 g; the 1 : 1 rule says 48 oz, which is 1,361 g, so it comes up 127 g short, about one cavity's worth. Beginner recipes tell you to melt "½ to 1 oz more base than the mold holds", and this is the reason: it's the density, not spillage. Even if melt and pour weighed exactly what water does, the rule would still be 4.1% short.
Densities differ between bases (clear, white, goat milk, butter-rich), and the data sheets leave them out, so measure the one you use. Fill one cavity with water and weigh the water. Pour the same cavity exactly level with melted base, let it set fully, unmold and weigh the bar. Bar grams ÷ water grams (× 0.998) is the density. The tool's "measure your own" box does the division and uses the result.
How much fragrance in melt and pour soap?
Less than most retailers say, if you go by the people who make the base. Stephenson, the manufacturer behind Crystal and many private-label bases, says "up to 3% additives, including a maximum of 1% fragrance". Hobby retailers range up to six times that:
| Source | As published | % of base | Per pound of base | Per 4 oz bar |
|---|---|---|---|---|
| Stephenson (base manufacturer), processing card | up to 3% additives, including a maximum of 1% fragrance | 1.00% | 4.5 g | 1.1 g |
| Bramble Berry | 0.3 oz per pound | 1.88% | 8.5 g | 2.1 g |
| Wholesale Supplies Plus | 3% (IFRA limits override) | 3.00% | 13.6 g | 3.4 g |
| CandleScience | 0.5 oz per 16 oz | 3.13% | 14.2 g | 3.5 g |
| Bulk Apothecary | no more than 1 oz per pound | 6.25% | 28.3 g | 7.1 g |
Grams of fragrance oil: each published rate applied to a pound (453.6 g) of base and to a 4 oz bar (113.4 g of base).
Stephenson's reason is the base, not the nose: it says levels of 3% or more may affect the color and clarity of the base, and too much fragrance leaves a soft, sweating bar that can seep oil. Retail rates are tuned for scent throw in a finished bar. A sensible reading: start around 1–2%, go to 3% only with a base that says it can take it, and treat 6.25% as the ceiling of anyone's advice, not a target. Whatever the rate, the fragrance's own IFRA limit for rinse-off soap caps it. The fragrance load calculator checks that, and explains why the IFRA percentage is of the finished soap. Vanilla turns melt and pour brown; vanilla-stable bases take up to 2% of vanillin-containing fragrance.
Color and extras count against the same allowance. Stephenson's is 3% in total, fragrance included; Wholesale Supplies Plus says its Crafter's Choice bases take 4–6%. Mica is typically up to ½ teaspoon per pound and clays around 1 teaspoon; the additive rate calculator turns spoons into grams. Put your fragrance and extras into the tool and it checks the total against both.
Melt and pour temperatures, by base
SoapmakingToolbox.com · melt and pour · base to fill a cavity
| Cavity holds | Base at 1.048 g/mL | In ounces | Range (1.00–1.12 g/mL) | Fragrance at 1% / 2% / 3% |
|---|---|---|---|---|
| 1 fl oz | 31 g | 1.09 oz | 30–33 g | 0.3 / 0.6 / 0.9 g |
| 2 fl oz | 62 g | 2.19 oz | 59–66 g | 0.6 / 1.2 / 1.9 g |
| 3 fl oz | 93 g | 3.28 oz | 89–99 g | 0.9 / 1.9 / 2.8 g |
| 4 fl oz | 124 g | 4.37 oz | 118–132 g | 1.2 / 2.5 / 3.7 g |
| 5 fl oz | 155 g | 5.47 oz | 148–166 g | 1.6 / 3.1 / 4.6 g |
| 6 fl oz | 186 g | 6.56 oz | 177–199 g | 1.9 / 3.7 / 5.6 g |
| 50 mL | 52 g | 1.85 oz | 50–56 g | 0.5 / 1.0 / 1.6 g |
| 75 mL | 79 g | 2.77 oz | 75–84 g | 0.8 / 1.6 / 2.4 g |
| 100 mL | 105 g | 3.70 oz | 100–112 g | 1.0 / 2.1 / 3.1 g |
| 125 mL | 131 g | 4.62 oz | 125–140 g | 1.3 / 2.6 / 3.9 g |
| 150 mL | 157 g | 5.55 oz | 150–168 g | 1.6 / 3.1 / 4.7 g |
Per cavity, filled level. Density default from Nature's Garden (31 g per fl oz of mold); measure your own base for better. Add ~5% for what stays in the jug.
Melt points (Stephenson product matrix)
| Crystal ST (clear) | 56 °C | 133 °F |
| Crystal WST (white) | 59 °C | 138 °F |
| Crystal HCVS (clear, vanilla-stable) | 54 °C | 129 °F |
| Goat’s milk | 53 °C | 127 °F |
| Shea butter | 61 °C | 142 °F |
| NS (clear, no-sweat) | 50 °C | 122 °F |
| WNS (white, no-sweat) | 53 °C | 127 °F |
| SLS free | 54 °C | 129 °F |
| SLES & SLS free | 51 °C | 124 °F |
| Triple butter | 51 °C | 124 °F |
| Oatmeal & shea | 49 °C | 120 °F |
Melt just past the melt point. Fragrance as it cools, below ~140 °F. Layers at 125 °F or below with an alcohol spritz between. Wrap bars as soon as they're set.
Melt points differ by base: Stephenson's own product matrix lists them from 49 °C (120 °F) for oatmeal and shea to 61 °C (142 °F) for shea butter base (all eleven are on the printable). Beyond that, advice disagrees. Soap Queen says bases burn above about 140–150 °F (the signs are a smell, cloudiness, yellowing and a thick, gloppy texture), and CandleScience says never exceed 160 °F. Stephenson itself processes hotter, melting to about 75 °C (167 °F) and pouring at 68–72 °C; its processing card prints 75 °C as "149 °F", which is simply a conversion error. The practical reading: melt just past your base's melt point in short bursts, never boil it, stir rather than heat to get the last lumps, add fragrance as it cools below about 140 °F, and pour layers at 125 °F or below with a spritz of 99% isopropyl alcohol between them.
How much does a melt and pour bar cost?
| Supplier | Base | 4 oz bar, 1–2 lb price | 4 oz bar, bulk price | Bulk + 3% fragrance |
|---|---|---|---|---|
| CandleScience | Stephenson HCVS (clear) | $1.00 | $0.80 | $1.04 |
| CandleScience | Stephenson white | $0.96 | $0.77 | $1.01 |
| Bramble Berry | Studio Clear | $1.50 | $1.00 | $1.24 |
| Bramble Berry | SFIC White | $1.75 | $1.06 | $1.30 |
| Wholesale Supplies Plus | SFIC White | $2.19 | $1.93 | $2.17 |
Base cost for 113.4 g (4 oz) of base at list prices seen 25 Sep 2026, before shipping; fragrance at an illustrative $2/oz. Prices change, so put yours into the tool.
So a 4 oz bar is $0.77 to $2.19 in base alone, depending mostly on whether you buy by the pound or the case. That's the number to compare with cold process: the cold process vs melt and pour guide costs both per 100 g of finished soap, and melt and pour stays the dearer route per bar however long you do it. For pricing to sell, the soap cost calculator adds packaging, labor and fees.
The beads of moisture on a finished bar are glycerin dew. Melt and pour bases are rich in glycerin, and glycerin is hygroscopic: it pulls water out of the air, most in humid weather. It isn't a fault in your batch. Wrap bars in plastic as soon as they're set, cool them at room temperature rather than in the fridge (condensation makes it worse), and in a humid climate choose a low-sweat base. Stephenson's NS and WNS, or SFIC's low-sweat bases, are made for it.