How many bars of soap in a loaf?
Loaf length divided by bar thickness, rounded down. A 10-inch loaf cut at 1 inch is ten bars; at 1¼ inch it is eight, and the odd quarter-inch becomes the end slice. Most molds sold as "10-inch" are 10 inches inside — check, because the outside can be 10½ and the end slices are where the arithmetic goes. Width and height do not change the count; they set the bar's face size and, with the thickness, its weight.
| Cut thickness | Bars from 10 in | Each at cutting | Each after cure | |
|---|---|---|---|---|
| 0.75 in | 13 | 107 g | 91 g | 3.2 oz |
| 0.875 in | 11 | 126 g | 108 g | 3.8 oz |
| 1 in | 10 | 139 g | 119 g | 4.2 oz |
| 1.125 in | 8 | 174 g | 148 g | 5.2 oz |
| 1.25 in | 8 | 174 g | 148 g | 5.2 oz |
| 1.5 in | 6 | 232 g | 198 g | 7.0 oz |
10 × 3.5 × 2.5 in loaf, 956 g of oils at 33% lye concentration with 3% fragrance — 1,391 g of batter, 1,187 g after losing 15% in cure. Computed by this site's engine.
Nothing leaves a curing bar but water. The recipe above carries 272 g of water for 956 g of oils; over four to six weeks roughly three-quarters of it evaporates, taking the batch from 1,391 g to about 1,187 g — a 15% loss, or about 20 g off each of ten bars. A recipe at a higher lye concentration has less water to lose and shrinks less; a "full water" recipe loses more and takes longer to get there. The tool uses your concentration, which is why it asks.
Standard bar sizes
The common handmade bar is about 3.5 × 2.5 inches on the face and 1 inch thick — roughly 8.75 cubic inches, 139 g at cutting and 119 g (4.2 oz) cured from the loaf above. "4 oz bars" usually means 4 oz on the label after cure, which takes a bar cut at about 4.6–4.8 oz. Guest bars run 1–1.5 oz; a big bath bar 5–6 oz. Thickness is the lever: for a heavier bar from the same loaf, cut thicker and accept fewer bars; for more bars, a taller mold beats a thinner cut, because bars under ¾ inch feel flimsy and warp.
In the US the net weight on a soap label has to be true when the bar is sold, not when it was cut (the rest of the US labeling rules are their own subject) — and a bar keeps losing water slowly for months. A bar that weighs 4.3 oz at six weeks may be 4.1 oz at a fair three months later. Label the weight you are certain of (4 oz, not 4.3), or use "net wt. 4 oz (113 g) minimum". The I need N bars mode above takes the label weight and works back to the oils so the batch is sized for the cured bar, not the wet one.
How much oil for a given number of bars?
Work backwards from the cured bar. Each gram of oil becomes about 1 + lye + remaining water + fragrance grams of finished soap — on the classic ratios, 1.241 g of cured soap per gram of oil. Twelve bars at 110 g is 1,320 g of cured soap, so about 1,064 g of oils — plus a little for end cuts and the bowl — and a mold that holds it: about 1,595 mL. The tool prints both and hands the oil weight to the recipe builder.
End cuts and trimming
The first and last slice of a loaf are usually uneven — a rounded edge, a bit of soda ash, a textured top — and many makers trim a quarter to a half inch off each end before cutting. Enter the total trim and the count drops accordingly; the trimmings are still soap (rebatch, confetti, samples). A wire cutter or a miter box with a fixed stop gives a consistent thickness; cutting by eye drifts a sixteenth per bar and the last one pays for it.