Weigh everything

Soap recipes are written by weight. A digital kitchen scale that reads in grams is the most useful tool you can own. Grams are finer than ounces, so small amounts like lye and fragrance come out more accurately.

1 ounce (oz)

• 28.35 grams

1 pound (lb)

• 16 oz = 453.6 grams

1 kilogram (kg)

• 1000 g = 2.205 lb

Ounces vs. fluid ounces

• An ounce (oz) is weight. You read it on a scale.

• A fluid ounce (fl oz) is volume. You read it on a measuring cup.

They only come close for water (1 fl oz of water weighs about 1.04 oz). Oils are lighter than water, so 8 fl oz of oil weighs less than 8 oz. When a recipe says "oz," weigh it.

Teaspoons and tablespoons are fine for small additives that recipes list that way, like clay or sodium lactate. 1 US teaspoon is 4.93 ml, and 1 US tablespoon is 3 teaspoons (14.79 ml).

Temperatures

Soap Queen notes that soaping at 140°F (60°C) or above can crack the tops of bars, and soaping at 100°F (38°C) or below makes soda ash more likely. Milk soaps are kept much cooler (see Lye Water & Other Liquids). The converter switches between °F and °C for you.

How much oil fits a mold?

1. Measure the inside length and width of the mold, and how deep you'll pour.

2. Multiply length × width × depth to get the volume.

3. Inches: volume × 0.40 = ounces of oils. Centimeters: volume × 0.70 = grams of oils.

4. Enter that oil total in the Lye Calculator.

Example: a 12 × 3.5 in loaf poured 2.5 in deep is 105 cubic inches, and × 0.40 = 42 oz of oils.

Round molds (PVC): use 3.14 × radius × radius × depth for the volume. Odd shapes: fill the mold with water and weigh it. Each ounce of water is about 1.8 cubic inches.

The factor assumes close to full water. With a water discount, or lots of additives, you may want 0.38 (or 0.65 metric) for a little less batter.

Sources - NIST: Approximate conversions from U.S. customary measures to metric. https://www.nist.gov/pml/owm/approximate-conversions-us-customary-measures-metric - Modern Soapmaking: How to Resize a Soap Recipe for Your Soap Mold (0.40/0.38 and 0.70/0.65 factors, PVC formula). https://www.modernsoapmaking.com/blog/how-to-resize-a-soap-recipe-for-your-soap-mold-imperial-and-metric - Classic Bells (Soapy Stuff): Estimate batter to fill a mold (water weight × 1.8). https://classicbells.com/soap/moldBatchSize.asp - Soapmaking Forum: How to figure how much oil for mold (12 × 3.5 × 2.5 example). https://www.soapmakingforum.com/threads/how-to-figure-how-much-oil-for-mold.2909/ - Soaping temperatures: Soap Queen, Soapy Mess-Up Quick Guide (soaping 140°F+ can crack tops; around 120°F reduces deep soda ash). https://soapqueen.com/bath-and-body-tutorials/tips-and-tricks/soapy-mess-quick-guide/