Whether you use a traditional cold process soap mold or some other container, you can calculate just how much soap you need to make to fill that mold. That way you don't end up having to throw any extra soap out. You calculate the volume of the mold in cubic inches, multiply by 0.4, which is the amount of the recipe that is dedicated to oils, and plug the total into an online lye calculator to figure each oil (based on its percentage in the original recipe) and the other ingredients. In short:
 Calculate the volume of the mold in cubic inches
 Multiply the volume of the mold by 0.4 to find the total amount of oils in the recipe. Calculate the amount of each individual oil using the percentages in your original recipe.
 Enter that amount of oils into an online lye calculator to get the final recipe quantities
Prepare the recipe as you normally would using the quantities from the lye calculator, and the soap should come close to perfectly filling the soap mold, whatever its size or shape.

Standard Square or Rectangular Molds
For square or rectangular molds, multiply the length by the width by the height in inches to give the volume of the mold in cubic inches. If you're not going to fill the mold to the top, calculate to the height that you want the soap. If the mold is 4 inches deep, but you're only going to pour 2 inches of soap into it, just use 2 in the calculation.
For example: If you have a log mold that is 15 inches long, 2.5 inches wide and 3.5 inches tall, and you plan to fill the mold to the top, multiply 15 X 2.5 X 3.5 = 131.25 cubic inches.
Then, take the total volume and multiply it by 0.4. 131.25 X 0.4 = 52.5 ounces of oils.
Then, using the percentages in your recipe, calculate each of the individual oils. Enter the oils in the online lye calculator to come up with the final recipe sized perfectly for that mold.

Round or Tube Molds
For round or tube molds, you calculate the volume a little differently. You multiply Pi by the radius of the circle squared and multiply that by the height of the mold.
It's not as hard as it sounds. The radius is 1/2 of the total width of the tube. For a 3inch tube, the radius is 1.5 inches. "Squared" is the number multiplied by itself, so 1.5 X 1.5 = 2.25.
Pi is a constant generally calculated at 3.14.
So, for a 3inch PVC tube mold that is 6 inches high, the calculation is:
3.14 (Pi) X 1.5 (radius) X 1.5 (radius) X 6 (height)
The volume of any cylinder is calculated using Pi X radius X radius X height.
The calculation gives you the volume of the container in cubic inches, which you then multiply by 0.4 to get the total amount of oils needed.
3.14 X 1.5 X 1.5 X 6 X .40 = 16.96 ounces of oils.
Then using the percentages in your recipe, calculate the individual oils. Put the oil totals into the online lye calculator for the final recipe quantities.

Water Method for Irregular Shaped Molds
If your mold is shaped irregularly or is otherwise difficult to measure, use the water method.
Fill the mold with water and then pour that water into a measuring cup. Multiply the number of fluid ounces of water by 1.8 to get the total cubic inches of the mold.
For example, if your mold holds 12 ounces of water, 12 X 1.8 = 21.6 cubic inches.
21.6 cubic inches in the mold X 0.4 = 8.64 ounces of oils in that recipe.
Enter the 8.64 ounces of oils—best to round up to 9 or at least to 8.6—allocated by the percentages of each oil in your recipe, and use the lye calculator to give you the final recipe.
NOTE: This method works because a fluid ounce of water weighs one ounce, which is not the case with other liquids.

Use an Online Calculator or Software Program
When all else fails, or you just want to double check your math, a few online calculators can calculate the recipe for your particular soap mold.
Using Summer Bee Meadow's Lye Calculator and Recipe Resizer, you enter the basic recipe first and then resize it to fit the mold size you desire.
Soapmaker Software is a complete software program for calculating recipes, resizing for different molds, calculating costs of ingredients and batches of soap.