The best way to measure ingredients for making soap is by weight. Therefore, you will need a good scale. Measuring by volume is not consistent or accurate. Inaccuracies in measuring become amplified when you are working with small batches. One ounce of oil more or less won't make a big difference in a batch that has a total weight of twelve pounds. But in a batch of one pound, one ounce short or extra is a huge difference and can make a batch far too alkaline or too oily. More important, if you are over in your measurements of caustics, your soap can turn out dangerously caustic.
When you shop for scales, economy shouldn't be your primary concern. You need accuracy, ease of use, and the ability to handle the amount of the ingredients in the batch size you are making. Look for a scale with a “tare” feature — meaning it is able to reset to zero as you add ingredients. Also look for scales that measure in grams as well as ounces. Divisions of ∕ ounce are the largest the increments should be. But if you can find a scale that measures in tenths of ounces, that's even better, so buy it. It will help you to be more accurate than measuring in ∕-ounce increments.
You can scrimp on many of your soapmaking supplies, but put that money into a good scale. It will repay you with confidence, security, and peace of mind over and over. For less than $100, you can buy an excellent scale that you'll have for years.
Digital postal scales available at office supply stores are the choice for many soapers. They usually run on 9-volt batteries, have the tare feature, weigh in ∕-ounce increments, and have a maximum of ten pounds. This set of specifications is sufficient for most home soapmakers.
It's a good idea to protect your digital scale in a zipper bag.