If you're creating your own soap recipes from scratch, and even if you're using a recipe from a book or website, it's VERY important to run your recipes through a lye calculator to make sure that your ratios of water, oils and lye are correct. Doing so will help insure that your batch of soap will come out just right.
Here are the best online lye calculators I've found.
The folks at Soap Calc continue to improve and streamline their online lye calculator. It continues to be one of the best and most comprehensive online lye calculators available.
If you're really serious about soap making and creating recipes, you'll want to invest (it's not that much) in Soapmaker Software. It's really a great program that does all of your lye calculations and more! Read a full review of the program.
This is THE lye calculator for liquid soap...and a pretty darn good one for bar soap too. It has a great "resize" feature, and allows you to easily print the recipes. Plus, they're a great supplier of soap making supplies!
Brambleberry's calculator is a great simple calculator - and has been revised to include liquid soap. You can only superfat up to 10% - but that's just fine!
One of the very first online lye calculators was at Majestic Mountain Sage. It got a reputation for having too much water in the recipes, but they've corrected that by now offering a range of water usage in the recipes. They've also updated the calculator for sodium lactate.
Another very basic calculator - but this one will calculate in grams or ounces.
Miller Soap, one of the oldest soap making websites on the net, has a collection of Microsoft Excel spreadsheets to use to create your own recipes. I've used many of them and they are great in that they allow you to customize and save your recipes however you want.