You could use a computer program like ProMash to work out your beer recipes. But for those of us who prefer simplicity to complexity, you may want to do your calculations with this incredibly useful slide rule. From John Palmer, author of "How to Brew", this 3-in-1 slide rule sports a gravity-volume converter, a gravity contribution calculator, and a hop bitterness (IBU) calculator. Indispensable for all-grain brewing or formulating your own recipes.