Use this page to determine the gluten/protein level of your flour, and to calculate the amount of vital wheat gluten to add to your flour to achieve a desired protein level.
For hobbyist bakers it can be difficult to find bread flour that has the appropriate gluten content. Recipes for things like bagels, sourdoughs, pretzels and more, can require higher gluten content than what might be found in your average grocery store flour section.
First, this basic application will help bakers determine the gluten content of flour that doesn't specify it on the package.
Second, when you need to increase the glutent content of flour you can add wheat gluten to it, which typically has a 75% gluten content. This application takes care of the somewhat complicated calculations needed to determine how much wheat gluten and flour to mix.