Determining what constitutes an appropriate challenge for your party of heroes can be a real chore. This app handles all the math for you, including modifiers for creature count and number of characters, so you can spend more time planning your adventure.
First, describe the character levels that comprise your party. The app can handle any reasonable number of characters, at levels up to 20. You could, for example, specify that your party is made up of three level-4 characters, one level-5 character, and one level-6 character.
Once your party is defined, the app will tell you the XP thresholds appropriate for challenges of easy, medium, hard, and deadly difficulties. Those thresholds become the basis for a couple of other features: Creature Count per CR, and Custom Encounter Difficulty.
Creature Count per CR is essentially a quick-reference chart which shows you the maximum number of creatures of each Challenge Rating that you can throw at the party to generate encounters of various difficulty. For example, suppose you want the 5-member party described above to have a Hard encounter with some Challenge 2 creatures, such as ogres. The chart will tell you that youll need to use 4 of those creatures.
Custom Encounter Difficulty is a calculator that allows you to define an encounter that consists of multiple, mixed CR creatures, and then tells you the difficulty of that encounter. To illustrate, consider again our 5-member example party. Using the Custom Encounter Difficulty calculator, you can create an encounter consisting of one CR1, two CR2, and one CR3 creatures, and it will tell you that this constitutes a hard encounter worth 3,600 XP.