D&D 5e Challenge Rating Calculator

This rates a monster you have built yourself, using the monster statistics table from the 2014 Dungeon Master's Guide. Enter effective hit points, armour class, damage per round and either the attack bonus or the save DC, and it finds the defensive rating from the hit points, corrects it one row for every 2 points the armour class is away from expected, does the same on the offensive side, then averages the two and gives the proposed challenge rating with its XP value and proficiency bonus. It also prints what a monster of that rating normally looks like, so an outlier is visible. The table is 2014 only and is a starting point, not a verdict.