tiLevel is an abbreviation for "true item level" and is a light weight mod that calculates how close an item is to its item level budget by displays a percentage in the tooltip. Each of the stats are weighted with consideration to the item's rarity. This fun mod let's players see how close (or far) away an item is to it's budget.
Note: The mod uses a reversed engineered formula to calculate the true item level and is not official Blizzard calculations. This means that if a item is way above or way below the assigned item level, don't storm Blizzard's walls!
tiLevel in Action
If you wish, you can see the manual calculation of an item by using the command:
/tilevel <item link>
This will show the weight of each stat and how tiLevel calculates each individual stat.
While tiLevel will calculate most items, it cannot calculate all. For example, there is no known formula to manually calculate the true item level of trinkets. Trinkets generally have random procs that make it very difficult to break down into a mathematical formula. Also, items that do not have stats will be unable to be calculated as well. This includes low level, bows, crossbows, wands, etc. The true item level is determined by weighing stats. If there are no stats, the true item level will not be able to be calculated.
For more information about reverse engineering item levels and item levels in general, Elitest Jerks has posted a very in depth thread that can be viewed here: http://elitistjerks.com/f15/t44718-item_level_mechanics/