Unit (distance)

Many League of Legends item and ability descriptions refer to distance measured in "units". There is no clear, official definition of what a unit is, but it can be inferred by the actual distance that things described in "units" can reach.

Visualizing units
In attempting to eyeball distance during a match, some have half-jokingly suggested visualizing the length in "Teemos", because Teemo is approximately 100 units wide.

It is also important to bear in mind that abilities are measured from the middle of your champion (as if you were looking directly down on top of him), and that autoattacks are measured from the "edge" of your champion (as if looking down on top of him, at a circle made from the outermost part of himself known as his "hitbox". This means that if Champion A has 650 attack range, and Champion B has a ability with a 650 range, A can attack B while still being outside of the range of B's spell.

One way to determine the exact distance of a given number of units is to trigger the range indicator for a number of units you know about. For example, if your champion has an autoattack range of 650 "units", during the match you can press the letter A, creating a red circle around you that is 650 units from the outside edge of your champion.

Many abilities, similarly, have a range indicator that appears when you press its key, but only if you have Quick cast turned off, or hover your mouse pointer over the button for your ability. Remember that this is measured from the middle of your champ, not the outer edge like autoattacks.

Distance factors
When calculating autoattack, even any change in the size of a champion counts. For example, as Zac becomes larger with his health, he can reach farther, as can Cho'gath. Even the size-increasing elixir increases your champion's attack range.

Bear in mind that distance units do not actually determine whether a champion's attack is melee or ranged. There is not a given point at which it ceases to become one and becomes the other. If a Champion A were marked "ranged", and Champion B were marked "melee", they would continue to be so even if Champion A's autoattack range became shorter than B's.