From TibiaWiki

The Experience Formula based on the Level is as follows:

,

where is the Level.

To find out how much experience you need to advance to the next level, simply substract the ammount of experience of the next level from the current ammount of experience that your character has. You can also find a premade experience table here or here.

If you want to know how many of the same creatures you need to kill in order to advance to the next level, you need to find out the ammount of experience needed for the next level as explained before, and divide that by the total experience points that the creature gives you. This can also be calculated here.

Mathematics behind the formula

The experience needed to advance from level x-1 to level x is

From this we can calculate the experience required to advance from level 1 to level x by the summation

Which can be rearranged to

By substituting the modified Gauss equations

and

We get

Which can be expanded to

Which simplifies to

This is the experience formula.
Or simple: (50 * (Lvl) * (Lvl) * (Lvl) - 150 * (Lvl) * (Lvl) + 400 * (Lvl)) / 3 For example, level 15: (50 * (14) * (14) * (14) - 150 * (14) * (14) + 400 * (14)) / 3 = 37,800 Use level - 1!(only here)