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 subtract the amount of experience of the next level from the current amount 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 amount of experience needed for the next level as explained before, and divide that by the total experience points that the creature gives you.
Mathematics behind the formula
The experience needed to advance from level x-1 to level x is
File:Exp2nl.png
From this we can calculate the experience required to advance from level 1 to level x by the summation
File:Summation.png
File:Rearrange.png (expanding)
By substituting the modified Gauss equations
File:GaussSquare.png
and
File:ModifiedGauss.png
We get
File:Substitution.png
File:Expanded.png (expanded)
File:Expformula.png (simplified)
Simplifying again gives us the experience formula,
File:Math1.png
Or simply:
(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)