Pokemon EXPThere are two formulas you should know when calculating the amount of experience your Pokemon have earned or need to advance in level.
Your PokemonAt Pokemon Random, this is the formula you use for current EXP needed.
Pokemon Level x 10 x Growth Rate Multiplier
The first two are simple enough, but here's the list of multipliers:
Erratic - .5
Fast - .75
Medium-Fast - 1
Medium-Slow - 1.25
Slow - 1.5
Fluctuating - 1.75
A level 5 Bulbasaur, with it's Medium-Slow Growth Rate will need 75 experience to level up.
5 x 10 x 1.5 = 75
For now, information on growth rates can be found in
http://www.serebii.net/pokedex-dp/(Will be listing them at 40 a night. That's a decent number.

Currently done through Johto and some of Hoenn)
I plan on adding a topic with the different Pokemon listed in their proper rate.
Your OpponentIf you've just defeated an opponent, use this formula to determine experience gained.
Pokemon Level x 10 x Pokemon Stage X Trained?
The first two are self-explanitory.
Pokemon Stage:Baby - .5
Basic - 1
Stage 1 - 1.5
Final - 2
Baby refers to anything in a baby-stage.
Basic refers to any basic-stage Pokemon that can evolve.
Stage 1 refers to any evolved Pokemon that can evolve one more time.
Final refers to any fully evolved or 1-stage Pokemon.
Trained:Trained Pokemon - 1.5
Wild Pokemon - 1