Here's an easy way to get an excellent approximation of the damage dealt by a Pokemon to another. The method given below doesn't even require that you know the actual stats of the Pokemon in question - only their base stats.

First of all, a chart converting a Base stat to the equivalent Damage stat:

Code:

BStat DStat
-----------------
5 2.19
10 2.67
15 3.14
20 3.62
25 4.10
30 4.57
35 5.05
40 5.52
45 6.00
50 6.48
55 6.95
60 7.43
65 7.90
70 8.38
75 8.86
80 9.33
85 9.81
90 10.29
95 10.76
100 11.24
105 11.71
110 12.19
115 12.67
120 13.14
125 13.62
130 14.10
135 14.57
140 15.05
145 15.52
150 16.00
160 16.95
165 17.43
170 17.90
180 18.86
190 19.81
200 20.76
230 23.62
250 25.52
255 26.00
Stat ±2 ±0.19
Stat ±4 ±0.38
252 EVs +3.00
HP +5.00
Atk/SpA x4.00

The above table is to convert the Def or SpD base stat. To convert the base HP stat, just add 5 to the corresponding number, and if you want to convert the base Atk or SpA stat, just multiply the number by 4. You'll first need to do the additions and subtractions before performing any multiplications or divisions in order for the method to work, however.

If the stat you want is not on the list (say you have a Pokemon with 88 SpA or 144 HP or 77 SpD or whatever), you add or subtract 0.19 if you want to add or subtract 2 from the stat, and twice that (0.38) if you want to add or subtract 4.

So 77 SpD corresponds to 8.86, the stat for 75, + 0.19 = 9.05.

144 HP corresponds to 15.05, the stat for 140, + 0.38, + 5 (since this is HP) = 20.43.

88 SpA corresponds to 10.29, the stat for 90, -0.19, x4 (since this is SpA) = 40.4 (remember you first perform the subtraction and then the multiplication).

If you have 252 EVs in a stat, you add 3. Simple as that. This means that you add 1/21, or 0.0476, for every 4 EVs you have. You can also understand it as dividing the EVs by 84 and adding that to the damage stat.

Any boosts in damage or in stats, be them due to nature, STAB, item, ability, or whatever are multiplied normally.

After doing all this, the percentage maximum damage dealt is then simply

Code:

Max Damage = (D[S]Atk x MovePower) / (DHP x D[S]Def)

The min damage is just 0.85 multiplied by the max damage. You don't even need to convert it to a percentage; the answer is already a percentage!

Let's give an example.

Quoting part of a post by MoP:

So we have Choice Specs Modest Kyogre with 252 SpA EVs using Thunder against 0/0 Palkia. Notice that he had to find Kyogre's attacking stat (438) and Palkia's HP and SpD stats (321 and 276).

The method above would work as follows:

Kyogre's base SpA is 150. 150 corresponds to a damage stat of 16.00. We add 3 due to having 252 EVs, so we have 19. We multiply by 4 since this is the SpA stat, so we have 76. We multiply by 1.1 since this is Modest, so we have 83.6. We then multiply by 1.5 due to Choice Specs, so we have 125.4.

Palkia's base HP is 90. 90 corresponds to a damage stat of 10.29. We add 5 since this is the HP stat, so we have 15.29.

Palkia's base SpD is 120. 120 corresponds to a damage stat of 13.14.

So max damage is (125.4 x 120) / (15.29 x 13.14) = **74.90%**. Min damage is 74.9% x 0.85 = **63.66%**. So I'm about 0.2% off the real damage.

Another example (taken from a post by Gothic Togekiss:)

We have the latest CAP Pokemon having 110 base Atk with 252 EVs in Atk and a beneficiary nature using Thunderpunch against a 124/252 Bold Milotic. Milotic's base HP and Def are 95 and 79 respectively.

So we have:

CAP6's base Atk is 110, which corresponds to 12.19. We have 252 EVs so we add 3: 15.19. Since this is Atk, we multiply by 4: 60.76. We multiply by 1.1 since it has a beneficial nature: 66.84. We further multiply by 2 since Thunderpunch is supereffective: 133.68.

Milotic's base HP is 95, which corresponds to 10.76. We add 5 since this is HP: 15.76. We also add 124 / 84 = 1.48 since it has 124 HP EVs, getting 17.24.

Milotic's base Def is 79, which corresponds to 8.86 + 0.38 = 9.24. We add 3 since we have 252 EVs: 12.24. We multiply by 1.1 since this is Bold: 13.46.

Hence, the maximum damage would be (133.68 x 75) / (17.24 x 13.46) = **43.21%**. The minimum damage would be 43.2 x 0.85 = **36.73%**. So we're only 1% off the real damage.

If you want a decent but simple damage calculator using just the base stats of the Pokemon in question, give this a try.