This is a SIMPLE damage calculation tutorial. If there is any other additional info you would like explained in this guide, either reply with a comment or send me a PM, and I'll be sure to answer any questions to the best of my ability.

Why do we do damage calculations?

-- To find the most efficient and least wasteful EV spreads required to achieve the desired effect- usually a 1-2HKO.

When do we do damage calculations?

-- As you are planning, revising, or editing a team.

First off, you need an accurate damage calculator. The Libelldra Forge calculator is my personal favorite, and I find it to be the most accurate. It has the widest variety of items, abilities, and scenarios that would influence damage, and gives you damage results in terms of both percentages and raw numbers, as well as whether or not an attack would 1-2HKO, and under which circumstances it may, such as after Stealth Rock damage, Sandstorm/Hail damage, or Leftovers recovery.

Libelldra Forge Calculator

Second, you need a resource website. http://serebii.net is my favorite site for looking up stats, becuase it's simple and easy to understand, besides being one of the most highly regarded Pokedexes on the internet. The battle simulator "Shoddy Battle" is also a valuable tool, as it lets you quickly and accurately assign and change EVs, letting you know just how big or low a stat can get with any given nature.

Serebii.net (Find the DPPt Pokedex on the left-side panel- all Pokemon are organized by Type for your convenience.)

Damage calculations allow you to check the damage of a particular Pokemon against another. As such, it lets you efficiently distribute EVs in the least wasteful manner, only using what is necessary to achieve the desired effect.

BASICS

Let's say you want to OHKO Heatran switchins against your Abomasnow with Earthquake.

You would start off punching in Abomasnow's MAXIMUM possible Atk (Max IVs, max EVs, and an Atk + nature, such as Adamant or Brave), against Heatran's predicted HP and Def, Earthquake being a PHYSICAL attack.

Let's say you're aiming to OHKO a particular Heatran set- the Choice Scarf Heatran. Looking up the set, ScarfTran has a Modest nature (Sp.Atk+/Atk-), which is neutral to Def. Be sure your numbers are correct! Modest Heatran has 292 HP and 217 Def. However, you want to add 31 to both numbers, because you always want to assume maximum IVs- that way, you don't undershoot how much you invest in Atk and you don't end up not getting that OHKO. Maximum IVs brings Heatran's HP and Def to 323 and 248, respectively- THESE are the numbers you want to calculate against.

Now, you want Abomasnow's Atk. Since you REALLY want to OHKO this Heatran, you start from the TOP and work your way down. We'll start off assuming you want to OHKO Heatran with a nature NEUTRAL to Atk, such as Rash (Sp.Atk+/Sp.Def-). Abomasnow's MAX possible Atk with a neutral nature is 283 (assuming you're using the Serebii.net Pokedex, you do NOT need to add 31 IVs to the MAX stat- they'll already be included). Looking up to the attack, Earthquake, you'll see that it has 100 power. So, now, you have all the numbers you need (at the moment.).

Now, when punching your numbers into the calculator, remember to include the typing information! Abomasnow is NOT a Ground type, so there is NO STAB bonus applied- however, Heatran's two types ARE both weak against Ground, making him 4x weak against Earthquake. (Using Libelldra Forge's calculator, Type 1 and Type 2 are both set to '2x.'). Finally, with all the numbers and typing information pumped in, we hit the "Calculate Damage" button, and see the results.

As it turns out, Abmasnow overshot his damage! Not only does he always OHKO Scarftran with Earthquake, but he does a little damage over! Job's done, right? Wrong! Now, we REDUCE the number of Atk points that Abomasnow has- remember, we're finding the most efficient number to get the job done! So, now it's all a matter of fine-tuning. Bring down the Atk stat in increments of, say, ten. If you STILL OHKO after you've dropped ten points, try dropping another ten! Keep going until you no longer OHKO 100% of the time. Once you hit that point, start bringing it back up ONE point at a time until you hit JUST the right number.

After doing this, we find that 278 Atk will suffice. That's FIVE points less than the Atk we started off with- multiplied by four (the number of EVs it takes to make one point in a stat), that's 20 EV points we get to play with among other stats. Normally, such a number would seem insignifcant, but in the end, even the smallest numbers can count for something.

ITEMS

Let's say that Abomasnow from above wants to be a MIXER, meaning he attacks on BOTH sides of the spectrum (Physical as well as Special). As such, he wants to really spread out his EVs so that he can hit hard on both sides, but is more concerned about the Special end. So, he wants to reduce the Atk EVs invest even FURTHER. The easiest way to increase his damage output besides EVs is ITEMS, and Life Orb is a perfect choice for mixing. As such, we can apply the Life Orb in the Libelldra Forge calculator, calculate damage with the CURRENT needed Atk to OHKO Heatran with EQ (278), and see what happens to the resulting numbers.

Now we see a BIG boost in damage- from 100% to about 130%. This gives us a great opportunity to further drop that Atk stat. Reduce as you did before!

After some HUGE deductions, the needed stat goes from 278, to a TINY 213! If you look, this is actually ZERO EVs and seven less IVs than max! This in mind, you still have all 510 EVs to spread around the rest of Abomasnow, whether it be for HP, Sp.Atk, or what have you. Items are a wonderful way of supplementing your damage output to allow for a more effective EV spread when needed, but use them sparingly, as often, there are better choices.

ABILITIES

There are some abilities that, in various situations, can either INCREASE or DECREASE damage. For instance, Charizard has the ability BLAZE- when his HP is at 1/3 its usual maximum, his Fire attacks get a boost in power. Often, you can use these abilities to your advantage! Be sure you include them in your calculations- experienced players and Pokedex sites (such as serebii) will tell you all you need to know about a particular ability's uses.

OTHER FACTORS

Other factors to include are shown in the Libelldra Forge calculator in plain sight. These factors aren't usually always in effect, but include little details that could greatly affect the outcome of damage, such as Reflect/Light Screen, which cuts Physical/Special damage in HALF, respectively, or Stealth Rock damage, which COULD mean the difference between a OHKO on your part, or a clean sweep on the enemy's.

One especially interesting factor to consider is WEATHER.

Rain Dance will increase the power of Water attacks by 1.5x, and could activate a plethora of Pokemon abilities, those of which including Swift Swim, Rain Dish, Hydration, Forecast, and Dry Skin. Besides that, the attack Thunder achieves 100% accuracy, and has a 30% chance to ignore Protect/Detect. It also cuts Fire damage in half.

Sunny Day will increase the power of Fire attacks by 1.5x, and activates a variety of abilties, including Chlorophyll, Solarpower, Leaf Guard, Flower Gift, Dry Skin, and Forecast. Solarbeam has no charge time under the Sun. Sunny Day cuts Water damage in half.

No other weather effects increase or decrease damage, with the exception of Sandstorm, which increases the Sp.Def of Rock types by one level (1.5x).

Last, but possibly MOST common are stat modifiers. There are TWELVE levels to every stat, ranging from -6 to +6. Every level increases or reduces a stat by 1.5x. For instance, when a Pokemon uses Swords Dance, it increases their Atk stat by TWO levels. 1.5 + 0.5 = 2.0. Essentially, every extra level raises a stat by another 0.5x. At max, +6 levels of a stat essentially multiplies the stat's value by FOUR times. These levels are included in the Libelldra Damage Calculator, allowing the calculator to do the work for you, so that you don't have to do the work yourself, multiplying out stats and such.