Four factors that change the outcome before you even look at stats
Two identical Pokémon, the same move, the same level, and yet one attack deals twice as much damage as the other. You won't see the difference in the stats; it's in what's happening around them: what the weather is, what terrain the attacker is standing on, whether the defender has put up a screen, and whether the attacker itself is healthy. Four things, four separate multipliers, and any of them can swing an exchange of hits. This article gives the exact numbers and, more importantly, the exceptions where the general rule doesn't hold.
Weather: two offensive, two defensive
Sun and rain affect offense: sun boosts Fire moves by half and weakens Water moves by half, and rain does the opposite. The multiplier applies to every move of that type, no matter who uses it. Blastoise's Water move in the rain gets exactly the same boost as a Water move borrowed by a Pokémon that isn't a Water type.
Sandstorm and snow don't touch offense at all; they raise defenses. A sandstorm raises the Special Defense of Rock-type Pokémon by half, and snow does the same for the Defense of Ice-type Pokémon. It's an easy difference to miss: a player looking for "good weather for attacking" under a sandstorm will find nothing, because that weather defends rather than attacks.
Terrain only works underfoot
The four terrains, Electric, Grassy, Psychic, and Misty, boost moves of the matching type by 30%, and Misty Terrain also halves the power of Dragon moves. Here's the condition that's easy to forget: the bonus applies only to Pokémon that are on the ground. A Flying Charizard in Electric Terrain gets nothing, because it's airborne, and the same goes for any Pokémon with the Ability Levitate, even a pure Electric type. Terrain checks not only the move's type but also whether anyone is actually touching the ground.
Moves that break the usual weather and terrain rules
A few moves have a rule stronger than the general bonus, which is why you can't calculate them with the formula above alone:
- Weather Ball and Terrain Pulse change type depending on the weather and terrain. Outside of them they're ordinary Normal moves; inside, they become Fire, Water, Rock, or Ice (Weather Ball), or Electric, Grass, Psychic, or Fairy (Terrain Pulse). The type change carries through to the same-type bonus and to effectiveness against the target, so it's not just cosmetic.
- Solar Beam loses half its power outside of sun, because in the game it charges for a turn that sun cuts to zero.
- Earthquake, Bulldoze, and Magnitude lose half their power in Grassy Terrain: an earthquake in a place that dampens tremors on its own.
- Rising Voltage in Electric Terrain and Expanding Force in Psychic Terrain double their power instead of getting the usual 30%, as long as the attacker is on the ground.
Burn: the only status that goes straight into the formula
Of all the status conditions in battle, only a burn changes the damage number: physical attacks are cut in half. Poison and sleep don't affect attack power at all, and paralysis slows a Pokémon down without weakening it. There's one trap here: a burn weakens attacks even on a critical hit. A critical hit ignores stat stages that work against the attacker and goes straight through the opponent's screens, but it doesn't cure an ailment. Those are two different mechanics in the game, and a crit skips only one of them.
The Ability Guts reverses this mechanic rather than just canceling it: instead of being weakened, physical attacks go up by half. A burned Pokémon with Guts hits harder than a healthy one. It's one of the few cases in the game where a status condition is something you actually want, not an accident to be healed.
Screens that a critical hit doesn't see
Reflect and Light Screen halve damage, each only for its own category of move, physical or special. Aurora Veil does both at once, but in the game you can only set it up in snow. All three share the same weakness: a critical hit goes through them as if they weren't there. It's the same rule that makes a crit ignore the opponent's raised defenses. To the game, a screen and a defense stage are the same kind of "fortification," and a critical hit skips it rather than breaking through by force.
The practical takeaway: setting up Reflect against a team built around a high critical hit rate does less than the number on paper suggests. The screen still protects against regular hits, just not against the ones that hurt the most.
Test it on your own team
All four factors (weather, terrain, status, and screens) are calculated together in the damage calculator, along with nature, IVs, EVs, Abilities, and held items on both sides. Instead of working out multipliers in your head, plug in a specific pair of Pokémon and see which circumstance actually tips the exchange. Weather that matches a move's type often means more than a ten-point difference in Attack.