What are the rules for mega evolutions in Pokémon go?

Solution 1:

This is mostly repeat from This is the Dave's answer but with some extra detail and subheadings.

List of current Mega evolutions, their initial Mega evolution cost, and their types if they have changed from their base forms:

100:

• Mega Beedrill
• Mega Pidgeot

200:

• Mega Abomasnow
• Mega Absol
• Mega Altaria (Dragon/Fairy)
• Mega Ampharos (Electric/Dragon)
• Mega Blastoise
• Mega Charizard X (Fire/Dragon)
• Mega Charizard Y
• Mega Gengar
• Mega Gyarados (Water/Dark)
• Mega Houndoom
• Mega Lopunny (Normal/Fighting)
• Mega Manectric
• Mega Slowbro
• Mega Venusaur

Mega evolution cost

The initial Mega evolution cost is either 200 or 100. After a Pokemon has previously Mega evolved, this cost is reduced by a factor of 5 (to 40 or 20, respectively) for that Pokemon only. For example a previously Mega Evolved Beedrill has a cost of 20, but all other Beedril still have a cost of 100 unless they have also Mega Evolved previously. This reduction remains even if the Pokemon is later traded.

Earning Mega energy

Mega energy can be collected for all Pokemon who can currently Mega evolve. The methods of collection are:

• By winning a Mega raid. The energy earned ranges from 50 to 90, with more energy earned by defeating the Mega raid faster.
• Completion of specific research tasks. 10 (20) is rewarded from some "Power Up Pokemon 5 (10) times" Field Research tasks, whereas the amount varies for Event Field, Special, and Timed Research tasks. These tasks give Mega energy for a specific Pokemon based on the visual appearance of the Mega energy item (which is often difficult to determine: see Bulbapedia for a list of symbols).
• Walking with a buddy of the same evolution species of a Pokemon that you have Mega evolved before. For example, if you have Mega Evolved Gyarados previously, any Magikarp and Gyarados can earn Mega candy as your buddy. Energy is earned at a rate of 5 Mega energy every km, but the mega energy is only rewarded when the buddy Pokemon rewards candy i.e. at 1, 3, 5, or 20 km. For example, if the distance is 3km, it will earn 15 Mega Energy every 3km but not before 3km is walked.

In-battle Mega evolution effects

All Mega Pokemon have altered Atk and Def which give overall boosted stats (currently all Pokemon have both Atk and Def stats improved, though when Mega Diancie gets released, it will likely lose Def). The Stamina stat will not change.

If a Mega evolved Pokemon is on the field in a raid or gym battle, all its and allied Pokemon's moves that share a type with the Mega evolved Pokemon will receive an attack boost until it leaves the field i.e. switches or faints.

Mega evolved Pokemon cannot be selected in teams in any PvP battles except for training against your team leader i.e. not against Team Rocket or other human players.

Outside-battle Mega evolution effects

Whilst you have an active Mega evolved Pokemon, all Pokemon caught of the same type as the Mega Pokémon reward +1 additional candy. +1 candy will also be rewarded when captuing a raid boss regardless of its type. A icon will appear on your profile on your friends' friend list, indicating that you have an active Mega evolution. If the Mega evolution Pokemon is your buddy, this Pokemon will be visible to other players in its Mega evolved form when they see your profile.

Mega evolution duration: 8 hours

Max Mega energy that can be stored per Pokemon: 2000

Solution 2:

I'll have a go at answering some of this, I think this is incomplete however.

Initial mega energy costs for Pokémon are 200 or 100. Once that particular Pokémon has evolved, it is unlocked and the cost of mega evolving further times is reduced to 40 or 20. Each species has their own energy.

200 candy needed for first mega evolution, then 40; Abomasnow, Amphardos, Blastoise, Charizard, Gengar, Gyarados, Houndoom, Venusaur. Manetric added March 2021

100 candy needed, then 20 Beedrill, Pidgeot

Source: 1, 2

Only the Pokémon you evolved has the mega energy cost reduction, not all Pokémon of that species.

Source: 3

Walking with your buddy earns 5 Mega candy every km, but ONLY if that Pokémon has Mega evolved before. The mega energy is given when the buddy pokemon gives candy, this depends on the Pokémon's candy walk distance (1,3,5,20 km). If the distance is 3km, it will earn 15 Mega Energy every 3km but not before 3km is walked. 6 This means that it will require 9km to evolve the first list and 4km for Pidgeot or Beedrill. 7

Your buddy doesn’t have to be the same Pokémon that Mega Evolved, any Pokémon in the evolution line will earn Mega candy after you have Mega evolved once.

Mega Pokémon are active for 8 hours.

Mega Pokémon give you +1 candy to all Pokémon caught of the same type during the time it’s active.

Mega Pokémon will give +1 candy on the catch of a raid boss, regardless of your mega type, during the time it's active.

When in a raid or gym battle, when a Mega evolved Pokemon is on the field, all allied Pokemon's moves that share a type with the Mega evolved Pokemon will receive an attack boost until it leaves the field i.e. switches or faints.

Mega energy is gained when battling Mega evolved Pokémon in Mega raids. The speed at which you defeat the Mega raid is linked to how much energy you receive. Mega energy is also gained in completing research tasks.

8

Mega evolution energy is capped at 999.

9

Additional energy gained from gyms in a set time: https://www.reddit.com/r/TheSilphRoad/comments/miv2i5/what_are_the_breakpoints_for_mega_candies_when/gt6w4f5

Time to beat the boss (Amount of energy) 0s-0:29 (90 energy) 0:30-1:14 (80 energy) 1:15s-2:29 (70 energy) 2:30-3:44 (60 energy) 3:45-4:59 (50 energy)

Some speculation about rules:

Beedril is gaining energy from spinning gyms 4. This is at a rate of 5 energy given on average every 8 spins 5.