How does the math in stat boosting moves and mega evolution work?

Question

Let's say a Pokemon has 100 base stat in regular form and 150 in mega form. Correct me if I misunderstood how this works. If the Pokemon uses swords dance in regular form, it's stat increases to 200. If it mega evolves after using swords dance in regular form, will the stat be 250 or 300?

Answer 1

All stat stages like swords dance and stat-enhancing items like choice specs and eviolite act on the stats in the Pokemon's summary screen, not the base stats (or the acid stats :P). The "summary screen stat" is calculated using one of these formulas.

So if a Pokemon has 100 base physical attack and max level, EVs, IVs, and nature in physical attack, then its "summary screen" physical attack would be 328. Swords dance would increase it to 656. Increasing base physical attack to 150 would increase the "summary screen" physical attack to 438. Having both 150 base physical attack and swords dance would increase it to 876.
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Comments

That is not the answer I am looking for. I'm not considering EVs and stuff like that since I just wanna clarify how this works. I am only asking which of the two choices I asked in the question, 250 and 300, if a pokemon with 100 attack stat gets a swords dance boost and then mega evolving.

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@Savitar TF you saying, he clearly says that the stat is neither 250 or 300, as it depends upon both the base stat and EVs & IVs. Your understanding of how stat boosts work is messed up.

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Anyway, the mega evolved Pokemon is treated as the same old Pokemon, but
1. The stats are recalculated
2. The ability changes accordingly
3. The type changes accordingly
So the above answer answers your question correctly

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If you have a Pokemon with 100 base physical attack, level 100, 0 EVs, 0 IVs and a neutral nature, then its "summary screen" physical attack is 205. One swords dance raises it to 410. Raising base physical attack to 150 raises the "summary screen" physical attack to 305. Having both 150 base physical attack and one swords dance raises it to 610.

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@Deathrider basically my question is if a stat boosting moved was use before mega evolution, will the stat get the direct increase or completely recalculated using the mega evolved stat. I'm not exactly sure how to describe it but the edited answer is what I was looking for.

@sumwun thanks. This is the one I was looking for