Boosts: XP, shards and luck multipliers

The rule

XP, shards and luck all use the same rule: every bonus is added, then you get one multiplier.

$\mathrm{multiplier} = 1 + \min\!\left(5,\ \sum \mathrm{bonuses}\right)$

So two ×2 boosts (+1 each) give ×3 and not ×4, and the maximum is ×6.

Where bonuses come from

The bonuses of the Double XP and Double Shards passes are +100% each. Luck has its own page: How luck works.