Are there explicit rules for combining numerical bonuses in 5th edition, or is it just left implicitly to "common sense"? There are special cases (e.g. damage resistance and vulnerability, choice of base AC formulae, identical spell/item effects) which are well-defined, but I can’t find a general rule.
Additive and multiplicative bonuses
Example: Longstrider and Haste for a creature with 30′ base walking speed.
Longstrider: You touch a creature. The target’s speed increases by 10 feet until the spell ends.
Haste: Choose a willing creature that you can see within range. Until the spell ends, the target’s speed is doubled […]
- Always multiply first: (30′ × 2) + 10′ = 70′
- Always add first: (30′ + 10′) × 2 = 80′
- Do it in the order the spells were cast
- Player chooses
- GM chooses
Multiplicative bonuses with one another
Example: Haste and Boots of Speed for a creature with 30′ base walking speed.
Boots of Speed: While you wear these boots, you can use a bonus action and click the boots’ heels together. If you do, the boots double your walking speed […]
- Doubling the second time just idempotently repeats the fact that it’s been doubled: max(30′ × 2, 30′ × 2) = 60′
- Doubling the second time means your speed is quadrupled: 30′ × 2 × 2 = 120′
- Doubling the second time adds another 100% of your base movement, resulting in an overall tripling: 30′ × (100% + 100% + 100%) = 90′. This was the rule in 3rd edition.