Baldur's Gate 3 does an exceptional job of explaining the intricate mechanics and nuances of D&D to players who are, for the most part, unfamiliar. All the while trying to keep these newcomers engaged with excellent gameplay and stellar voice acting.
But, condensing so much information into tight little packages has the downside of players sometimes missing out on knowing about a core mechanic of the game. One of the mechanics that often goes misunderstood is Upcasting. Here is how it works.
What Is Upcasting
Following the rules of the Dungeons & Dragons ruleset, Upcasting in Baldur's Gate 3 is a spell mechanic that allows a spellcaster to cast a lower-tier spell with a higher-tier spell slot, typically for increased damage and stronger effects.
Let's take the example of Magic Missile, a level 1 evocation spell that deals 3d4 + 3 damage at level 1. Casting this spell allows you to send out three darts of Magic Missile at whichever targets you choose within range to deal 3d4 force damage per blast, hence the +3.
Now, if you upcast this same spell with a level 2 spell slot, Magic Missile will create an additional "Missile" that does an additional 3d4 damage. Which means it goes from 3d4 + 3 to 3d4 + 4. And if cast with a level 3 spell slot, it would become 3d4 + 5, and so on.
Which Spells Can Be Upcast
Barring some small exceptions, all spells can be upcast, but not all of them get a bonus if cast with a higher-tier spell slot. A good example of this is Misty Step, a teleportation spell that allows the caster to travel to a visible spot within 18 meters. It doesn't matter if you cast this spell with a level 1 spell slot or a level 6 spell slot; the results will remain the same.
In-game tooltips are your best friends here, as they directly indicate whether or not upcasting a spell will have any additional benefits to the caster. In most cases, players will want to conserve their higher tier spell slots and only upcast spells that give additional benefits like more raw damage; however, there are exceptions.
To see in-game tooltips, select a spell that can be upcast. The hotbar will show you your entire list of available spell slots that spell can be cast with. Hover over any of the spell slots and a tool tip will pop up with the description of what the spell does.
At the bottom of the tooltip, you will see a section dedicated to Upcasting. Here, it will be clearly stated what the effects of Upcasting are in the case of that particular spell. If there are no special effects, the tooltip will say so.