How many rounds does a spell with a duration of 1 minute last?

Question

If a concentration spell lasts for one minute (60 seconds, 10 rounds), when does one start counting down how much time it has left? Does the round you cast it in count towards how long it stays active?

First example, what if I use Aura of Vitality, which uses an action to cast then a bonus action can be taken to heal a target? The spell lasts for 1 minute, or 10 rounds. So on the turn I cast it I can use a bonus action to immediately use it, then do I get 9 more rounds or 10 more?

What about spells like Moonbeam, which don't take effect on your turn? Moonbeam activates when a creature enters it or starts its turn there. If you're first to go in a round then you'll get 10 uses out of it, but if you're last does that mean you only now get 9 uses out of it since it acts during an opponents turn?

Answer 1

It lasts for 10 turns of the character who cast it, including the turn it was cast on, and expires just before the character takes their 11th turn ... assuming of course that the character's concentration doesn't get broken.

So you get the following setup in combat:

1. Round 1: Character casts the spell, and it remains active for the remainder of this round
2. Round 2: Spell still active
3. Round 3: Spell still active
4. Round 4: Spell still active
5. Round 5: Spell still active
6. Round 6: Spell still active
7. Round 7: Spell still active
8. Round 8: Spell still active
9. Round 9: Spell still active
10. Round 10: Spell still active
11. Round 11: Spell fades as soon as the casting character's turn comes up

To address Moonbeam specifically:

Moonbeam creates an obstacle on the battlefield, and you can force one or more creatures start their turn in the first round by making it appear in their space.

Answer 2

Some 1 round spells, e.g. Blade Ward, have "Until the end of your next turn". others, such as Ray of Frost, have "until the tart of your next turn". Some, such as Chill Touch, even have both!

That is, a 1 round spell, cast during your turn on round 1, generally lasts until your turn on round 2 (1+1=2), but whether the start or end varies.

However, for longer durations, such as 1 minute (10 rounds), the section on Duration (p. 203) does not specify a general rule.

A good rule of thumb for GMs to follow, for a spell that lasts 1 minute, is to get 10 turns worth of benefit from it.

So for Haste (following this rule of thumb), which gives you an extra action and extra movement, including on the turn you cast it, as well as an AC boost, A GM may allow it to last until the start of your 11th turn.

You get to take an extra action on turn 1, and turn 10, but not turn 11; the AC boost protects you after your action on turn 1, until after your action on turn 10, right up until you start turn 11 (but not during your turn 11).

In contrast, a GM might allow Bless to last until the end of your 11th turn, so you get 10 turns of benefit from it (rather than only 9)... Or they might decide, for simplicity, that long duration spells are always the start of turn 11.

In both cases, a spell with a duration of 10 rounds, cast on your turn in round 1, should last to your turn on round 11 (1+10=11), with the GM determining if it is the start or end of your turn 11.

Suggestions applying the rule of thumb to the two spells you mention:

For Aura of Vitality, as you could use it on the turn you cast it, I would allow it to last up until the start of your turn on round 11, although in practice it has no effect after your 10th bonus action during your turn on round 10.

For Moonbeam, it mentions using an action on each of your turns to move it; so I would allow 10 actions, until the end of your turn on round 11. The would allowing you to move it on your 11th turn, but doing so would be redundant as it would cease at the end of your 11th turn (just after you move it). It would potentially affect enemies 10 times: their first turn after you cast it (i.e. the rest of round 1, up until your turn in round 2), right up until your turn in round 11.

Answer 3

Technically, it lasts until the end of the 10th round.

With actions that last more than one round^{[Citation needed]}, the description states

[...] until the end of your next turn.

So, following the flow of events, your action (on round 1) is to cast the spell. Your action is now over, and the countdown begins. For the next 10 rounds, you spend your action to maintain concentration¹ on the spell, which then expires at the end of the round.

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^{1. You can spend your action doing normal things, obviously, so long as you specify that you are maintaining concentration on the spell}