How many rounds does the average combat encounter last?

Question

I am trying to create a simple way to track ammunition for my upcoming sandbox game. In this system, players will buy ammunition in (for lack of a better word) "packets". One packet of ammunition contains enough for one encounter. To work out the price of a packet, all I need to do is multiply the cost of one arrow (or bolt, etc.) with the number of arrows fired in the average combat encounter. I understand that number of arrows used will double when extra attack is gained. I can work around that easily by making players with extra attack spend two packets per encounter.

(Why bother with ammunition at all if I'm going to simplify it this much? Because I also have a simplified encumbrance system, so this ties into that. Players have to decide how many fights' worth of ammunition they are willing to carry.)

However, to be able to work this out, I need to know how long the average encounter is. When I say "average", I mean a medium or hard difficulty encounter in which 4–6 characters face off against a similar number of opponents. In other words, what might be considered the bog-standard, run-of-the-mill encounter.

I would be particularly interested to hear from someone who has been recording the length of each encounter (as unlikely as that may be).

Answer 1

Five. It follows roughly a Boltzmann-distribution, where the maximum is at around 4-5. It is our statistics from around 200 combats.

(This how a Boltzmann-distribution looks. But in our case, the maximum is at around 4-5, not at 1, as on this picture.)

About the methodology:

Typically I don't write my actual stats in my character sheet, instead we have a table where the stats (mainly, the HPs) are recorded after every round. I used these tables. I have also some other surprising results (for example, spellcasters lose HP with around a similar mean rate as the fighters, but much more unpredictable). There is also a lot of encounters which is not a regular combat, although it uses combat stats (for example, assassin kills somebody in a single round), these weren't counted.

Warning: in our experience, high-level combats last much shorter. These stat is coming mainly from low-level parties. To produce a more clear statistics, at least some of them should be balanced and normalized by the party level, this was not done.

Answer 2

The DMG seems to implicitly estimate a combat to last about 3 rounds.

DMG p.278, Overall Damage Output:

To determine a monster's overall damage output, take the average damage it deals with each of its attacks in a round and add them together. [...] If a monster's damage output varies from round to round, calculate its damage output each round for the first three rounds of combat, and take the average.

Similarly, DMG p.281, Monster Features section on estimating the effect of Regeneration on CR calculations:

Increase the monster's effective hit points by 3 x the number of hit points the monster regenerates each round.

To me, the two of these indicate the expected length of an encounter is approximately 3 rounds... or at least, the expected duration of any given creature's effect on it is 3 rounds.

There's a distinction there, but in personal experience, 3 rounds matches up with an average combat duration for fights that aren't extremely difficult, extremely easy, and/or don't have an extreme amount of outnumbering on one side (I play on Roll20, and my DM has an initiative script/plugin that keeps track of the passing of rounds, so I can reliably see how long a combat has lasted at the end).

It can take longer if there's a decent amount of hide-and-seek going on (maneuvering around obstacles, etc.), but those tend to just cause more rounds of movement rather than more rounds of combat.