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I'm introducing the following house rule:

A round lasts 5 seconds.

What would be the effects of such a house rule? Would there be any? Would this break the game?

Out of the top of my head, I cannot think of any actions/spells/mechanics that rely on the duration of a round.


Disclaimer: The original question was a bit broader and was closed twice as opinion-based/off-topic.

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    \$\begingroup\$ Why are you changing this length if you don't think it has any effect? Why risk breaking something? \$\endgroup\$ – SeriousBri Jul 3 at 17:46
  • \$\begingroup\$ @SeriousBri This more of a hypothetical question. I want to know whether the round duration of specifically 6 seconds is meaningful or if it is an arbitrary chosen number that does not affect the game otherwise. \$\endgroup\$ – Aventinus Jul 3 at 17:49
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Many spells would have functionally longer durations

Lots of spells have durations specified in minutes. In a six-second-round time system, a minute translates to 10 rounds, whereas in a five-second-round time system, the same time period becomes 12 combat rounds. Functionally, this will make these spells last about 20% longer than they otherwise would.

This probably isn't game-breaking at all, since it would be extremely rare that a combat encounter in 5e lasts more than ten rounds anyway - the effective duration of minute-length spells is pretty much "this fight".

It would have made the original version of XGtE's healing spirit even more powerfully abusable than it was before by adding a couple more rounds of free healing, but luckily that's been errata'd to restrict the amount of total healing the spirit can offer.

Running speed in tactical time is slightly increased

Much as with spell durations, if speeds are unchanged and a character can travel as far in 5 seconds as they previously could in six seconds, their movement speed in tactical time is about 20% better than it was before.

Again, this is unlikely to be of significant consequence since the speed of all creatures has increased by the same amount, and long-distance movement outside of combat is handled by different rules that measure in hours or days, not combat rounds. (But it will help your players if they're aiming to try and set sprinting records relative to real-world human accomplishments.)

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    \$\begingroup\$ "it would be extremely rare that a combat encounter in 5e lasts more than ten rounds anyway" - This could make a difference, however, for a DM who uses initiative order and the action economy for scenes which are not strictly combat. For instance, if adventurers are exploring stealthily, or evading enemies after being spotted, or participating in an extended chase scene. In such a case, a scene could easily go through many quick rounds of movement and ability checks, and then collapse into standard combat. \$\endgroup\$ – recognizer Jul 5 at 3:50
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1. Some encounters in published adventures are may be written assuming six seconds per round.

This is probably the most significant problem. I can give a couple examples. From Storm King's Thunder, there is an encounter involving timed hazards:

Characters who have a passive Wisdom (Perception) score of 14 or higher notice gaps in the ceiling, suggesting the presence of two hanging blocks of stone. Characters who search for traps and succeed on a DC 14 Wisdom (Perception) check also notice these blocks, which constitute the temple’s outer defenses.

Each block is a 40-foot-tall, 40-foot-wide, 20-foot-thick slab held up by mechanisms buried in the mountainside. When the lever in area 2A is moved to the down position, the block of stone closer to the entrance (area 1) falls, sealing off the tunnel. When the lever in area 2B is moved to the down position, the inner block does the same thing. Each block takes about 6 seconds to drop to the floor, allowing time for creatures to get out of the way.

At 6 seconds per round, the party has a single turn each to get out of the way of the hazard. At 5 seconds per round, they potentially have two rounds (how are you going to adjudicate 1/5th of a round?).

We see a similar issue with an encounter in Tomb of Annihilation:

With a successful DC 15 Wisdom (Perception) check, a character discerns that the entire floor of the corridor is a single pressure plate. The adamantine propeller has AC 20, 30 hit points, and immunity to all damage except force damage. It thunders into motion whenever more than 20 pounds of pressure is placed on the corridor floor. Once activated, the propeller spins up to full speed in 6 seconds. If the weight is removed from the floor, the blades take a full minute to slow to a stop.

While the propeller is spinning up or slowing down, a creature can leap through a gap between two blades with a successful DC 20 Dexterity (Acrobatics) check. On a failed check, the character takes 33 (6d10) slashing damage as it passes through the blades.

Again, a timed hazard, where a 5 second turn potentially gives the party more time to avoid it.

2. It makes the Acquisitions Incorporated spell gift of gab a bit odd to rule:

When you cast this spell, you skillfully reshape the memories of listeners in your immediate area, so that each creature of your choice within 5 feet of you forgets everything you said within the last 6 seconds.

When used in combat, casting the spell would retcon anything you said on your last turn. If a turn is five seconds, now it can include words from the last two turns.

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    \$\begingroup\$ I think the simple solution to these issues is to understand that many effects were designed around the "6 seconds per round" design. So in making this change, you should also consider adjusting any other effects that seem to work in 6-second intervals (12, 18, etc.) and tune them down to 5 second intervals instead. This goes along with Carcer's answer that you're also effectively increasing the rounds per minute, so some effects will last more rounds than normal. \$\endgroup\$ – MirrorImage Jul 5 at 4:53
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One impact of this a mismatch between spell and ability durations measured in rounds and those measured in minutes - in the event of a long combat lasting 10 rounds, a spell like Bless would end in 6-second rounds but would have 2 5-second rounds left.

It would probably be sensible to change any 1-minute duration effects to 10 rounds; longer durations, like the various 10-minute spells, are unlikely to be directly affected as in practice they'll be primarily non-combat time anyway, as I think it's reasonable to assume it's rare to see 100 rounds of combat time tracked that precisely.

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  • \$\begingroup\$ Excellent point about spells that last 1 minute. \$\endgroup\$ – Aventinus Jul 3 at 17:50

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