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One of my players wants to say his monk can kill a T-Rex in a single blow, by simply ramming it. The pitch goes like this:

  • As a wood elf he has a base speed of 35 feet.
  • The Mobile feat increases this to 45.
  • Being 18th level monk adds +30, so we're at 75 ft.
  • His wizard friend casts Haste on him. Being under the haste spell doubles his speed, to 150, and adds an action.
  • Dash doubles the 150 speed to 300.
  • Having two actions from Haste means he can Dash twice, so that's 600 feet of movement.
  • Finally, he can spend 1 ki point for Step of the Wind and thus Dash one more time as a bonus action. So we arrive at 900 feet of movement, in a turn lasting 6 seconds.

Now do the math: 900 feet per six seconds equates to 102 mph.

Something being hit by an object weighing as much as Medium humanoid that is traveling 100 mph, well, Ouch. But never mind killing a T-Rex. My question is, can he really go 100 mph? Every DM instinct I have says, this has gotta be wrong. Is he allowed to do this?

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    \$\begingroup\$ I hope your friend is aware that he's going to kill himself as well. Hitting a solid object at 100mph is quite fatal to humanoids. \$\endgroup\$ – Erik Oct 31 '18 at 6:24
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    \$\begingroup\$ The question in the title and at the end of the post are opposites of each other (can he do this? vs. is this wrong?). This can lead to slight confusion and I recommend you reconcile this difference. \$\endgroup\$ – Szega Oct 31 '18 at 8:47
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    \$\begingroup\$ Dash 1 = +150, Dash 2 = +300, Dash 3 = +300... Dash 3 either should be 600, or they should all be 150. I don't understand the logic of how Dash 2 is affected by 1, but 3 isn't affected by 2. \$\endgroup\$ – Tezra Nov 1 '18 at 17:13
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A level 18 hasted wood elf monk can move at 68 mph.

The speed with haste is correct, but you make some mistakes with the Dash action. Dashing does not double a creature's speed, rather it grants a creature extra movement equal to its speed[1]. Therefore, the hasted wood elf monk can move 600 feet per turn[2], or 68 mph.

However, that's not even close to the top speed a player character can reach. For example a hasted Tabaxi monk can move 1120 feet per turn[3]. Several other spells, magic items, and class dips can push that figure well over 2000 feet per turn.

Regardless, that does not mean a character can or should be able to collide with another creature for massive damage. D&D 5e certainly has no rules that handle said collisions. The closest thing is falling damage, but that is only meant to handle creature-to-ground collisions, not creature-to-creature.


  1. The Dash action says:

    When you take the Dash action, you gain extra movement for the current turn. The increase equals your speed, after applying any modifiers.

  2. Speed = (35+10+30)*2 = 150. Feet per turn = speed * (movement + 3 dashes) = 150*4 = 600.
  3. Speed = (30+10+30)*2 = 140. Feet per turn = speed * (movement + 3 dashes) * (feline agility) = 140*4*2 = 1120.
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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – doppelgreener Nov 1 '18 at 16:12
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No

Unless I missed something, this is not how dash works.

Having two actions from Haste means he can Dash twice, so that's 600 feet of movement.

Dash says

When you take the Dash action, you gain extra Movement for the current turn. The increase equals your speed, after applying any modifiers.

Note that Dash does not increase your speed, it just gives you extra movement. Your speed is 150 ft at that point, and so is your movement. Using 3 dashes (two from actions and one from bonus action) will increase your movement by 3*150 = 450 ft to a total of 600 ft, which equals 68mph and is, therefore, less than 100 mph.

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    \$\begingroup\$ @ValleyLad mind you, the monk would also take massive damage ^^ \$\endgroup\$ – PixelMaster Oct 31 '18 at 6:31
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    \$\begingroup\$ Also, RAW, damage would be... zero. Not amazing I guess. \$\endgroup\$ – HellSaint Oct 31 '18 at 6:39
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    \$\begingroup\$ Yeah this sort of thing worked in 3.5, because there were a couple of options for converting speed to damage (most notably Tornado Throw from Book of Nine Swords). In 5th edition there's nothing that'll let you do that so far. \$\endgroup\$ – Tacroy Oct 31 '18 at 7:41
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    \$\begingroup\$ You could use fall damage rules for damage of running into something at a high speed, falling from ~150ft gives ~70mph impact, which would be 1d6 per 10ft fall, thus 15d6 for running into something at that speed (perhaps to both creatures) \$\endgroup\$ – user2813274 Oct 31 '18 at 12:50
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    \$\begingroup\$ Actually, the question is, as stated, "My question is, can he really go 100 mph? Every DM instinct I have says, this has gotta be wrong. Is it?" - as OP mentioned in the comment, a question about the damage itself would be a separate question. There are a few implications on ruling any kind of "impact damage", e.g. an orc can easily move 90 ft in a round, being a CR 1/2 creature, if he deals, say, 3d6 damage of "impact", two orcs simply running into a character might kill him in a supposedly not-hard encounter. \$\endgroup\$ – HellSaint Oct 31 '18 at 16:58
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No, this would not do damage by RAW

One of the core principles of the game is that abilities do exactly what they say they do, and nothing more. Anything else is strictly a matter of GM interpretation.

Dash has no mechanic for causing damage. Haste has no mechanic for increasing damage directly, although it may happen as a result of allowing an extra attack.

Therefore, any damage would be at the GM's discretion, and I'll note that the smaller creature is likely to end up worse off from that collision.

As a matter of GM discretion

You could make this a learning experience for your players: just because you can, don't mean you should. It's up to you whether your campaign runs this way though; some players will not be happy with "surprise" fatalities.

Using a real world example, terminal velocity for a human in free fall is 100-150 MPH, which puts his ~100 MPH speed in a reasonable range for applying maximum falling damage (20d6) due to the impact.

Since there's no rule for this scenario, you could split the damage to 10d6 to each creature, divide it up unevenly between them, or apply the full 20d6 to each.

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    \$\begingroup\$ I'd take the mass of the creatures into consideration. A T-Rex is Huge, and the Elf is Medium. So to me, that's 2d6 for the T-Rex, 18d6 for the monk. \$\endgroup\$ – MarkTO Oct 31 '18 at 22:27
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    \$\begingroup\$ The ground, of course, also takes 20d6 from a fall. We just usually ignore it. \$\endgroup\$ – Jack Aidley Nov 1 '18 at 10:25
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    \$\begingroup\$ @MarkTO I've got a LOT more mass than a baseball, but when I get hit by a baseball traveling 68 mph, I think it hurts me quite a bit more than the baseball. \$\endgroup\$ – Question Marks Nov 1 '18 at 14:02
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    \$\begingroup\$ Yes, this is not simply a matter of mass, but of "hardness" or "toughness", which are very confusing physical properties. \$\endgroup\$ – Pedro Lamarão Nov 1 '18 at 14:05
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    \$\begingroup\$ The ground, is, of course, generally, planet Earth. \$\endgroup\$ – Pedro Lamarão Nov 1 '18 at 14:05
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This is turning into the "extraordinary ability in one area assumes extraordinary ability in other areas to make it work" dilemma.

For example, a superhero with super strength can punch through a car and rip out the engine. But, unless they have super-hard skin, their hands will turn into hamburger. So, just because they have the strength to do something doesn't mean they have the other abilities to pull it off.

So, for the monk, if the player wants them to move that fast: fine. I would start adding in damage over time to them for moving so fast.

  1. Feet are not designed to move 100 mph, so the feet would get shredded if bare-footed.

  2. Player will argue they're wearing foot covering (leather boots, sandals, etc). Foot wear is not designed to hold up to running 100mph. And, that speed would generate a lot of friction (both inside the foot wear to the feet, and possibly on other areas of the body as the clothing or leather parts on their body would rub and cause friction burns quite quickly from all the pumping of arms and legs.)

  3. One way to alleviate this would be to assume that the ki-spent Step of the Wind ability literally makes them light as air, so instead of running super fast, they're simply able to fly forward greater distances since their leg muscles are now pushing something light as air instead of normal body weight. (Think of Crouching Tiger Hidden Dragon, where they're using their ki to "fly" through the air and run great distances.) If that's the case, then it could alleviate any idea of friction burns and damage to the body from great speeds. But, if that was the case, I would say they have to pick a direction and stick with it during the turn, because something moving at that great of a speed can't change direction at the drop of a hat - especially if Step of the Wind is having them soar into the air and glide quickly. You can't simply turn in mid-air. So, if you want to allow them to have great speed like that, then the consequence is lack of maneuverability; they pick a direction, head that direction, and can't change it until several rounds later or something.

We're basically having to argue the laws of physics in a fantasy setting. It would be like debating whether the fly spell magically gives people oxygen bubbles over their faces in order to fly high altitudes (to which, if it's argued it did, then magic users would be abusing that to fend off gas traps and stuff).

The way I view this situation is that the players is sounding like a munchkin and min/maxer, focusing on the "can I?" instead of the "should I?"

If they want to do it, and the rules add up to allow it, you have to think of the repercussions of it; there has to be balance in the game. Either the character starts taking some kind of damage from pushing the human body far beyond normal limits, or they suffer severe movement restrictions once they get going.

In regards to the T-Rex: I wonder if the player is thinking their monk has Iron Fist or something, and could use it to punch the T-Rex (like they're running at 10 0mph and just jut an Iron Fist out to hit the T-Rex). Of course, my idea of Iron Fist is from old-school 2nd Edition Oriental Adventures AD&D monks, so maybe your player's monk doesn't have Iron Fist or something.

There's just a lot going on with what you're player is trying to do, and as a GM I would consider it ok or not ok based on context and situation.

For example:

  • The player wants to run 100 mph to go warn some people that enemies are coming: ok, fine, I'd let it fly.

  • The player wants to run 100 mph to punch a T-Rex to death: no, not fine; you get mushed to goo.

The problem with letting something like this be ok one time is the player gets it in their head that it's ok for anything they can dream up, so they'll start dreaming up abusive ways to use it (e.g. punching a T-Rex). So, it's often better to just either nip it in the bud right now, or come up with some rules to let them know there's a pro/con situation to using it (like damage or movement restrictions).

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Every DM instinct I have says, this has gotta be wrong. Is he allowed to do this?

D&D and all the various flavors of it are fantasy systems about super-human characters. You're describing a mystically trained mythological being close to the peak of possible advancement boosted by a magical spell.

68mp/h is nothing. At this level these are people who can catch bullets, strap an unconscious dinosaur to their back and climb a cliff, rip open castles with their bare hands and cast spells that reshape the world.

It feels wrong because you know how fast that is from driving a car. But think about all the other things you can do and you will see that it is just one more thing that makes sense in a game but doesn't in real life.

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