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Monster speed in BECMI-like rules systems is written as, for example:

  • Move 90' (30')

What is a reasonable way of converting such a movement rate to modern D&D, such as the fifth edition (but 3rd, 4th and Pathfinder are also fine)?

I am not afraid of mental arithmetic. The solution need not replicate current speeds, but rather should, as input, take usual values from monster entries in BECMI (and related OSR rulesets), and as output, give out a speed value that is reasonable in D&D 5, so that fast creatures remain fast and slow remain slow, without giving speeds that are outrageous by standards of D&D 5.

The speed need not be rounded to 5' increments; that is part of reasonable mental arithmetic, where it even needs to be done, as I do not always play on a grid.

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  • \$\begingroup\$ @Zachiel: The Rules Cyclopedia says: "The first number, usually 120', is the number of feet the character moves per turn [10 minutes] at a very cautious walking pace indoors; out-doors, the unit of measurement is tripled so that 120' becomes 120 yards per turn. […] The second number within parentheses is the movement rate per round [10 seconds] in feet; this number is often called "encounter speed" and is the number used during combat. Outdoors, the encounter speed would be 40 yards as opposed to 40' in this instance." \$\endgroup\$ – Ilmari Karonen Oct 2 at 22:34
  • \$\begingroup\$ … Apparently "120' (40')" is the standard speed for PCs, and in general the encounter speed for PCs is ⅓ their normal speed — but since it's measured per round rather than per turn, and since a turn equals 60 rounds, it's actually 20 times faster! The Cyclopedia explains this by the slow "normal" speed including "mapping, peeking around corners, resting, and so forth." Also, apparently "running at full speed (toward or away from an enemy)" allows movement at 3x encounter speed (i.e. normal speed, but per round, not per turn), but this can only be sustained for 30 rounds before exhaustion. \$\endgroup\$ – Ilmari Karonen Oct 2 at 22:50
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Converting from BECMI movement stats to D&D 5e Speed stats starts with looking at what the numbers in each are like, and then making sure they're converted in a way that recognises that playability is the goal rather than something mathematically pure (though if we can have both, great).

A comparison of movement rates in BECMI and D&D 5e

If you survey the per-round move* and Speed statistics in BECMI and 5e, you'll see there's a much wider range of ground speeds in BECMI than in 5e: the benchmark (human PC standard) in BECMI is 40' and ranges from a high of 70' (a panther) to lows of 3' (a giant bat, gray ooze, or shrieker) and even 1' (a green slime; considered a mere trap in 5e). D&D 5e has a much more compressed range of ground speeds, with Speed 30 ft. as the human benchmark, and the MM showing a range from Speed 60 ft. at the high end (various; for examples, an allosaurus or a skeletal horse) to Speed 5 ft. at the low end† (a grounded bat).

This gives us two scales, one wider and one more compressed, each with slightly different middle values (40 vs. 30). A straight conversion is the first thing that might seem obvious, but some details from the survey should give pause. Comparing some monsters directly shows us that there isn't a straight conversion rate that would make conversions match up for monsters that are in both. For example, we can compare a gray ooze and a bat in each. A gray ooze in 5e is an okay Speed 10 ft., not the slowest thing in the game, while a BECMI gray ooze is a glacial 3'… but this makes a bat faster on its feet than the ooze in BECMI (move 10') while in 5e the bat's Speed 5 ft. is slower than the ooze.

That doesn't mean that a conversion factor isn't possible, but it does mean that any conversion method used on BECMI monster move rates will give results that don't quite fit the rates that D&D 5e monsters have been assigned.

Fortunately, what that means is that some ideal of accuracy isn't quite achievable, so something approximate is the best possible, and is going to have to be good enough to be playable.

A rough conversion

A rough and ready conversion is just to knock off 10' from BECMI move stats for anything 40' and up, giving a range from Speed 60 ft. to Speed 30 ft. Then for move 30' to 10', knock 5 off BECMI move stats for a range from Speed 25 ft. to Speed 5 ft. Anything lower, just set it to Speed 5 ft. and move on.

This has the advantage that human-speed opponents in BECMI end up with Speed 30 ft. in 5e, and the slightly-less-than-human-speed opponents in BECMI end up with Speed 25 ft., which is what D&D 5e assigns to those two classes of opponents. These make up a large proportion of BECMI monsters, so having the categories match up like that is nice.

It also means that you can use BECMI monsters on or off a grid as readily as native 5e monsters.

An alternative, simpler option to consider: no conversion

It's not entirely unreasonable to just use the BECMI per-round speed directly as a D&D 5e Speed. Most monsters have either 40' or 30' of movement, which is as much or a bit more than the typical 5e adventurer.

If you're planning on using exclusively BECMI monsters, then these slightly higher speeds can just be part of the reality that the adventurers have to deal with.


* The number in parens is the per-round move; e.g., 90' (30') is a creature that can move 30' during a combat round. The first number isn't used in combat, so we can ignore it.

† This low excludes monsters with a Speed 0 ft. rating to indicate monsters that are either immobile or which can only use some other movement mode such as flight; in BECMI, these monsters simply have no (ground) move statistic at all.

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Let's start with a baseline human PC. In 5e, a Human has a base walking speed of 30 feet per round (6 seconds in 5e), which corresponds to a brisk walking pace (5.5 km/h or 3.4 mph) in real life.

According to the Rules Cyclopedia chapter 6, BECMI PCs (who are human by default, unless they pick dwarf, elf or halfling as their character class) have a movement speed of "120' (40')" by default. The first number is the "normal speed", which is used outside combat and measured in feet per turn (10 minutes!) indoors, or in yards per turn outdoors, while second number (which, at least for PCs, is normally one third of the first) is the "encounter speed", which is used in combat and measures in feet per round (10 seconds in BECMI) indoors, or in yards per round outdoors. When "running at full speed (toward or away from an enemy)" this speed can be tripled, but only for up to 30 rounds (5 minutes) before exhaustion sets in.

Leaving aside the questionable realism of the BECMI movement speeds,* this suggests a hopefully reasonable conversion rule:

  • To convert a 5e movement speed to BECMI, multiply the 5e speed by four to get the normal speed, and divide this by three to get the encounter speed.
  • To convert a BECMI movement speed to 5e, take the normal speed (i.e. the first number) and divide it by four — or, equivalently, multiply the encounter speed by 3/4. (If these yield different results, take whichever seems more sensible.)

The rule above works perfectly for normal humans. Let's take a look at a few other select creatures and see how well in fits:

  • Elf: 5e (PC race) speed 30'; BECMI 120' (40'). Same as humans in both editions. The conversion formula works perfectly.

  • Halfling: 5e (PC race) speed 25'; BECMI 90' (30'). In both editions halflings are a bit slower than humans. Converting the BECMI speed to 5e using the formula above would give 90' / 4 = 22.5', which rounds up to 25'.

  • Dwarf: 5e (PC race) speed 25'; BECMI 60' (20'). BECMI dwarves are even slower than halflings. The converted speed would work out to only 15'.

  • Goblin: 5e speed 30'; BECMI 90' (30'). In this case, the 5e speed directly matches the BECMI encounter speed — but this also means that 5e goblins are as fast as humans, whereas BECMI goblins are 25% slower. If you wanted BECMI-like slow goblins in 5e, you could use the conversion formula and round their speed up to 25' as with the halflings.

  • Bear: 5e (Black / Brown / Polar) speed 40' (climb 30'); BECMI (Black / Grizzly / Polar / Cave) speed 120' (40'). Like goblins, BECMI bears are slower than their 5e counterparts, having the same movement speed as humans. Again, using the conversion rule above would give you slow bears with a speed of 30', whereas just using the BECMI encounter speed directly would make your bears as fast as in 5e. The latter does seem somewhat more realistic to me, but it's ultimately your choice.

  • Bat: 5e speed 5' (fly 30'); BECMI 9' (3'), fly 120' (40'). The fly speeds match perfectly; the walking speeds don't, but when was the last time you saw a bat walking on the ground?

  • Giant Bat: 5e speed 10' (fly 60'); BECMI 30' (10'), fly 180' (60'). This time the fly speeds don't match; converting from BECMI to 5e would give a fly speed of only 45'. However, looking at their other stats, the BECMI giant bats seem to be noticeably weaker creatures than their 5e counterparts anyway, e.g. having only 2d8 HP in BECMI vs. 4d10 in 5e.

  • Horse: BECMI horses come in four types with different speeds:

    • Draft Horse: 90' (30') → converted 5e speed 22.5'
    • War Horse: 120' (40') → converted 5e speed 30'
    • Pony: 210' (70') → converted 5e speed 52.5'
    • Riding Horse: 240' (80') → converted 5e speed 60'

    5e also has the same four types of horses, but they only come in two speed categories: 40' (Pony / Draft Horse) and 60' (Riding Horse / War Horse). Of the four types, only the riding horses match exactly (which is nice, since they're the fastest kind); the rest are off in various ways. BECMI ponies are quite a bit faster than their 5e counterparts, while the draft and war horses are slower. (In fact, BECMI war horses have the same speed as a walking human, while draft horses are considerably slower; in 5e all horses are faster than humans.) Not knowing much about horses in real life, I can't really say which system is more realistic.

  • Dragon: BECMI dragons come in three size classes with distinct movement speeds:

    • Small: 90' (30'), fly 240' (80') → converted 5e speed 22.5' (fly 60')
    • Large: 120' (40'), fly 300' (100') → converted 5e speed 30' (fly 75')
    • Huge: 150' (50'), fly 360' (120') → converted 5e speed 37.5' (fly 90')

    In 5e, Young, Adult and Ancient dragons have a speed of 40' (fly 80'), which is fairly close to the converted speeds of large and huge BECMI dragons — a little faster on the ground, roughly similar in the air. Wyrmlings in 5e have a speed of 30' (fly 60'), which is pretty close to the BECMI small dragons (though again a bit faster on the ground).

  • Hydra: 5e speed 30' (swim 30'); BECMI 120' (40'). Surprisingly few surprises here. The hydra has the same speed as a human in both editions, and the conversion formula works perfectly.

  • Griffon: 5e speed 30' (fly 80'); BECMI 120' (40'), fly 360' (120'). The walking speed matches exactly; the converted flying speed would be 90', which is a little faster than in 5e, but not by much.

  • Gray ooze: 5e speed 10' (climb 10'); BECMI 10' (3'). 5e oozes are a lot faster than in BECMI; the converted speed of the BECMI ooze would be only 2.5' per round. (Also, this is the first BECMI monster I found where the encounter speed is not exactly one third of the normal speed, although in this case this is only due to rounding.)

  • Gelatinous cube: 5e speed 15'; BECMI 60' (20'). An exact match, once again.

Based on this rough and unscientific sample, I'd say that the rule of dividing the BECMI "normal speed" by four indeed works reasonably well, insofar as it gives reasonable speeds and should yield similar combat dynamics when facing typical PCs.

To be fair, just using the BECMI "encounter speed" (which is normally 1/3 of normal speed) directly, as suggested by SSD's answer (which they posted while I was writing this one) wouldn't be too badly off either. In a few cases it actually gives a better match to the corresponding 5e monster — but in those cases the relative speeds of the monsters compared to humans are also off, so it can also be argued to be just a content difference between the editions.


*) The Cyclopedia tries to justify the slowness of the "normal speed" by explaining that it includes "mapping, peeking around corners, resting, and so forth." Even so, an indoor speed of 220 meters (720 feet) per hour seems rather glacial, and an outdoor speed of three times that isn't much better. Nothing, however, justifies the fact that, as far as I can tell, a normal BECMI character running at full speed outdoors in combat can apparently run a mile in less than two and a half minutes and keep running for another mile after that!

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