OD&D/FMC Metric Conversion

On the Itch page for Fantastic Medieval Campaigns, LtPinback asked if I would consider including metric conversions for the game. Here is my attempt at converting the 1974 ruleset to use metric units, taking into consideration not only the fictional distances in the game-world, but also that the game as written is meant to be played using a physical ruler (in this case, a 30cm ruler).

By the way, if you missed it, Version Beta of FMC is available now and (as always) free to download! Click the link above to check it out, and I hope you find it interesting and/or useful.

Tabletop Scale

As y’all know, the basis of movement in OD&D/FMC is the twelve-inch ruler whose length represents different distances in different contexts. I’d go as far as to say that the tabletop scale is more important than any of its conversions (tens of feet, tens of yards), and that the latter are ultimately just justifications for using a ruler. All concerns are secondary to physical, tabletop scale.

Characters have four degrees of speed: 12", 9", 6", and 3". As LtPinback pointed out, using a 30cm or 300mm ruler, we can convert these speeds easily and proportionally:

Imperial (in)	Metric (mm)
12	300
9	225
6	150
3	75

But that’s just the tip of the iceberg.

Outdoor Encounters

In Chainmail, 12" represents 120 yards when participating in outdoor battles. For our case, we might say 25mm represents 10m, such that:

Tabletop (mm)	Outdoors (m)
300	120
225	90
150	60
75	30

Keep in mind that our goal is not to convert every single literal measurement in the game, e.g. using 110m which is about equal to 120 yards. Rather, our goal is to use scales which are intuitive and justifiable with tabletop-size measurement tools.

Is it kinda funky that we’re using a number like 120 in the metric system? Don’t think about it too much. This is what you’re signing up for.

Underworld Exploration

In OD&D/FMC, 12" represents 120 feet when exploring dungeons. That’s not all: dungeons are typically drawn using 1" (~25mm) squares which represent 10’ by 10’ areas. This means that, really, the idea is you’re traversing 12 squares on the map each move (though you also get 2 moves per turn—I know, it’s annoying).

With our metric ruler, we have a length of 300mm which we use to move around 25mm figures on 25mm squares in 75mm increments [1]. 120 feet is equal to 40 yards which is about the same in meters. Okay. Slightly annoying.

Let’s try 25mm represents 3m, so that 300mm represents 36m. That is kinda nice since 3m is approximate to 10 feet, but it’s damn ugly. Nope.

Tabletop (mm)	Underworld (m)
300	36
225	27
150	18
75	9

How about we say that 75mm represents 10m, so that the maximum speed becomes 40m (~131 ft)? That is a much prettier number, but it means that each 25mm square represents a third of 10m (3⅓m). I don’t think this is as bad since you’re not really worried about the size of individual squares as much as making sure that distance in general makes sense and is intuitive.

Tabletop (mm)	Underworld (m)
300	40
225	30
150	20
75	10

Neither of these feel very satisfactory, and both show the limitations of converting a twelve-inch-ruler tabletop system to a metric one. I like having the overall numbers be nice and rounded like in the second table, rather than being told “You can move 36 meters”—I know it’s abstract, but who’s counting meter by meter?!

Even Gary asks you to use basically 3⅓ foot squares in combat for AD&D since he divides the movement rate the same way (letting 12" represent 40 feet). That is, everything is already awful; why care? Besides, the second scale guarantees that underworld exploration occurs at a scale that is exactly one third of outdoor combat.

Overworld Exploration

OD&D/FMC is somewhat quirky for having, of all things, 5-mile hexes. Usually we’re accustomed to seeing slightly larger 6-mile hexes, which are easily convertible (and often converted) to 10km. 5 miles is closer to 8km, which is not bad since you don’t really count up miles or km; you just count up hexes which represent those distances.

8km hexes or 10km hexes? Honestly, whatever is easiest for you. There’s no reason that OD&D/FMC hexes are 5 miles except that they are, and you might as well swap those with 6-mile hexes. Moreover, movement rates in the wilderness do not correspond at all with the typical movement rates that individual characters have. This is on one hand because it’s the speed of the party as a whole, and on the other hand because you don’t need a ruler to count hexes on a game board.

My theory is that OD&D/FMC hexes are 5 miles wide because it’s easier to count in increments of 5 than 6. So, why not 10km?

Conclusion

There’s a lot of thirds because the imperial system loves thirds, while this is not so friendly with the base-10 math that metric measures use. This is kind of a quirk of converting between the two, especially when using something whose math is so grounded in feet and yards. If there were a metric unit equal to ⅓m, we would have none of the problems mentioned above. Instead, it's like we're having to convert everything to yards while pretending that feet aren’t a thing.

Here is my final conversion table of tabletop inches to tabletop millimeters, along with the conversion of that to outdoor combat and underworld exploration. Remember that hexes don’t interface with tabletop scale in OD&D, so they are not listed below.

Tabletop (in)	Tabletop (mm)	Outdoor (m)	Underworld (m)
12	300	120	40
9	225	90	30
6	150	60	20
3	75	30	10
1	25	10	3⅓

Sincerely, I hope this is helpful! I’d really like input from others since I may end up including something like this in FMC. It’s just something I think has to be done very intentionally, to avoid messing up the internal logic of the game or also making something unintuitive for human players.

Endnotes

[1] Here I’m referring to the bases having diameters of 25mm.