Unless your application somehow interacts with the real world, there is no reason to have any units at all - let the user decide what they want 1.0 units correspond to.

But if you need to be 'compatible' with the real world (eg. you want your values to be same as in the real world) I would use metric because it seems to be the most universal when it comes to more scientific stuff, however I would only do that internally. The interface might benefit from a conversion factor which you can change to get IO in whatever units you want.

Often applications let you choose what unit system is used for display - I dont know if they internally use one of them or if they use custom units.

I can back #Waterlimon's advice:

Unless your application somehow interacts with the real world, there is no reason to have any units at all - let the user decide what they want 1.0 units correspond to.

But something confuse me:

1) metric coordinates where 1.0 is 1 meter
2) other than metric, (i think for example about scale where 1.0 = 1 cm,

Are you writing that 1cm is not metric? because it is.

Yeah, if you dont have any performance or other implementation dependent reason to not use 1 engine unit = 1 meter such as:

-You are simulating very small or big things (atomic level/universe), might be better to use another scale

-You have some very important element which is some specific size, might make sense to make that size equal 1.0 (eg in tile based game you might want 1 tile = 1.0 units even if the tile would not be 1 meter)

You should probably use meters. If you think 1 cm is better as 1 unit instead of 0.01, might as well use it for aesthetic reasons.

Of course, you can always use multiple different scales for different systems but that would probably overcomplicate it.

Ultimately, you can use whatever you like, but unless you have a reason not to, 1 unit = 1 metre is the one that'll be least surprising to future coders that use your codebase. If your game world is particularly large/small, then use something more appropriate - e.g. my current game is at a global scale, and you can't zoom in to a more human scale (think Civilization), so I have made one unit equal 1000km.

Centimeters are a bit odd, because they don't fit into the pattern of 10^3 increments that are used for SI units, so if I were doing a game that was very zoomed in, I would probably go for millimeters rather than cm. Not that it's likely to be a problem once you're used to it.

I'll never forget the pain of having to use imperial measurements from a legacy codebase though, trying to figure out someone elses physics code is hard enough without having to deal with feet and slugs.

i hesitate beetween metric and centimetric (the other one when
you just call 1.0 as an 1u (unit, where there i was no stating correspondence between this unit and real world at all) i consider now bad one i was using it before but i was getting lost in it)

Unless your application somehow interacts with the real world, there is no reason to have any units at all - let the user decide what they want 1.0 units correspond to.

It's mainly a concern for the artists - when they build a human, do they make them 1.8, 180 or 5.9 units tall (m/cm/ft)?

However, it also affects some of the programmers on the team too. Is the value for gravity 9.8, 980, or 32.17? When designing the physics system, what size object can you define as being too small or too large, and then be able to make the simulation behave in a stable way by assuming that most objects will be in this range of sizes? How does the audio guy implement the doppler shift on moving sound sources -- are those velocities in m/s, km/h, ft/s, etc...? The guys designing the streaming world system need to know what range of units they'll be working with -- the floating point accuracy of positions that are 10km from the origin differs wildly depending on whether you're using cm, m, km, etc... Whether or not they need to use hierarchical or 64-bit coordinate systems may depend on the choice of units!

So it's pretty important for everyone on a game team to have a common understanding of what the game units are. A good engine will let you change these choices from game to game though.

It’s not your choice what 1 is. It’s up to the egomaniac lead artist for each game by each team.

In my experience, the technology lead has made the decree of universal scale, forcing the art teams to adapt to His will