Help with density/volume calculations?

[Deus Zed Machina]

Ok, so I'm making some parts that hold fuel, but the resources' "density" values are confusing me. I've tried using the standard D=M/V formula, but the numbers I get from that don't seem right. How would I go about figuring out how many units of a given resource fit in a cubic meter of space?

[Streetwind]

Density in KSP isn't really density in the physics sense, because KSP does not deal with volume in any way, shape or form.

The density value in the RESOURCE_DEFINITION node is a mass, given in metric tons. It is the mass of one of this resource. "One what?" Just one. KSP doesn't deal with volume units.

So if a tank has 400 Oxidizer in it, then that is a mass of propellant equal to 400 times the density value from the definition node.

 

Edited June 2, 2017 by Streetwind

[PocketBrotector]

Streetwind is correct in that the in-game units for resources in KSP are completely arbitrary. This is partly a legacy design from when the game was much simpler, although it's worth noting that resources themselves don't necessarily have a set volume - they can be compressible gases, incompressible liquids or solids, or something that doesn't have a volume at all (electric charge). 

However, there is a volume key in the resource definitions. This is the number of liters per unit, and it's provided so to facilitate modding - resource-switchable tanks, etc. The stock resources are mostly 5 liters/unit, while most mod resources are 1 liter/unit. You can take a look at the stock tanks' heights and radii to calculate their apparent external volume, then compare that to how much LFO (or whatever) they hold, to get a sense of how much of that external volume typically represents usable internal volume. For the 2.5m tanks, it's about 80%.