[barelyauser]

Since you know how much fuel you start with, can't you integrate something like the time spent firing the engine and use that as gauge?

[dr_orpheus]

Yes, and that is what most satellites due to estimate fuel. But any errors in that measurement can accumulate. See my other response on this thread for more details.

[WalterBright]

I'm sure they thought of that, so there must be some reason why it is inaccurate.

[tekla]

No, since engines aren't perfect and wastes propellant. Also fuel boils off in space.