I'd like to suggest that this fine website that I have consulted for years include concentrations as well.
As well, what about a simple scale unit converter regardless of measurement type. For example, milli to micro to nano to pico etc....
It doesn't matter if we're talking about meters or liters, having a unit scaler would be applicable to a wide range of measurements, including those that are not covered on this site to date.
Keep up the good work!
Thanks,