As far as I know, (and I may be wrong, it wouldn't be the first time
) the kv rating equals turns per volt. So a 1000kv motor running on a 3S LiPo, or 11.1 volts, would translate to 11.100 rpm, however, battery voltage is higher at peak charge, so you would want to take that into consideration. In other words, a fully charged 3S is actually about 12.6 volts, so a 1000kv motor on a freshly charged 3S would actually be around 12,600 rpm, and drop as the voltage decreased.
That's easier for me to calculate than more complicated mathematical formulas.
Of course, these numbers are all at full throttle.
If i'm incorrect, someone please let me know, that's how I was taught.