PT100 - 39250 Cubic fit formula
Assuming you're talking about the old-fashioned so-called "American standard" RTD, they're seldom used these days. You can find Callendar-Van Dusen equation coefficients for those, and some other types, on this National Instruments web page. 



If your RTD is 50 ohms, just scale it (double the measured resistance) so it fits the 100 ohm equation.