If you have a random thermistor (like a random Engine Coolant Temperature (ECT) sensor or a random Intake Air Temperature (IAT) sensor), you can calculate the coefficients for the Steinhart-Hart Equation, which provides excellent curve fitting, based on three meaurments: low temperature, mid-range temperature and high temperature.

Right from Wikipedia:
T low (F) R low (Ohm)
T mid (F) R mid (Ohm)
T high (F) R high (Ohm)

C language formula:

Now, with these coefficients, you can calulatate the temperature for any given resistance:
R (Ohm)
Result (F)
Result (C)

Copyleft Andrey 2013

External links:
Voltage Divider calculator