The β Constant (expressed in Kelvins, K) of an NTC thermistor reflects relationship of Zero-Power Resistance with temperature and is expressed by the following formula for any two resistance-temperature points.
 
Where:
β is the Beta Constant [K]
T₁ is the absolute temperature [K] at any point
T₂ is the absolute temperature [K] different from _T1
R₁ is the resistance [Ω] with zero power at T₁
R₂ is the resistance [Ω] with zero power at T₂
The  Constant increases slightly as temperature increases, so it is calculated from two predetermined temperature points, T₁ and T₂, as required by the application. Typical temperatures used are 25°C/85°C, 25°C/50°C, 100°C/200°C, etc. with 25°C/85°C being the most common temperatures used.
The temperature difference should be in the range of 50°C to 100°C. If T₁ and T₂ have a difference less than or greater than this range, the error in the calculated  β Constant may be too great for a specific application - please see Fig. 2.

Tolerances of β Constants are shown in Table 2 with temperature sensing normally selected from F to K and temperature compensation from J to L. The material of the Stetron thermistor determines its resistance and β Constant for a particular package size. Generally, a larger β Constant produces a higher Zero-Power Resistance.