According to Heisenberg’s uncertainity Principle, it is impossible to measure  calculate exactly both the position and momentum of an electron simultaneously. This principle is based on the wave – particle dual its of particle. 
Mathematical form of Heisenberg’s principle is $\Delta x.\Delta p\ge \frac{h}{4\pi }$ (OR) $\Delta x.m\Delta v\ge \frac{h}{4\pi }$   
 $\Delta x=$ Uncertainty in finding position of an electron 
 $\Delta p=$ Un certainly in finding momentum of an electron
 $m=$ mass of electron $=9.11\times {10}^{-31}kg.$