How to calculate resolution of radar?
The range resolution is normally some fraction bandwidth of the pulse. The fraction will depend on how you measure it i.e. null to null, 3dB points, 6 dB points etc. Also the windowing (Hamming, Kaiser) used will reduce the resolution i.e. increase the number. For a chirp pulse with the bandwidth you stated the null to null width is given by

$$ \delta r = \frac{c}{2BW}=\frac{3\cdot 10^8}{2\cdot  40\cdot 10^9 }=3.75 \cdot 10^{-3} \textrm{ m } $$

You can reference the following links:


High Range Resolution Techniques
Understanding Millimetre Wave FMCW Radars, 
Radar Basics