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MemoryInUse

MemoryInUse[]
gives the number of bytes currently being used to store all data in the current Mathematica kernel session.
MemoryInUse[$FrontEnd]
gives the number of bytes used in the Mathematica front end.
Current memory use in the kernel:
Storing a million elements of t takes additional memory:
Current memory use in the front end:
Current memory use in the kernel:
In[1]:=
Click for copyable input
Out[1]=
In[2]:=
Click for copyable input
Storing a million elements of t takes additional memory:
In[3]:=
Click for copyable input
Out[3]=
 
Current memory use in the front end:
In[1]:=
Click for copyable input
Out[1]=
Even though there is no visible output, the result is still assigned to Out[2], so memory is used:
Add an expression after the ; to prevent the large result being assigned to Out[4]:
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